We've talked about transplanting fall crops in the late summer heat, but what about sowing those seeds directly into the garden? Soil temperature is a real concern here, so today we'll spend a few minutes reviewing the optimal germination temperature for direct-sown fall garden crops and a few tricks for reducing the soil temperature more quickly.
Resources:
Taylor Precision Products Standard Grade Thermometer
Soil Temperature and Seed Germination (psu.edu) Ep. 142 - Planting by Soil Temperature vs Air Temperature: When's the best time to plant?
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Welcome back, my gardening friends, to another focal point.
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Friday Episode Let's spend just a few minutes together reviewing
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A snippet of information from A. Previous episode highlighting a
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new topic. Or quickly focusing on a current
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event in the food and. Agriculture world, let's get
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down and dirty. A couple of weeks ago we talked
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Tips and tricks for transplanting fall crops in the
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late summer heat, but we didn't mention anything about direct
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sowing seeds into the garden At the same time, If you're
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anywhere in, I would say zone 5 or warmer, definitely zone 6 and
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above, you likely are going to have some considerations when it
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comes to planting seeds directly in the ground, and that's mainly
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soil temperature. Many of the plants that we're
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growing in the fall are obviously cool weather loving
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plants. As such, a lot of them just
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don't like to germinate in soil temperatures above a certain
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range. Now surprisingly, when we look
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at things like broccoli and other brassicas, which you would
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think would really like it cooler, they actually like a
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soil temperature that's higher than what you might think for
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germination, like as warm as 85 Fahrenheit.
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But in most areas, we're not starting brassicas directly in
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the garden unless we're using them as greens.
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We're starting them indoors. So things that we generally
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plant directly in place in the garden for a fall harvest or
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even for overwintering are things like beets, carrots,
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lettuce, radishes, spinach, even turnips.
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Now, some of these can handle warmer soil temperatures.
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Others are going to see a rapid decline in germination
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percentage. The warmer that the soil is, up
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to the point where you will have no germination at all, and the
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seed is essentially cooked right there in the soil.
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So if you're in an area where the summer sun is strong and
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your daytime temperatures are still routinely above 80
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Fahrenheit, then you're going to want to have a soil thermometer
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on hand before you decide to start planting those seeds.
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I will link to the one that I use in the show notes.
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They are very inexpensive and they are great to have on hand
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at all times. The best time to take the soil
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temperature is around mid morning and the best depth for
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this is at about the 2 to 4 inch mark.
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Soil temperatures are generally the coolest just before dawn.
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By about mid morning though, 10 or 11:00 AM, the sun's come out.
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It's warmed up the soil. Just enough to be a bit more
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representative of what the daily average is.
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The warmest soil temperature during the day is around 3:00 in
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the afternoon and in the late summer.
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You may want to test your soil temperature at this time too, if
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you can. If your temperatures are extreme
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for even a couple of hours in the afternoon, it can lead to
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seedling death if the seeds do manage to sprout in those
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temperatures. And if you have dry sandy soils,
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your soil temperature can actually get hotter than your
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surrounding air temperatures. The rest of us generally have
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soil temperatures that are cooler than our air temps during
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the day. But that soil does retain that
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heat of the day, and so it's actually going to be warmer than
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the air temperatures at night. In the spring, we're usually
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looking for like an average soil temperature to guide us, but in
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the late summer, I'm a bit more concerned with the extremes.
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So ideally we want soil temperatures that are not going
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to be above the highest level or the highest temperature for the
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preferred range for each of our most common fall crops.
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So turn up. Sort of.
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Tend to be the exception to this.
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Their preferred soil temperature range for germination is between
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60 and 95 Fahrenheit, or 15 to 35 Celsius.
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So in all but the hottest areas and those areas that have those
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very dry sandy soils, most of us are going to be OK to plant
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turnips just about anytime in the in the late summer.
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Where we start to run into a problem is with things like
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carrots and kale beets, Swiss Chard and radishes.
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They do not do well in temperatures around and soil
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temperatures around 85 Fahrenheit or 29 Celsius the
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closer you get to that mark. The lower your germination
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percentage is going to be, so they typically are going to do
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better. Closer they are to about 65
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Fahrenheit or 18 Celsius. 7075 is good, you know, up to about
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23 Celsius. That's really kind of the ideal
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mark. Anything after that and you're
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going to start to see a a lower germination percentage, and then
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anything above that 85 Fahrenheit or 29 Celsius mark,
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you're likely to see very little germination.
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The other, even more sensitive ones are things like lettuce,
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mustard, peas if you're trying to do fall peas.
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And spinach, These do not like temperatures, soil temperatures
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above 75 Fahrenheit or 23 Celsius.
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So that is pretty cool, especially when we're talking
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about the late summer garden soil.
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Spinach especially, for some reason, really tends to like it
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closer to like that 65 Fahrenheit or 18 Celsius
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temperature mark. It just seems to be very, very
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finicky. And taking temperatures in all
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of your beds where you plan to plant is a really good idea too.
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If I were to walk out into one of my raised beds right now that
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is in the full sun all day long, I can tell you if there isn't
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any mulch in place on that soil, the temperature at about the 2
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inch depth in that soil is going to routinely be at between 90
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and 95 Fahrenheit. But if I go into one of my in
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ground beds that has a. Deep layer of mulch.
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The soil temperature at the same time of day is likely to be
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closer to 75 Fahrenheit, right? This is partly because there is
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more moisture retained in that soil, because it has a larger
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mass than what we have in the raised containers, but also
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because it has that deep layer of mulch.
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This is why it's important to test the temperature in each
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location prior to planting, and to plan what's going to go
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where. Based on these findings, you can
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choose those more sensitive crops in the beds that you know,
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have the lower soil temperature. This is also really good
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information too, to take note of for our garden journals, because
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that way we can get a better plan for next season for what
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happens in those beds. So how can we get our soil
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temperatures to cool down a little faster toward the end of
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the summer so that we can get those fall seeds in?
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A lot of us are. Sort of racing the clock right
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now to get stuff in the ground so that we know that it
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germinates and it comes to maturity before say, our first
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frost hits. Well, of course mulch is our
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friend here. OK, so So as long as you're
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using like a light colored mulch, like straw, you're going
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to be keeping the hottest of the summer sun's rays off of the
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soil surface. That is going to help.
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But that does take some forethought.
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You have to make sure that you have that mulch in place.
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So the other way to do this to to cool the soil is to add
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moisture. Now we talked about making sure
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that you are watering your garden beds really well before
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you do transplants. Well, part of the heat in the
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spring goes towards drying out that wet soil.
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The same concept applies now in the late summer and the early
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fall. In most areas with high heat.
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We also have very little rainfall and those soils have
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likely begun to dry out. So if you can manage to water
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that soil very thoroughly, it is going to cool it down by several
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degrees. Trap that moisture in then with
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some mulch and then you should start to see the average soil
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temperature begin to drop. Then to go one step further, you
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can add some shade cloth over top of that bed and that is
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going to further protect the soil from the sun's rays and
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lower that temperature even further.
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It's a multistep process, but if you need to get things in the
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ground around a certain time to avoid a fall frost, then these
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are the steps you'll likely need to take other than just waiting
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to protect the seeds that you need to germinate in place.
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Thanks for joining me on this focal point Friday.
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I'll be back again on Tuesday for another.
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Regular episode of the Just Grow Something podcast, so until next
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time, my gardening friends, keep on cultivating that dream garden
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and we'll talk again soon.