And, welcome back. The last time we took the soil temperature in the garden in my part of Upstate New York, it was 32.9 degrees F. on March 7th. Hope Is A Thing With Dirt
On Friday, March 20, it was, as you see above, 43.4 degrees. That’s with several 50-60 degree days behind us, but very chilly freezing nights. It is now way above freezing. And there is an onion shoot in the photo, too. Time to get out the seeds, right?
Mmmmmm, no. There ARE some seeds that you could put into the ground now and if you protected them a little bit, they’d come up, but as you see from the chart below, it would take them a very, very long time to do that. Even something like Beets or Lettuces, which are very hardy and would germinate at 41 degrees, would take 42 days and 49 days respectively to germinate.
Percentage of Normal Vegetable Seedlings
Produced at Different Temperatures* **
Numbers in ( ) are the days to seedling emergence. Number in red = optimal daytime soil temperature for maximum production in the shortest time.
| Crops | 32ºF | 41ºF | 50ºF | 59ºF | 68ºF | 77ºF | 86ºF | 95ºF | 104ºF |
| Asparagus | 0 | 0 | 61(53) | 80(24) | 88(15) | 95(10) | 79(12) | 37(19) | 0 |
| Beans, lima | 0 | 0 | 1 | 52(31) | 82(18) | 90(7) | 88(7) | 2 | 0 |
| Beans, snap | 0 | 0 | 1 | 97(16) | 90(11) | 97(8) | 47(6) | 39(6) | 0 |
| Beets | 0 | 53(42) | 72(17) | 88(10) | 90(6) | 97(5) | 89(5) | 35(5) | 0 |
| Cabbage | 0 | 27 | 78(15) | 93(9) | 0(6) | 99(5) | 0(4) | 0 | 0 |
| Carrots | 0 | 48(51) | 93(17) | 95(10) | 96(7) | 96(6) | 95(6) | 74(9) | 0 |
| Cauliflower | 0 | 0 | 58(20) | 60(10) | 0(6) | 63(5) | 45(5) | 0 | 0 |
| Celery | 0 | 72(41) | 70(16) | 40(12) | 97(7) | 65 | 0 | 0 | 0 |
| Cucumber | 0 | 0 | 0 | 95(13) | 99(6) | 99(4) | 99(3) | 99(3) | 49 |
| Eggplant | 0 | 0 | 0 | 0 | 21(13) | 53(8) | 60(5) | 0 | 0 |
| Lettuce | 98(49) | 98(15) | 98(7) | 99(4) | 99(3) | 99(2) | 12(3) | 0 | 0 |
| Muskmelon | 0 | 0 | 0 | 0 | 38(8) | 94(4) | 90(3) | 0 | 0 |
| Okra | 0 | 0 | 0 | 74(27) | 89(17) | 92(13) | 88(7) | 85(6) | 35(7) |
| Onions | 90(136) | 98(31) | 98(13) | 98(7) | 99(5) | 97(4) | 91(4) | 73(13) | 2 |
| Parsley | 0 | 0 | 63(29) | 0(17) | 69(14) | 64(13) | 50(12) | 0 | 0 |
| Parsnips | 82(172) | 87(57) | 79(27) | 85(19) | 89(14) | 77(15) | 51(32) | 1 | 0 |
| Peas | 0 | 89(36) | 94(14) | 93(9) | 93(8) | 94(6) | 86(6) | 0 | 0 |
| Peppers | 0 | 0 | 1 | 70(25) | 96(13) | 98(8) | 95(8) | 70(9) | 0 |
| Radish | 0 | 42(29) | 76(11) | 97(6) | 95(4) | 97(4) | 95(3) | 0 | 0 |
| Spinach | 83(63) | 96(23) | 91(12) | 82(7) | 52(6) | 28(5) | 32(6) | 0 | 0 |
| Sweet Corn | 0 | 0 | 47(22) | 97(12) | 97(7) | 98(4) | 91(4) | 88(3) | 10 |
| Tomatoes | 0 | 0 | 82(43) | 98(14) | 98(8) | 97(6) | 83(6) | 46(9) | 0 |
| Turnips | 1 | 14 | 79(5) | 98(3) | 99(2) | 100(1) | 99(1) | 99(1) | 88(3) |
| Watermelon | 0 | 0 | 0 | 17 | 94(12) | 90(5) | 92(4) | 96(3) | 0 |
On the other hand – the fact that they would germinate is a great indicator that if you had little beet plants or little lettuce plants already germinated, you could get them into the ground and if you protected the bed with heavy duty row cover (like Remay ™ or clear plastic), as long as you got some sun, they’d jump right up there. Even if you got some very cold nights or even some snow, they’d be protected and would grow.
But perhaps you want to grow something like tomatoes or peppers. Check out the chart above: you will need the soil temperatures to be in the high 50s to get really good germination. So, even if you have tomato or pepper plants, you would not want to put them out until the soil temperatures are a lot higher.
You, however, are one of those folks who wants to get the jump on your neighbors – or perhaps you live in the Intermountain West or northern Maine or Upper Michigan or the Adirondacks – you have a very short window to get your tomato plants into the ground, growing as fast as they can, setting blossoms as fast as they can so that you can have something other than green tomatoes by August.
You need to get that soil as warm as you can, as quickly as you can.
This is where black plastic comes in. It’s not too soon, even in my part of the world in Upstate New York, to get black plastic onto the beds. Certainly, it will take a while to get the soil up to the high 50s to put tomatoes and peppers in, but check the chart again (actually, go to the link – the chart is much prettier there – print this out and put it into a plastic sleeve or tape it to the fridge door or something; I found this chart and think it is a really useful thing to have). If I put black plastic out right now, today, with little plants already started for all those veggies that give good germination under the 41 degree column, I could plant those through the plastic, protect them with clean plastic or heavy duty row cover and get a jump on the season. With my handy-dandy meat-thermometer-that-I-changed-to-a-soil-thermometer, as the weather gets warmer and we get more sun, I can check the soil temps — and put the next column’s veggies in and so on.
At the other end of the season, starting in August, you can use the same technique – take off the black plastic and help the soil start to cool down, so that you can start putting in your lettuce, cabbages, etc. for the fall. They like cooler ‘feet’ (look how they respond at the higher end of the temperature scale – the hotter the soil is, the poorer they will do) and will appreciate the cooler soil and will germinate better for you (if you direct seed) and grow better if you put plants in.
What sort of black plastic do you need? Well, in a pinch, probably a black garbage bag would do the trick, but that plastic is not made to handle the sun and it would fall apart pretty quickly. You’ll want to go to your hardware store or home center and get a roll of heavy duty black plastic. Unroll a single thickness over the bed, anchor it down with soil, rocks, old pipe or whatever you’ve got. If you have cool weather crops, you can start planting them if the temperature is right; if you have to wait for a bit, then keep checking the soil temperatures every couple of days to see how it is doing.
