How hot must soil be to kill seeds?

wild_foragerSeptember 14, 2008

I just planted a bunch of kale, beets, spinach and chard yesterday in large pots. Now today it's in the low 90's and the soil in my pots skyrocketed. The temperatures in the soil were in the mid 90's. Will this harm my seeds a great deal? It's okay if some of them don't germinate, and if all else fails I can replant.

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jimster(z7a MA)

I can give you a very general answer. Probably someone else has a specific answer.

It surprises me, when planting seeds during hot weather, that seeds aren't killed when the soil gets hot. Sometimes between waterings the soil gets dry temporarily and very hot to the touch, yet even shallowly planted seeds almost always survive. This makes me think seeds can withstand a lot of heat. It must be the case with wild plants, who have no caretaker looking after them.

There is no way of knowing whether this applies in your situation, with the things you are growing, but I would be optimistic.


    Bookmark   September 14, 2008 at 6:37PM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

I'd share Jim's optimism. Testing shows that 140 degrees in a compost pile will kill seeds but i can't say if those results would apply equally to shallow planted seeds.

But you may want to consider moving those pots to a partially shaded area because once the seedlings sprout, heat tolerance is greatly reduced. What won't kill a seed, will quickly kill a young seedling.


    Bookmark   September 14, 2008 at 7:16PM
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They might not be killed, but they won't germinate if they aren't in their target temperature range.

    Bookmark   September 14, 2008 at 9:18PM
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There has been some fairly dramatic cooling in the last day or two. I assume this will increase chances of germination, but I'm wondering whether the day of intense heat may have harmed the seeds in a different way. How susceptible are seeds like chard and kale to mold? There was definately moisture, so I know the seeds got water, but if they get water and don't germinate do they rot, and if so how quickly?

    Bookmark   September 16, 2008 at 7:41AM
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Different types of seeds have different needs regarding temperature required for germination, and temperature which will prevent germination. Many seeds won't sprout in soil temperatures below seventy degrees.

Water helps to open the seed coat so the seed can sprout. Don't worry, you will likely get some germination. If they don't germinate, the seeds will eventually decompose.

Some seeds require rather harsh environmental/seasonal conditions to erode the seed coat sufficiently for germination. Sometimes a thick-coated seed will lay dormant for a year or two before sprouting. Gardeners usually plant such seeds in fall, so winter conditions will erode the seed coat. Or, they scarify the seed coat by pricking with a pin or placing seeds and small pebbles into a jar and shaking until bits of the seed coat begin to fall to the bottom of the jar.

I planted kale seeds recently. They sprouted within a week.


    Bookmark   September 17, 2008 at 2:24AM
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Actaully my kale have sporuted, much quicker than I imagined. I was thinking they wouldn't sprout for a while if at all. Apparently they didn't mine the heat. Beets and cahrd take a few mroe days anyway so I expect to see them soon. And peas.... well, there's no way to know. They're fickle.

    Bookmark   September 17, 2008 at 7:56AM
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jimster(z7a MA)

Peas and beans are notorious for rotting in wet soil if they don't germinate quickly enough. Because they are large seeds, you can dig around and examine a few to see if they are sprouting.


    Bookmark   September 18, 2008 at 1:38PM
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It does appear that some are starting to come up. The chard is actually coming up the least. I've only got a few so far... I hope it does better soon. Every kale seed has sprouted apparently!

    Bookmark   September 19, 2008 at 10:51AM
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