How do we know as gardeners when the appropriate time is to plant those seeds or transplant our seedlings? There are guidelines on the backs of seed packets and tables and charts online and in books that tell us the supposed correct date to plant those seeds. But this generic information is usually based on the average last frost date for our area. If you’re in an area that doesn’t often get a frost or if you live in a frozen tundra that date can be deceiving. Plus, that information doesn’t take into consideration the fluctuations we see in our temperatures and weather patterns each season or the ever-changing climate. In nature, seeds just know when to sprout at the right time based on the cycling of the seasons. There proper germination is based on factors like moisture levels, light availability and, most importantly, soil temperature.
Today we’ll talk about optimal soil temperatures for both cool season and warm season crops, for both seed germination and transplant growth, how that compares to our average air temperatures, how to properly test your soil temperature and where to find historic soil temperature data for your area so you can more effectively plan your planting dates. Let’s dig in.
Karin Velez [00:00:01]:
This is positively farming media. Hello, my gardening friends, and welcome back to the Just Grow Something podcast. Many of us here in the northern hemisphere are scurrying to get our spring gardens planted right now. And I know some of you are desperately waiting for the snow to melt. And then there are those of you in the warmer areas who've either already gotten your early summer crops transplanted or you are working on that now. But it how do we know as gardeners, when the appropriate time is to plant those seeds or transplant the seedlings? Now, there are guidelines on the backs of seed packets in tables and charts in online and in books that tell us the supposed correct date to plant those seeds. But this generic information is usually based on the average last frost date for our area. Well, if you're in an area that doesn't often get a frost or if you live in a frozen tundra, that date can be deceiving.
Karin Velez [00:00:56]:
Plus, that information doesn't take into consideration the fluctuations we see in our temperatures and our weather patterns each season or the ever changing climate. Now, in nature, seeds just know when to sprout at the right time based on the cycling of the seasons. Their proper germination is based on factors like moisture levels, light availability, and, most importantly, soil temperature. So while using the last frost date or our average air temperatures might be a good guideline to start with, a better method for knowing when it's actually time to sow those seeds or transplant those plants is the soil temperature. In my area of west central Missouri, the last frost date is around the middle of April. I've seen many a gardener get into a rush and plant out their tomatoes right after that frost date into very cold, very wet soil. And those plants sat there and waited for the soil to warm up before doing anything. In fact, plants that went into the ground a full four weeks later than those April plants produced fruit at almost the exact same time.
Karin Velez [00:02:02]:
And the plants were healthier and more able to fight off the diseases that we inevitably face here every single season. Today, we'll talk about optimal soil temperatures for both cool season and warm season crops for both seed germination and transplant growth, how that compares to our average air temperatures, how to properly test your soil temperature, and where to find historic soil temperature data for your area so you can more effectively plan your planting dates. Let's dig in. Hey, I'm Karen, and I started gardening 18 years ago in a small corner of my suburban backyard. When we moved to a five acre homestead, I expanded that garden to half an acre, and I found such joy and purpose in feeding my family and friends. This newfound love for digging in the dirt and providing for others prompted my husband and I to grow our small homestead into a 40 acre market. Farm. When I went back to school to get my degree in Horticulture, I discovered there is so much power in food and I want to share everything I've learned with as many people as possible on this podcast.
Karin Velez [00:03:02]:
We explore crop information, soil health, pests and diseases, plant nutrition, our own nutrition, and so much more in the world of food and gardening. So grab your garden journal and a cup of coffee and get ready to just grow something. So the pace around here is pretty frenetic right now. The temperatures have warmed up just a little bit, and so we are slamming all of our spring crops into the ground. One of the things that took me a really long time to get into my head is that I typically was planting my early spring crops too late and my summer crops too early. Once I figured out that our temperatures here can fluctuate drastically in spring, which can sometimes cause early crops to bolt, and I figured out what I needed to do to make them tolerant of freezes, I started being able to plant those crops much earlier and have had very good success. I also had to get out of the mindset that rushing to get the warm weather crops in as early as possible was going to get me an earlier harvest and a higher yield, because that was patently false. And both of these realizations had everything to do with soil temperature.
Karin Velez [00:04:19]:
When we look at direct sowing seeds in the spring, the coldest temperature at which any seed will generally germinate is 40 degrees Fahrenheit or ten Celsius, with a handful that will sprout in temperatures as low as 35 Fahrenheit. The warmer that soil is, the faster that seed is going to germinate up to a certain point at which the temperature actually becomes detrimental to the seed. I generally aim for around 45 degrees Fahrenheit for my early crops, and this includes transplants. My goal with transplants is to get them in the ground early enough that they can grow substantially before we start getting any spikes in our air temperatures. I want them far enough along that if we see temperatures jump up into the 80s in a few weeks, which is pretty common for us in the spring, they would be less likely to bolt. This may mean planting cabbage and other brassicas as early as the first week of March, which is what I did this year. But I based this decision on the soil temperature. The soil temperature the first week in March was consistently averaging 45 degrees Fahrenheit, while the air temperatures had been in the 70s that week.
Karin Velez [00:05:28]:
So I took the leap and I got the first cabbages planted. I knew that we would still have some severe cold weather, but the plants I put out were large enough and hardened off enough that I thought they would tolerate a hard freeze. So when we dipped to 17 Fahrenheit overnight for two nights in a row. The following week, the majority of those plants did just fine with a simple row cover. The only ones that were damaged were plants that had gotten exposed when heavy winds pulled the edge of the row cover off in the middle of the night. But even those plants are able to bounce back because the root zone was definitely warmer than the air temperatures. And this is why soil temperatures are so important. Air temperatures can change very quickly whether it drops below freezing with a temperamental spring storm or it heats up substantially.
Karin Velez [00:06:17]:
On a sunny day, soil temperatures change more slowly because of the mass of the soil and the moisture content. So on a sunny day, any exposed soil surface will warm up much more quickly than if it's covered really well with mulch. This is one reason why we mulch our garden beds to keep that soil temperature constant. But this is also why we want to plant based on soil temperatures rather than air temperatures alone. Just because we've gotten past the last frost date doesn't mean the soil temperatures are conducive to planting those heat loving plants. If you look at the packet instructions for starting those seeds, you'll see a recommendation for the soil temperature for germination. Let's use tomatoes for our example. The lowest temperature at which tomato seeds will successfully germinate is right around 50 degrees Fahrenheit, or ten Celsius.
Karin Velez [00:07:09]:
That's the bare minimum, and that's usually going to result in slow and very spotty germination. Ideally, they like a temperature much closer to 70 Fahrenheit or 20 1. Commonly listed optimum temperature for tomato seed germination 85 Fahrenheit or 29 Celsius. For that seed to germinate in about 24 hours, with almost 100% success. That's why we have more success, and our tomato seeds and other heat loving plants sprout more readily and more consistently when we put them on a heat mat. We're warming up that soil above the ambient air temperature to help that seed get going. The more difficult it is for a seed to germinate, the more likely it will struggle as a seedling, and the more likely it will have difficulty fighting off pests and diseases. The repercussions of that slow germination are that long lived.
Karin Velez [00:08:04]:
Okay, but that's seeds. What does this mean for our plants? What temperatures are best for transplanting seedlings out into the garden? Well, those recommendations for seed starting also hold true for the optimal temperature for plant roots. And it makes sense if we think about it. Why would a seed out in nature sprout at one temperature but prefer a different temperature at the root zone? So if we look at the optimum temperature of 85 degrees Fahrenheit for a tomato seed, we can assume that a seed that has been on the soil surface from fallen fruit the season before is going to be in about that top inch of the soil. This, of course, is the zone where the sun is going to warm the soil the fastest. So if the optimum temperature for Germination is 85, then the optimum soil temperature for growth is the soil temperature a few inches below that. Generally speaking, a soil's temperature at a depth of two inches will be about 33% lower than the surface temperature. And at four inches, the temperature is 66% lower than the surface temperature.
Karin Velez [00:09:11]:
So for our tomatoes, this means where the seed germinates at 85 Fahrenheit, the root zone when it starts to grow will be at 57 Fahrenheit. So if we're transplanting our tomato plants at a soil temperature that is less than 57 Fahrenheit or 14 Celsius, we're not getting our plants off to the best start. And since we're generally going to be planting those plants at about the three to four inch mark, this is even more important. The general recommendations for cool weather plants are a minimum soil temperature of 60 degrees Fahrenheit, or 15 Celsius for Germination, 40 to 50 Fahrenheit, or four to ten Celsius for transplanting, and an average air temperature of 60 to 85 Fahrenheit, or 15 to 29 Celsius for growth. Now, for our warm weather plants, these increase to 75 Fahrenheit, or 24 Celsius for germination minimum of 50 Fahrenheit or ten Celsius for transplanting. But really they'll struggle at anything lower than 60 Fahrenheit and a minimum average air temperature of 75 Fahrenheit, or 24 Celsius for optimum growing. So what happens if we choose to transplant into soil that is cooler than ideal for our warm weather plants? The research has shown us that colder soil temperatures impede the root growth of any plant species that is sensitive to frost, like our tomatoes. In fact, the roots of tomato plants stop growing at 50 Fahrenheit, and that root growth is slow to recover after the roots finally do warm back up.
Karin Velez [00:10:54]:
Less than 2 hours of root exposure at that temperature is enough to cause damage. And it takes more than 8 hours at those warmer soil temperatures for the plant to recover. So if we're rushing out there to get our plants in the ground as quickly as possible, and the soil is not warm enough, not only are we not getting an early harvest, we're likely getting a later one and one that won't produce as much. Now, I know my northern gardeners are cussing me out right now because many of you just don't have a choice in the matter. You have a very short growing season and need every single daylight hour to get a crop of tomatoes or peppers or whatever other warm weather crop you're trying to grow. I get that. Stick with me, though, because here in a minute, we're going to talk strategies for warming that soil up artificially to get you where you need to be. Okay, so that's the case for waiting for soil temperatures to warm up in the spring.
Karin Velez [00:11:49]:
What about waiting for soil temperatures to cool down? There are a lot of places where you quite literally cannot grow much of anything in the summer because it's just too hot. What temperatures are too warm for planting? At what point will seeds not germinate and essentially cook in the soil? We'll talk about that and how to mitigate that problem after the break. It's time to get moving on the gardening season, and the folks at Elmirt have us covered. From potting mix to worm castings, their fantastic plant juice and bloom juice, even their new Kelp Mist foliar spray, they've got what we gardeners need for starting and growing plants of all kinds. They've even got gift bundles for the beginning gardener or the plant lover in your life. Listeners of this podcast get a free bottle of Bloom juice with any purchase from Elm Dirt. So head to Justgrowsomethingpodcast.com dirt and use code justgrow at checkout for your free bottle. The link is in the show notes so if you're in an area where most gardening has to come to a screeching halt in the summer because of high temperatures, you're facing a different problem than we are here in the spring.
Karin Velez [00:13:05]:
You are likely getting those plants in as early as possible to get a crop before the heat of the summer hits than trying to get more plants in for the fall for a second round of gardening. And even those of us in the Midwest are trying to time our fall planting so it's not too warm when they go in, but not too late that we hit a frost before our plants mature. So how warm is too warm for our plants and our seeds? You might be surprised. Many of our heat loving plants will germinate in temperatures as high as 100 degrees Fahrenheit or 38 Celsius. Plants like turnips, corn, okra, melons, squashes and cucumbers will sprout quickly at those temperatures a few degrees cooler, and we're looking at even more plants like beets, carrots, radishes, Swiss shard, tomatoes, peppers and beans. But once again, we need to remember we're talking about the mass of soil here. So even if our air temperatures start to cool off a little bit as we head towards fall, it's going to take longer for the soil to follow suit. So from early summer on, our soil temperature is going to typically be much higher than the air temperature.
Karin Velez [00:14:15]:
And this can be affected by the soil type that you have and the moisture content. For example, sandy dry soils heat up very quickly. So if the air temperature is in, the soil temperature is likely already well above 100, and any soil temperature above 105 Fahrenheit is detrimental to seeds and plant roots. So for fall planted crops, you'll likely have good success planting from seed rather than transplants, especially for crops that mature fairly quickly. But if you're looking to transplant seedlings, you'll need to be more careful about the soil temperatures when you plant. So how do we test our soil temperature properly? In order to get the average temperature and how can we speed up the heating or cooling process to facilitate planting? The best time to take the soil temperature is mid morning, and the best place is at the two to four inch mark. Soil temperatures are generally coldest just before dawn, but by about 10:00 A.m., the sun has come out to warm up the soil just slightly to be a bit more representative of the daily average. The warmest soil temperature during the day is around 03:00 P.m., so a mid morning reading should give us the most accurate picture of our average.
Karin Velez [00:15:40]:
I usually do this at least two to three days in a row just to be sure we're hitting the goal temperature consistently. Now, I know not everybody is at home at 10:00 A.m. To go and test their garden soil. So here's the next best thing. Take the reading in the morning before you leave, and then take it again when you get home. Add those temperatures up and divide by two to get your average for the day. Now do this four or five days in a row to get your true average. Now, you can test the soil temperature by having a dedicated soil thermometer, which is what I have.
Karin Velez [00:16:11]:
They're pretty inexpensive, and I'll link to the one that I have in the show notes. Yes, it's an Amazon affiliate link, and I'll likely make like $0.15 if you order it, but it really is the one that I use. You can also use a compost thermometer, which has a longer probe, or you can even use a probe thermometer that you use for cooking. I have a digital one from Weber that I use, and that would make a handy soil thermometer. And I have another handheld digital meat thermometer that would work, too. A little dirt is just going to clean right off. No matter which type of thermometer you use, you want to be sure that you're hitting that two inch to four inch level beneath the soil surface. That's why I like the soil thermometer I have because it's four inches long, and I can just push it down until the display is resting just above the soil surface.
Karin Velez [00:16:57]:
And I know I'm measuring in the right place in the soil and I can keep it with my garden supplies at all times, which means it's ready to use. Now, there are tricks that we can use to modify our soil temperatures, either warmer in the spring or cooler in the fall to facilitate getting our seeds and plants in the ground at the ideal time. Remember I said that mulch is our friend when it comes to maintaining even soil temperatures? Well, if you're trying to get that soil to warm up, there are a few tricks to help you push the boundaries a little bit. Number one is to pull back any mulch that you have already down on the ground during the day to allow the sun's rays to penetrate the soil surface. Soils are typically cooler in the spring coming out of winter, partly because of soil moisture. Now, as that soil warms up, some of the heat that accumulates is being used to dry out that wet soil. The remainder of that heat is what goes towards raising the soil temperature. So if you can move back any mulch to let that heat in, then put it back in place at night before the sun goes down, you can trap that heat in the soil.
Karin Velez [00:18:04]:
Heat absorbed on a warm day actually continues to pass down through the soil layers the next day. So this uncovering and covering is going to have a cumulative effect, raising the soil temperature more quickly. You can also use clear plastic coverings over top of your beds to intensify the heat and significantly raise those temperatures. So placing PVC hoops over your beds and putting clear plastic over that will really get those temperatures spiking and hold it in for longer. Then once your soils are warm enough, you can start planting seeds and use those protective covers to protect the emerging seedlings. Your warm weather plants are still going to need to wait until the air temperatures are conducive to growth. But by getting a head start, the soil temperatures will be ready to go. At the same time, the air temperatures are now on the other end of the spectrum.
Karin Velez [00:18:55]:
How can we get our soil temperatures to cool down faster in the fall? Mulch is our friend again here too. So long as you are using a light colored mulch like straw, you'll be keeping the hottest of the summer sun's rays off the soil's surface. That will help. So if you have beds where you plan to plant in the fall dark landscape, fabric as your mulch is not going to be the best idea. Go with something light. The other way to cool the soil is to add moisture. I mentioned that part of the heat in the spring goes towards drying out the wet soil. Well, the same concept applies in the late summer and early fall.
Karin Velez [00:19:32]:
In most areas with high heat. We also have very little rainfall and the soils have likely begun to dry out. Mulch is going to help with this, but if you can manage to water that soil thoroughly, it will cool it down by several degrees. Trap that moisture in with the mulch and you should see the average soil temperature begin to drop. Now adding some shade cloth over top of the bed will further protect the soil from the sun's rays and lower that temperature even further. It's a multistep process, but if you need to get things in the ground around a certain time to avoid a fall frost, there are the steps that you'll likely need to take to protect the seeds or the plants that you put in the ground. So when we're planning out our seed starting or our transplant schedule for the spring or the fall. If we're relying on soil temperature, then how do we plan? How do we figure out when to start those seeds or buy those plants? We don't want them ready too early, and we don't want to buy them too late.
Karin Velez [00:20:31]:
Enter Historical Data now, I'm not expecting you to be out there taking measurements of your garden soil and jotting that information down in your garden journal every week so that you have that data to look back on. I'm pretty thorough in my notes, and even I don't do that. I do know, though, that if I planted on a certain date, my soil was above a certain temperature. So technically, you can rely on your own notes to get a general idea of when to start seeds and plan for transplants. But there are also people who are paid to gather this data for us. Your local extension website may have historical soil temperature data for your area specifically, but I generally use a soil temperature map from Greencast online. I'll link to it in the Show Notes. It is a fantastic free tool where you can punch in your location and it will give you current and historical data for the soil temperature at that location at two inches below the soil surface and show you the five year and ten year averages.
Karin Velez [00:21:35]:
It's what I base my anticipated planting dates on for everything from sweet corn to tomatoes in the spring and all the late summer crops going in in the fall. And if I'm unsure whether a reading that I'm getting from my garden is normal for this time of year, I can look at the Green cast data to see if it's an unusual year or if maybe I'm just misremembering from previous seasons. I really do love accurate data, so hopefully I have convinced you not to be in too much of a hurry to get your warm season plants in the ground. If your growing season isn't super short and given you a few tips to get the soil warmed up or cooled down if you really need to get those seeds or plants in the ground. I know we all get anxious, but sometimes waiting really is the best thing for our plants and for our harvest. Until next time, my gardening friends. Keep on cultivating that dream garden and we'll talk again soon. You just finished another episode of the Just Grow Something podcast.
Karin Velez [00:22:40]:
For more information about today's topic, go to justgrowsomethingpodcast.com where you can find all the episodes, show notes, articles, courses, newsletter, sign up, and more. I'd also love for you to head to Facebook and join our gardening community in the Justgrowsomething Gardening Friends Facebook group. Until next time, my gardening friends keep learning and keep growing.