Fall-Planted vs. Spring-Planted Onions: Choosing the Right System for Your Garden - Ep. 317

[00:00:00] If you have been suffering through scorching heat like we have been here in West Central Missouri right now, that point in the evening where the light kind of just starts to go gold just before sundown is when everything maybe looks a little more forgiving than it did at 2 o'clock in the afternoon. Like, at 2 in the afternoon, things are looking really ugly. At least they are around here.

[00:00:25] But in the evening, you can see that the tomatoes are still sort of hanging on and the zucchinis are still doing their thing and the cucumbers are tattered but they're still producing. And maybe right next to those beds is the one where you are preparing to tuck garlic cloves in in a couple of months after we talk through the variety selection on last episode.

[00:00:46] You may also be looking at some empty or partially empty beds where the crop is over and done and the beds need to be cleared. And this really may not feel like the time of year that you want to be doing that. I mean, you've already prepped a bed for garlic, right? Right? And who wants to go through just doing that prep all again right away to prepare for something else?

[00:01:11] Well, it might be time that fall planted onions enter this conversation. And this is an argument that you can have with yourself pretty much every September. Do you take the effort to clear and prepare a space to use for onions right now, let them sit through the winter and pull bigger bulbs next June?

[00:01:33] Or do you leave it open and just put it to bed for the rest of the year and then just plant onions the normal way come spring? Welcome back to Just Grow Something. I'm Karin Velez, market farmer, horticulturist, and someone who has practiced both of these techniques the past several years. And that argument is exactly what we are going to tackle today. Overwintering onions has gotten a lot of attention in gardening forums, especially online in the last several years. And for good reason.

[00:02:02] If you do it right, it can genuinely get you bigger bulbs earlier in the season. And documenting my way of doing this is actually one of my most frequently watched YouTube videos. So there is a lot of interest in this. But the biology behind it is more specific than just plant in the fall instead of the spring.

[00:02:27] And the cost of getting this wrong is a bed that maybe bolts on you or, you know, at the very least just sits up, you know, tied up for the better part of the year producing not much of anything at all. So I want to walk you through what's driving that, which varieties the research says are worth the risk, and how to decide whether this system belongs in your garden at all. Let's dig in.

[00:02:57] So if this show is useful to you in any way, shape, or form, following or subscribing helps more than you might think. So please, if you have a moment, take a moment to do that in your podcast player or on YouTube if that's where you are watching me right now. Please and thank you. Also, a quick note on something that I failed to talk about in the garlic episode last week, and that is the option of saving your own garlic as seed garlic. I was listening back to that episode, and I cannot believe that I didn't mention that.

[00:03:27] Yes, if you have your own garlic that you harvested this year and you want to set aside some of that harvest to replant as your own seed stock for this season, absolutely go for it. So long as you didn't experience any pest or disease pressure, you should be good to go. Always choose the best bulbs to replant, okay? We plant the best. We eat the rest.

[00:03:50] So I have about five pounds of our softneck garlic saved from this season, cured and waiting to go in the ground when the time is right, in addition to the five pounds of hardneck garlic that I've ordered from our local growers' co-op that does bulk ordering of seeds and supplies. So if you have some beautiful specimens of garlic that you don't mind planting rather than eating, and you have cured that garlic to hold until it's time to plant, then go for it.

[00:04:20] But if you've seen any signs of pests or diseases in that crap or in that soil, do yourself a favor and just order fresh seed stock. I know it's expensive, but it truly is not worth the risk. And yes, if you're doing the math from last episode, that's enough garlic to plant around 500 row feet of garlic. And even that may not be enough, but it's all I'm willing to do at this point.

[00:04:44] We planted roughly that amount last year, and we still sold out within about six weeks of harvesting. I mean, garlic truly is a staple. And on a final note, if you have ordered from larryscoffee.com slash justgrow here recently, and you've gotten your free gift, and you've tried your first round of their coffee, please, please, please reach out and tell me what you think. I told you that this is like my current coffee obsession. I accidentally let myself run out.

[00:05:13] And while waiting on my next order to show up, I ground up some other beans that I had on hand, and now I am truly tasting the difference between my other standbys and larrys. But I also went down a rabbit hole for my next order, and I realized that larrys also has a whole line of coffees for a cause. They have five different blends that each give back to one particular organization, like, you know, one that does research into the root causes and treatments of mental illness.

[00:05:42] One that's benefiting the official nonprofit partner of the Great Smoky Mountains National Park. And even one that is helping a differently abled young lady with her dream of owning her own coffee shop. Come on. What? I love this company more and more every time I dive into them. So larryscoffee.com slash justgrow is how you can support each one of these organizations if you choose.

[00:06:07] And you can support this show, and you get your own fair trade free gift. I will leave the link in the show notes. It is just such good stuff. So let's start with the day length science that runs the whole Onion Show. An onion does not decide to start forming a bulb until it gets the proper number of hours of daylight.

[00:06:37] Okay. So basically, once that number of hours, that number of daylight hours crosses a variety specific threshold, the plant starts to shift its energy from growing leaves to swelling of bulb. There are three categories of day length. There are short day onion varieties. Okay. They start forming bulbs at between 10 to 12 hours of daylight. There are intermediate varieties, and they need 12 to 14 hours.

[00:07:03] And then there are long day varieties that need 14 or more days before that bulbing process kicks in. Now, the reason that that threshold matters so much is because the size of the Onion Bulb that you eventually harvest is very much determined by how much leaf growth the plant puts on before that day length trigger hits.

[00:07:28] So big, healthy top growth at the time of bulb initiation means a bigger bulb. A little plant that gets triggered into bulbing too early just doesn't have the leaf area to build much of anything under the ground. And so it forms a very small, often mostly unusable bulb, no matter how long you leave it in the ground afterwards.

[00:07:52] We have to remember that the bulb, the part that we eat, is actually the storage organ for that onion. The plant's whole goal is reproduction. And onions are biennials, which means they generally flower in their second year unless they are under extreme stress. So that onion needs to grow a bunch of green tops, take advantage of the photosynthesis fed by the sun and the nutrients from the soil.

[00:08:20] And when it's time to go dormant, it sends that energy from the green tops down into that bulb to store during dormancy. And then once it breaks dormancy, it will send up some new green growth, and then it sends up flower stalk, and it drops seeds. And now the cycle is complete, and that onion will rot into the soil to feed the seeds that it just dropped. We, as omnivores, who've learned to cultivate plants for our consumption, interrupt this process, okay? We want that storage onion for ourselves.

[00:08:50] So our whole goal is to get those leafy tops as big as possible before that day length hits the right time for that plant to want to store its energy. And that gives us a nice big bulb that we can dig up and we can store. Now, picture a short day onion, okay? This is one that bulbs at around, let's just say, 11 hours of daylight, whether it's planted in the fall or the spring, okay?

[00:09:18] Let's picture this in Salem, Oregon, which sits at 45 degrees north latitude. That variety would hit its 11-hour trigger around February 25th. At that point in a northwest winter, the plant is still very small, okay? It's going to bulb anyway because the day length tells it to, and so the result is going to be a very small, underwhelming onion.

[00:09:46] Now, take a long day variety at that same location, one that needs about 15 hours of daylight, and it's not going to hit that trigger until around May 18th. By then, the plant has had months of additional growing time to build up that leaf mass, and it produces a large, well-formed bulb, okay?

[00:10:11] Same location, same planting date, very, very different outcomes. And the entire difference between those two comes down to which day length category that variety belongs to. So this is why the seed catalogs will label these onions as short day, intermediate day, or long day because it does make a difference.

[00:10:34] It is an important decision that you make when choosing an onion variety because you have to match that label to your latitude, okay? So as another example, storage onions grown in Minnesota are generally long day types that require 14 or more hours of daylight because that's what the latitude delivers.

[00:10:55] If we were to plant a short day variety up there, it's going to trigger that onion into bulbing absurdly early with nowhere near enough leaf growth behind it. Now, here in Missouri, where I am, we sit in that very broad sort of intermediate to long day band, right? So we're north of the line where short day varieties make sense, but we're not always as far north as like the classic long day recommendation.

[00:11:24] My latitude is just shy of 39 degrees north. So I can choose either intermediate or long day varieties, regardless of whether I plant them in the fall or the spring, which, spoiler alert, is why I do both, right? But not every location is suited for fall planting. Some require fall planting because spring wouldn't get you the amount of green tops that we need.

[00:11:49] So if you're trying to figure out what works best for your area, there's a shortcut. I will leave this info in the show notes, but pause me right now if you need to grab a pen and write this down. I'm going to give you the shortcut, okay? You ready? Okay. Short day onions. If you're sitting at latitude 25 degrees and 35 degrees north, okay? Your day length trigger is between 10 and 12 hours.

[00:12:17] So we are talking usually places with mild winters, USDA zone 7 plus. And these are generally planted in the fall to mature in the late spring, okay? Now, if you're sitting at latitude 32 to 42 degrees north, your day length trigger is going to be 12 to 14 hours for your intermediate day onions, okay? This is like the central U.S.

[00:12:47] Generally, these are planted in the spring or the very, very early spring in certain areas. These are things like superstar candy, red candy apple, okay? Long day onions. If you are sitting at latitude 37 degrees to 47 degrees north, you want long day onions. The day length trigger on these guys is between 14 and 16 hours. So these are our northern latitudes.

[00:13:13] These also are often, you know, sweet or specialty or the very, very long storage type onions. Remember those ones I just talked about up in Minnesota, right? These oftentimes can be planted in the spring depending on the climate. Other places, they are planted, I'm sorry, they're often planted in the fall depending on your climate. But other areas are going to be in the spring depending on how cold your winters get, okay? So these are things like Walla Walla, Copra, and Elisa Craig.

[00:13:41] And again, like I said, where I'm at, at 39 degrees, I can choose between long day and intermediate day. And which ones I plant in the spring and which ones I plant on the fall will depend on a few other conditions that we'll talk about here in a minute. Again, I will leave all that information in the show notes so you can look at it at a glance or copy and paste if you want to. Now, day length explains bulbing, right? But it does not fully explain bolting.

[00:14:09] And bolting is the reason that overwintering onions is a little bit of a different proposition than spring planting. So for that, we need a second mechanism, and that is vernalization. Wait, what do you mean you heard that word before? Yes, we heard that word last week when we were talking about garlic, right? Remember, vernalization is a period of cold that is needed for a plant to take a specific action. And in garlic, it was the differentiation of the bulb.

[00:14:37] For onions, it's a little bit different. Remember, we said onions are biennials. So in a normal, uninterrupted life cycle, they spend their first year building that bulb. They sit dormant through the winter, and then they flower and set seed in their second year. That flowering switch is what is triggered by vernalization. Again, the sustained period of cold that tells the plant that winter has happened,

[00:15:03] and now it's safe to shift into reproductive mode. Vernalization in biennial crops generally needs six to eight weeks of temperatures below 50 degrees Fahrenheit. Of course, you know, the specifics of this is going to vary based on the crop and the variety. For our purposes with onions, what this means is when you overwinter an onion,

[00:15:27] you are deliberately exposing a living plant to exactly the cold exposure that can flip that reproductive switch. If the plant is too large going into that cold, meaning it has already built up enough size that it reads as a mature plant rather than a juvenile one, it is far more likely to respond to vernalization by bolting.

[00:15:53] And when I say bolting, what I mean is sending up that hard, woody flower stalk instead of continuing to build that bulb. This is why overwintering onions have to go into the winter fairly small, okay? So if you're planting from transplants, we're talking about the width of a pencil. If you're talking sets, we really only want them about a quarter of an inch across. A plant that reads as juvenile, okay? It survives the cold.

[00:16:21] It sits through the winter and then it responds to spring's increasing day length by bulbing rather than the cold it just experienced by flowering, okay? If it's too big before the winter sets in and has crossed that sort of size threshold where it thinks that it's mature, then that vernalization can trigger bolting or flowering instead. So we want to avoid planting too early in the fall, specifically to reduce that bolting risk.

[00:16:51] We don't want the plants to root in and then take off growing vigorously before winter sets in and get too big before they go dormant. So with fall plants or with fall onions, we've got two different mechanisms that are kind of working on two different timelines. The day length decides when the plant bulbs once it gets the chance to.

[00:17:13] Bernalization decides whether the plant bolts instead of bulbing based on how large it was when it went through that sustained cold exposure. Overwintering asks a plant to survive winter at the size where these two systems could go either way,

[00:17:32] which is why variety selection and planting timing matter so much more in the fall than they do for a spring planted crop because that spring planted crop is never going to face the vernalization window at that vulnerable size. So here's where we're going to bring the university research into the conversation.

[00:18:01] This is where I want to get specific because me telling you some varieties overwinter better than others isn't really especially helpful if I don't tell you which ones, right? So the most detailed data set that I found comes from a multi-year overwintering trial run at the University of New Hampshire working out of Woodman Farm in Durham. This is Zone 5B, so it is legitimately a little bit of a colder climate.

[00:18:26] In their 2014-2015 trial, they evaluated 18 onion varieties. They planted them at multiple dates from mid-August transplants through mid-January as sets, and they compared the performance in both low tunnels and high tunnels. Now, I know a lot of us aren't using low tunnels or high tunnels, but they're worth mentioning because they lend a layer of protection.

[00:18:53] In my experience, that level of protection can be achieved through mulch because that is what we do. I am not planting my onions in any kind of, you know, protected structure, okay? Now, when they say transplants, they're generally talking about baby plants, something that was recently sprouted from a seed about the size of a pencil. And when we say sets, we're talking about the little mini bulbs that have had their growth arrested and forced into dormancy for storage and then planting, okay? Just so we're on the same page here.

[00:19:20] So they planted transplants in mid-August and they planted sets in mid-January. And I think they did a few other plantings in between there as well. A few things came out of that research that I think are pretty useful for backyard gardeners. First, regardless of variety or the planting site or the planting date, nearly all of the onions survive the winter itself just fine. So when I have gardeners ask me, well, hey, I live in such and such place, but we actually get, you know, freezing solid winters.

[00:19:51] Is it too cold in your climate to try, you know, to overwinter onions? Winter kill is not the risk here. I tell people just add more layers of mulch to your colder winters, but absolutely go for it. I get messages like this after people watch my YouTube videos all the time. Bolting was the risk in this trial, not winter kill or rot, okay? Second, and this is the part that matters for variety selection.

[00:20:19] Some varieties bolted at really unacceptably high rates, almost no matter what the researchers did. Candy, Century, Pontiac, Cabernet, Red Coach, Pinot Rouge, and Chianti. All of those varieties showed high bolting percentages at early transplant dates. And even when planted later to reduce that risk, they just tended to produce only really small bulbs like under four ounces.

[00:20:49] So the research team's conclusion was that those varieties are not recommended for overwintering. Full stop. Regardless of how well-suited they might be to a spring planting in your area. And I have seen this in my own garden. I no longer try to plant my candy variety onions in the fall as sets to overwinter. I plant them as transplants in the spring.

[00:21:13] On the other end, a specific group of varieties showed excellent winter survival and very low bolting rates even at the earlier supposedly riskier transplant dates. So the yellow varieties were Bridger, Gatekeeper, High Keeper, Keepsake. Are you seeing a trend there? Toughball. And then there was two numbered ones like T440 and T448, which we will never see in a catalog, right?

[00:21:40] Along with a couple of red varieties, Desert Sunrise and Electric. These are the varieties that have been selected for overwintering performance. And now that data actually backs it up. And a version of Bridger is one that I have had very good success with overwintering as sets in my own gardens. Again, a result of a university trial that I have been able to duplicate in my own gardens. Now, Walla Walla.

[00:22:10] Okay, this is probably the most famous onion in this conversation, especially if you've spent any time in the Pacific Northwest gardening circles. Walla Walla basically landed kind of smack dab in the middle. And that has also been our experience here on the farm. In the trials, it showed a really high percentage of bolting at the earlier transplant dates, but it produced excellent results, like bulbs over half a pound when they were plated later in the planting window.

[00:22:39] So the researchers basically said that Walla Walla is a riskier choice than the dedicated overwintering varieties, but essentially that the timing precision matters more with Walla Walla than it does with a variety that is bred specifically for this system. So we tend now to also just plant the Walla Walla in the spring if we're growing it at all.

[00:23:04] Planting date itself made a difference regardless of the variety they were planting. Earlier planting generally increased that bolting risk across the board, but for the varieties with that strong bolt resistance, the earlier planting also produced the largest bulbs because those plants got more fall growth in before winter shut things down. And that is sort of that tension that sits there right at the center of this whole system.

[00:23:32] If you push the planting earlier, well, you're going to risk more bolting, but you also, you know, get bigger bulbs if the variety can handle it. If you push the planting later, you're reducing that bolting risk, but you're also giving the plant less time to size up before dormancy, and that's going to cap how large of a bulb you can get even from a bolt-resistant variety. So the takeaway here, I think, is that the varieties matter a lot for overwintering.

[00:24:02] So grabbing whatever onion seed or set happens to be labeled for fall planting without kind of checking whether it's one of those bolt-resistant ones, you know, may set you up for a bed full of bolting onions in April instead of harvesting bigger bulbs in June. But if we can wait a little bit in our planting window, then we have a much better chance of getting those varieties through without the risk of bolting. But if we really, really, really do want those larger bulbs,

[00:24:30] and that's part of the reason why we're planting in the fall, then we should look for those ones that reduce that bolting risk and try to plant them just a little bit earlier. Now, let's talk about the cost of taking up that bed space. If you're transplanting onion seedlings or sets in October

[00:24:59] and you're harvesting bulbs the following June, that bed is committed for somewhere in the neighborhood of 9 to 10 months. Compare that to a spring planted crop, okay? A transplant's going in as soon as the soil can be worked, generally early March in a lot of the Midwest, with a harvest by mid-summer. That's a crop occupying the bed for roughly 4 to 5 months, not 9 or 10.

[00:25:27] So if you listen to episode 296, where we talked about succession planting and never leaving a bed empty, you might already know where I'm going with this, right? A bed that is tied up with overwintering onions from September or October through June is a bed that cannot host a fall crop. It also cannot host an early spring crop, and it cannot be a part of any succession sequence for most of the year.

[00:25:54] Even if you're not planting until late October or early November, there is still a little bit of a trade-off. And that's not a small trade-off in a garden where space might be limited. And for a lot of home gardeners, space is the limiting resource, right? So this means overwintering onions is a system with some real opportunity cost. And that cost needs to be weighed

[00:26:22] against what you would otherwise be doing with that bed. If you've got garden space to spare, or if you're specifically trying to get onions harvested earlier than a spring crop could get them, well, then that trade might make complete sense to you. If you're working a small raised bed setup where every square foot needs to pull its weight across multiple crops in a season, well, tying that space up for the better part of a year

[00:26:50] for a crop that spring planting could also give you, maybe just a few weeks later, that might be worth thinking through really carefully before you commit to this. Now, the alternative to this is to choose the corners of your beds for your overwintered onions and then interplant the rest of your crops around them in the spring. Onions make a great companion to a lot of crops, okay? I routinely tuck green onions into all of my beds just willy-nilly to get some insect protection

[00:27:19] and to fill the space, to choke out the weeds. And I also get like a small crop of something in as little as 30 days. You can do this with full-sized bulb onions too, so long as you leave enough space for the bulb to grow and the things to be harvested around it without the onions being disturbed. So don't take the bed occupation as a true drawback until you really think through your garden plan a little bit.

[00:27:43] There's also a little bit of a regional research gap here too. Most of the detailed research on overwintering onions comes out of places with either genuinely mild, or maritime winter like the Pacific Northwest, or from controlled research systems that are using low tunnels and high tunnels for winter protection like that New Hampshire trial, right? University of Missouri Extension's guidance on onions

[00:28:13] only describes the standard spring-planted system and doesn't even cover an overwintering protocol for our specific climate the way it does for garlic. Now, it's not a sign that overwintering can't work here in Missouri. It absolutely works because I do it every year. But it is a sign that if you try it without any kind of season extension protection, you're maybe doing so with less regional research behind you

[00:28:41] than like a grower in Oregon or in coastal New England has. And that means that your first attempt in your own garden is likely a genuine trial, okay? Not necessarily a bad thing, but it's also not a guaranteed success. Of course, is anything actually a guaranteed success in our garden? Not in my experience. So where does this leave you? If you garden somewhere with reliably mild winters, okay, the Pacific Northwest, the coastal Southeast,

[00:29:09] similar maritime or mild winter climates, overwintering with a proven variety like Bridger or Desert Sunrise or even Walla Walla planted at the right size and the right time is very well supported by both university trials and the regional extension recommendations. There are also some newer intermediate long day cultivars that have shown really good promise, even in the traditionally less suited Southeast

[00:29:37] when you match it with the correct planting timing. Now, if you're gardening in a colder, more continental climate like ours here in Missouri or anywhere in that broad transitional to really cold winter band, without season extension infrastructure like low tunnels, overwintering onions can be an experiment that may take a few tries to get right. The New Hampshire research shows it can be done. My own gardens show it can be done,

[00:30:07] but it may require specific bolt-resistant varieties and paying careful attention to the transplant size and the timing, and in most cases, some form of winter protection, even if that's just a heavy layer of mulch. Spring planting is the reliable default for most home gardeners, and there is no shame in that being your answer. It doesn't ask you to gamble a planting

[00:30:34] on a fertilization response that you can't fully control, and it also frees that bed up for a fall crop the year before, maybe a winter cover crop, an early spring crop the same year, and that matters if you're working with limited space. Now, the approach that I have taken with our own gardens is this. I plant onion sets around mid-November, generally after we've had our first few sets of frost, but before the ground really starts to freeze solid.

[00:31:04] Essentially, at the same time I'm planting our garlic, whenever that ends up being, okay? I mulch really heavily, and then I walk away. I plant transplants of different onion varieties in late February or early March, whenever the soil can be worked. I automatically assume that I will be pulling a ton of those fall planted onions as green onions or what we refer to as spring-sized onions, okay?

[00:31:33] This means I have a continuous harvest of early green onions and some baby bulbs that can be used for both the bottoms and the tops, and then I will have a full harvest of fully bulbed onions from my spring planting. That way, if any of my fall planted onions start to show signs of bolting, they're pulled early and they're used up, and the remaining fall planted onions and all of the spring onions are left behind to be full-sized. That is the methodology that has worked for us

[00:32:03] the past several years, and that way I'm not disappointed by either crop. Some years, none of them bolt, and I'm just thinning a lot of them out as green onions. Other years, the entire crop of fall onions bolts, and then my spring-planted ones are the only ones that make it to maturity. So if you do want to try overwintering for the first time, my honest advice is to go for it, but to treat it as a trial planting in a corner of a bed

[00:32:32] rather than your entire onion crop for the year, choose one of those research-backed bolt-resistant varieties rather than just grabbing whatever, you know, is available for a fall planting. Get your transplants in at pencil diameter size if you're using transplants or very small sets, and do it before we're heading into the cold, but as soon, you know, as late as you possibly can, okay? That is the setup, I think,

[00:33:02] that gets you the best shot at getting the earlier, bigger bulbs that is kind of the whole point behind doing this. And if that sounds like too much of a gamble, then stick with the spring planting and call it good, or do like I do and do both. Regardless of which system you choose, walk out to whatever bed you're thinking about putting onions in and just ask yourself honestly, can this space afford to be tied up for nine months out of the year,

[00:33:31] or do I need it back by late winter, early spring for something else? That answer alone might make this decision for you. In the show notes, I have linked to every reference from today's notes, the day-length physiology, the full New Hampshire variety trial data, which is actually very interesting if you dig into it. I really encourage you to read that one. And like the regional extension guidance for both overwintering and spring planting. Next week, we're shifting gears

[00:34:00] to something that works in that exact bed space gap that we just talked about, cover crops, and how to use them to protect and build the soil in the beds that aren't tied up growing something like onions this winter. Until next time, my gardening friends, keep on cultivating that dream garden, and we'll talk again soon.