Proper Soil Temperatures for Transplanting - Ep. 193

[00:00:00] The most frequent question I'm seeing right now in our local gardening groups in the Kansas City

[00:00:04] metro area is, is it safe to plant? Most of these gardeners are concerned with their tomato, pepper,

[00:00:12] cucumber or squash plants or anything else that likes the warmer weather. But some of them are

[00:00:16] also questioning their cool weather plants too. And that's a legitimate question. The confusion

[00:00:23] is real right now here because even though we're supposed to just now be hitting our last frost

[00:00:30] date, the air temperatures have been unusually warm for most of April and that can lull us into a

[00:00:37] false sense of security. Highs in the 80s Fahrenheit and lows only dipping into like the 50s

[00:00:44] Fahrenheit can make you feel like it's time to put those heat loving plants out into the ground.

[00:00:50] It may also make you feel like you've missed the window on planting your cool weather crops like

[00:00:55] lettuces or snap peas. Using the last frost date in spring or our average air temperatures might be

[00:01:04] a good guideline to start with when figuring out when to plant the garden. But a better method

[00:01:10] for knowing when it's actually time to sow those seeds or transplant those plants is the

[00:01:17] soil temperature. Even though the air temperatures may be warmer than usual, the soil knows the truth.

[00:01:24] If that soil is cold and wet or might be that way in the next 10 days, your tomatoes or peppers

[00:01:31] or whatever are not going to be happy sitting in that chilled soil no matter how warm the

[00:01:38] daytime air temperatures get. But your sugar snap peas and your lettuce might be absolutely

[00:01:44] thrilled. So today on Just Grow Something we'll talk about optimal soil temperatures for both cool

[00:01:50] season and warm season crops for both seed germination and transplant growth, how to properly

[00:01:57] check your soil temperature and where to find historic temperature data for your area so you

[00:02:02] can more effectively plan your planting dates and not be lulled into that false sense of security.

[00:02:10] Let's dig in. Hey I'm Karen. I started gardening in a small corner of my suburban backyard and now

[00:02:16] 18 years later I've got a degree in horticulture and operate a 40-acre market farm. I believe there

[00:02:21] is power in food and that everyone should know how to grow at least a little bit of their own.

[00:02:26] On this podcast I share evidence-based techniques to help you plant, grow,

[00:02:30] harvest and store all your family's favorites. Consider me your friend in the garden.

[00:02:35] So grab your garden journal and a cup of coffee and get ready to Just Grow Something.

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[00:04:29] And quick reminder about the question of the month for April. What have you struggled with

[00:04:37] the most in terms of garden maintenance and did you find a solution? This can be anything from

[00:04:44] maintaining soil nutrients to keeping up with the maintenance of raised planters or trellises,

[00:04:51] proper mulching, keeping up with specific plant diseases, whatever your garden needs that you

[00:04:57] just struggle to give it or that you used to struggle with and you found a solution.

[00:05:03] Answer the question from right there in Spotify if that's where you're listening or

[00:05:07] apply to this week's email newsletter on Friday with your answer and go sign up for that newsletter

[00:05:13] on my website if you're not already on it. Drop that answer in the Facebook group or

[00:05:18] send me a DM on social media. You have until April 30th to give me your answer and maybe we can find

[00:05:25] some solutions to these problems together. Okay, I totally understand the desire to get a jump

[00:05:37] on early planting of warm weather crops, especially if you are in a climate that has a very

[00:05:43] short growing season for these heat loving plants. I didn't always plant by soil temperature,

[00:05:50] but I think that I've always realized there was a difference in the results. I just didn't realize

[00:05:57] it was the soil temperature that was causing that difference. I think I attributed most growth issues

[00:06:03] to air temperature and that was way off base at least in most instances. Once I recognize

[00:06:10] the importance of soil temperatures, I conducted an experiment like any good home gardening scientist

[00:06:17] should. I planted tomatoes in mid-April and then I planted the remaining tomato plants

[00:06:26] four weeks later in mid-May which is when I usually plant them. The plants that went in

[00:06:32] the ground in April didn't grow nearly as quickly as those planted in May. The April plants just

[00:06:39] kind of sat there taking more time to adjust to the transplanting. They stayed shorter for longer.

[00:06:45] They didn't start to produce their blooms nearly as quickly while the ones that went into the much

[00:06:52] warmer May soil were often running within just a few days and those April plants produced fruit

[00:07:00] at just about the exact same time as the May planted ones. The May plants were visibly healthier

[00:07:10] and they were more able to fight off the foliar diseases that we inevitably face here every

[00:07:15] single season in our tomatoes. That was all it took for me to slow my roll in the spring

[00:07:22] and start paying attention to the soil temperatures and the temperature trends

[00:07:28] from year to year to use for my planting. This isn't just with my warm weather plants but my cold

[00:07:35] season ones too and the preferred temperature changes depending on whether or not I'm sowing

[00:07:42] seeds directly into the soil or if I'm transplanting started seedlings. Let's look at this

[00:07:50] from the standpoint of direct sowing seeds to start with.

[00:07:58] When we look at direct sowing seeds in the spring, the cold's temperature at which any vegetable

[00:08:04] garden seed will reliably germinate is 40 degrees Fahrenheit or 4 Celsius. Lettuce is one of those.

[00:08:12] It will germinate just about any time the soil is not frozen. Cool season transplants like brassicas,

[00:08:20] head lettuces, pock choy, coal robbie and celery do best with a minimum soil temperature of 40 degrees

[00:08:27] Fahrenheit with a minimum air temperature of 60 degrees Fahrenheit or 15 Celsius.

[00:08:34] But just because they prefer the cooler temperatures doesn't mean warmer soil won't help

[00:08:40] especially with seed germination. The warmer that soil is up to a certain point, the faster that seed

[00:08:47] is going to germinate. I generally aim for about 45 degrees Fahrenheit for my early crops

[00:08:54] and this includes those transplants. My goal with transplants is to get them in the ground

[00:09:00] early enough that they can grow substantially before we start getting spikes in our air

[00:09:05] temperatures. I want them far enough along that if we see temperatures that jump up into the 80s Fahrenheit

[00:09:11] which is pretty common for us in spring, it's happening this week, they would be less likely to

[00:09:17] bolt. This may mean planting cabbage and other brassicas as early as the first week in March

[00:09:23] in some years, other years it's not until the end of March. This year was somewhere in between.

[00:09:30] If you look at the packet instructions for starting your seeds, you'll see a recommendation for the soil

[00:09:35] temperature for germination and that's a great guideline to follow. The more difficult it is

[00:09:40] for a seed to germinate, the more likely it will struggle as a seedling and the more likely

[00:09:46] it will have difficulty fighting off pests and diseases. Remember, seeds are just baby plants

[00:09:54] in a little package with a sack lunch. That sack lunch has all the energy that baby plant needs

[00:10:01] to push itself out of that package and push up through the soil so it can get more energy from

[00:10:08] the sun. But if it has to expend more energy than anticipated in breaking through that soil

[00:10:16] because it's too cold, well then it's depleted some of its energy stores that would normally

[00:10:22] be reserved for fighting off plant diseases and insect pests. So, for the best seed germination,

[00:10:29] wait to plant them outside until the soil conditions are right even if the air temperatures seem to

[00:10:35] be perfect. Okay but that's seeds. What does this mean for our plants? Our transplants?

[00:10:41] What soil temperatures are best for transplanting seedlings out into the garden? Well those

[00:10:48] recommendations for seed starting also hold true for the optimal temperature for plant roots and

[00:10:54] it makes sense if we think about it. The top few inches of the soil out in our garden are going

[00:11:00] to warm up more quickly than the lower few inches so the top few inches would be where the seed

[00:11:07] would germinate if it dropped naturally out in nature. The few inches below that is where

[00:11:14] the root zone of the plant is. So if we looked at the optimum temperature of say 85 degrees Fahrenheit

[00:11:20] for a tomato seed, we can assume that a seed that has been on the soil surface from fallen fruit

[00:11:29] the season before will be in the top inch of the soil. This of course is the zone where the sun

[00:11:36] is going to warm the soil the fastest. So if the optimum temperature for germination is 85

[00:11:42] Fahrenheit then the optimum soil temperature for root growth is the soil temperature a few inches below

[00:11:50] that. So generally speaking a soil's temperature at a depth of two inches will be about 33% lower

[00:12:00] than the surface temperature and at four inches the temperature is 66% lower than the surface

[00:12:08] temperature. So for our tomatoes this means where the seed germinates at 85 Fahrenheit the root zone

[00:12:16] when it starts to grow will be at 57 Fahrenheit. So if we're transplanting our tomato plants at

[00:12:25] a soil temperature that is less than 57 Fahrenheit or 14 Celsius we're not getting our plants off

[00:12:32] to the best start. And since we generally are going to be planting those plants about three to four

[00:12:38] inches down this is even more important. The general recommendations for cool weather plants

[00:12:47] are a minimum soil temperature of 60 degrees Fahrenheit or 15 Celsius for germination, 40 to

[00:12:53] 50 degrees Fahrenheit or 4 to 10 Celsius for transplanting and an average air temperature

[00:13:00] of 60 degrees Fahrenheit or 15 Celsius for growth. Now for warm weather plants these soil

[00:13:06] temperatures increase to 75 Fahrenheit or 24 Celsius for germination, a minimum soil temperature of

[00:13:15] 60 degrees Fahrenheit or 15 Celsius for transplanting and a minimum average air temperature

[00:13:23] of 75 Fahrenheit or 24 Celsius for optimum growing. So what happens if we choose to transplant

[00:13:33] into soil that is colder than ideal for our warm weather plants? The research has shown us

[00:13:40] that colder soil temperatures impede the root growth of any plant species that is sensitive to

[00:13:47] frost like our tomatoes. In fact the roots of tomato plants stop growing at 50 degrees Fahrenheit

[00:13:55] and that root growth is slow to recover after those roots finally warm back up again. So

[00:14:01] less than two hours of root exposure at that temperature is enough to cause

[00:14:08] damage and it takes more than eight hours at those warmer soil temperatures for the plant

[00:14:14] to recover. So if we're rushing out there to get our plants in the ground as quickly as possible and

[00:14:21] the soil is not warm enough, not only are we not getting an earlier harvest we're likely getting a

[00:14:28] later one and one that won't produce as much. Air temperatures can change very quickly whether

[00:14:37] that's dropping below freezing with some weird spring storm or heating up substantially

[00:14:42] because we have a few sunny days in a row but the soil temperatures change more slowly and this is

[00:14:49] because of the mass of the soil and the moisture content. So just because we've gotten past the last

[00:14:57] frost date doesn't mean the soil temperatures are conducive to planting those heat loving

[00:15:04] plants just yet. Now I know if you have a very short growing season and you need every

[00:15:12] single daylight hour in the summer to get a crop of tomatoes or peppers to maturity

[00:15:18] then you want those plants to get in the ground as early as possible. I get it.

[00:15:23] Stick with me because here in a minute we're going to talk strategies for warming that

[00:15:27] soil up artificially to get you where you need to be and I'm going to teach you how to take

[00:15:33] your soil temperatures properly so you're not fooled into thinking they're warmer than they

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[00:17:02] So how do we test our soil temperature properly in order to get the average temperature and

[00:17:09] how can we speed up the heating process to facilitate planting?

[00:17:17] The best time to take the soil temperature is mid-morning and the best place is at

[00:17:22] the 2 inch to 4 inch mark. Soil temperatures are generally coldest just before dawn but by 10 a.m.

[00:17:29] the sun has come out to warm up the soil just slightly to be more representative of the daily

[00:17:34] average. The warmest soil temperature during the day is around 3 p.m. so a mid-morning reading

[00:17:40] should give us the most accurate picture of our average. Do this at least two to three days in

[00:17:46] a row to be sure you're hitting the goal temperature consistently. If you're simply not home at 10 a.m.

[00:17:54] multiple times per week to test your soil here's the next best thing. Take the reading in the morning

[00:18:00] before you leave and again when you get home add those temperatures and divide by two to get

[00:18:06] your average for the day. Do this four or five days in a row to get a true average.

[00:18:16] So you can test the soil temperature by having a dedicated soil thermometer which is what I have

[00:18:23] or you can just use a meat thermometer it's entirely up to you and your sensibilities if you

[00:18:29] decide to use that thermometer for meat again after that. Basic dedicated soil thermometers

[00:18:35] are pretty inexpensive and I will link to the one that I have in the show notes I think I paid less

[00:18:39] than 10 bucks for it. You can also use a compost thermometer if you already have one which has

[00:18:46] a longer probe on it no matter which type of thermometer you use you want to be sure that

[00:18:52] you are measuring at that two to four inch level that's why I like the soil thermometer that

[00:18:57] I have because it's four inches long and I could just push it down into the soil until

[00:19:02] the display is resting just above the soil surface and I know I'm measuring in the right place in the

[00:19:09] soil every single time and I can keep that thermometer with my garden supplies at all times ready to use

[00:19:15] I don't have to go like rummaging through kitchen drawers somewhere to look for a meat thermometer.

[00:19:21] Now there are tricks that we can use to modify our soil temperatures either warmer in the spring

[00:19:29] or cooler in the fall to facilitate getting our seeds and our plants in the ground at the ideal

[00:19:34] time I know I talk about mulch ad nauseam but mulch is our friend when it comes to maintaining even

[00:19:45] soil temperatures right so if you are trying to get that soil to warm up there are a few

[00:19:52] tricks to help you push the boundaries a little bit number one pull back any mulch that you have

[00:20:00] already down in the garden during the day to allow the sun's rays to penetrate the soil surface

[00:20:06] soils are typically cooler in the spring coming out of winter partly because of soil moisture

[00:20:14] so as the soil warms up some of the heat that accumulates is being used to dry out that wet

[00:20:20] soil only the remainder of that heat is what goes toward raising the actual soil temperature so

[00:20:27] if you can move back any mulch to let that heat in and then put it back in place at night before

[00:20:34] the sun goes down then you can trap that heat in the soil fun fact heat that is absorbed

[00:20:42] on a warm day actually continues to pass down through the soil layers the next day so this

[00:20:51] uncovering and covering of the mulch of the soil is going to help have a cumulative effect

[00:20:58] so it's going to raise the soil temperature more quickly now if you don't have the time to be

[00:21:03] moving mulch back and forth every day you can also use clear plastic coverings over top of

[00:21:09] your beds to intensify the heat and significantly raise those temperatures so by placing like pvc

[00:21:16] hoops over your beds and then putting clear plastic over that you will really get those

[00:21:22] temperatures spiking and it will hold it in for longer your warm weather plants are still

[00:21:29] going to need to wait until the air temperatures are conducive to growth but by getting a

[00:21:34] head start the soil temperatures will be ready to go at the same time the air temperatures are

[00:21:42] so when we're planning out our seed starting or our transplant schedule for the spring or even the

[00:21:48] fall if we're relying on soil temperature how do we plan how do we figure out when to start

[00:21:55] those seeds or to buy those plants we don't want them ready too early and we don't want to buy

[00:22:01] them too late and most seed starting instructions my own included talk about starting seeds like

[00:22:07] six weeks before last spring frost or transplanting two weeks prior to the spring frost or whatever

[00:22:14] but if the soil temperatures aren't ready at that point how can we put that as a date on our

[00:22:18] planning calendar enter historical data and no this time i'm not even talking about our garden journals

[00:22:27] i am not expecting you to be out there taking measurements of your garden soil temperatures

[00:22:35] and putting that information down in your garden journal every week so that you have the data

[00:22:39] to look back on i'm a data nerd and even i don't do that okay i do know though that if i planted

[00:22:47] on a certain date my soil was above a certain temperature because that's the technique that

[00:22:54] i use so you technically can rely on your own notes to get a general idea of when to start

[00:23:01] seeds and plan for transplants if you are planting based on soil temperatures but if this is new for

[00:23:09] you or gasp oh you're not keeping a garden journal well you're in luck there are people

[00:23:17] who get paid to gather this data for us your local extension website may have historical soil

[00:23:24] temperature data for your area specifically but i generally use a soil temperature map from

[00:23:31] greencast online i will link to it in the show notes i have linked to it before it is a fantastic

[00:23:37] tool where you can just punch in your location via zip code and it will give you current

[00:23:45] and historical data for the soil temperature at that location at two inches below the soil surface

[00:23:54] and it will show you the five year and ten year average if you're interested in that

[00:24:00] it is what i base my anticipated planting dates on for everything from sweet corn to

[00:24:07] tomatoes in the spring and all the late summer crops going into fall and if i'm not sure whether a

[00:24:16] reading i'm getting is normal for this time of year like when i go out and i test the soil

[00:24:21] temperature myself i can look at their data to see if it's an unusual year or if maybe i'm just

[00:24:27] misremembering something from previous seasons gosh i really do love good data

[00:24:32] so hopefully i have convinced you to slow down a little and consider all the factors that go into

[00:24:42] planting your garden both the air temperatures and the soil temperatures and given you a few

[00:24:48] tips to get the soil warmed up if you really need to gut those seeds or those plants into

[00:24:54] the ground early i know we all get anxious but sometimes waiting is the best thing for our plants

[00:25:00] and our harvest until next time my gardening friends keep on cultivating that dream garden

[00:25:06] and we'll talk again soon thanks for listening to another episode of just grow something podcast

[00:25:12] for more information about today's topic and to find all the ways you can get in touch with me

[00:25:16] or support the show go to just grow something podcast dot com so when we're planning out

[00:25:24] our seed starting or our transplant schedule for the spring or even the fall if we're a lot

[00:25:30] we're lying again a second time we're not if we're relying until next time my gardening friends

[00:25:40] keep learning and keep growing