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Maximum soil temperatures for germination

SaladWhen I realized that soil temperature was the limiting factor keeping spring crops from sprouting early, I felt like I'd discovered the scientific way to time my spring plantings.  I probably should have realized that there was a flip side to the soil temperature coin --- many fall crops have trouble germinating in hot summer soil.

Vegetable
Optimum temp. (degrees F)
Maximum temp. (degrees F)
Beans, Snap
80
95
Cabbage
85
100
Carrots
80
95
Corn
95
105
Cucumbers
95
105
Lettuce
75
85
Muskmelons
90
100
Okra
95
105
Onions
75
95
Parsley
75
90
Peas
75
85
Peppers
85
95
Pumpkins
95
100
Spinach
70
85
Squash
95
100
Swiss chard
85
95
Tomatoes
85
95
Turnips
85
105
Watermelons
95
105

This summer has been considerably warmer than average, but I assumed that as long as I kept my fall plantings well watered, they would grow.  Not so.  I managed to sprout a single spinach seedling out of three beds, only a third of my peas came up, and I wasn't able to germinate Asian greens and lettuce until I planted new areas at the shady end of the mule garden.

Soil temperatureI finally thought to pull out my soil thermometer one morning at the end of August, at which time I discovered that the earth's temperature two inches down ranged from 65 degrees Fahrenheit in the more shaded front garden to 68 degrees Fahrenheit in the sunny mule garden.  Checking back at 6 PM when the sun had pounded down all day, the mule garden soil temperature had risen to 81 degrees.  Yes, mulched spots were considerably cooler --- clocking in at the low to mid seventies --- but my fall plantings have all gone into bare soil since the seedlings are too small to push aside mulch.  A few weeks earlier when I was vainly planting spinach seeds, I suspect the daytime soil highs easily exceeded the 85 degree Fahrenheit absolute upper limit for spinach germination.

So what's the solution for getting fall vegetables to germinate in the middle of a hot summer?  Lowering soil temperature is probably the reason that some old gardening books recommend placing mulch or old boards over your fall plantings --- you just have to remember to uncover your seedlings as soon as they sprout.  Another option is to plant fall vegetables in a cool, shady location, but that idea will backfire when winter cold nips those spots early.  Perhaps shady beds could be used as nursery plots for larger fall vegetables, and the seedlings transplanted out into the main garden later?  Or maybe there's a way to rig shade cloths to keep the soil temperature in range?  I'd be interested to hear how those of you who live with this sort of summer weather every year get your fall garden to sprout.

Our chicken waterer kept the flock healthy despite four months with highs in the 90s.



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I water the bed to be seeded with cold spring water, and plant the seeds when the sun is not directly on the bed. Then I put four chunks of firewood at the corners, and lay a sheet of plywood over the bed. I suppose I could just lay it directly on the bed though. I just thought the airspace might prevent conductive heating. But then again, plywood doesn't conduct heat all that well...

I think that planting the seeds in the evening is better, since they will have about 12 hours at germination temperature before it warms up again. I have no data to back that one up though.

Comment by Eric in Japan Fri Sep 9 08:24:52 2011
I like your idea of using the plywood up on something --- that would make it much more likely that I wouldn't forget about the seedlings and that they'd die waiting for light. Plywood sounds a bit unwieldy, though. I'd like to come up with some sort of shade that could be tossed on the bed quickly and easily, a bit like quick hoops for winter heat.
Comment by anna Fri Sep 9 13:15:05 2011
what about a frame made the way you make you ultra light gates, with quick hoop fabric stapled to it? It should be much lighter than plywood but still provide shade.
Comment by Anonymous Fri Sep 9 16:10:45 2011
If you can arrange it (not this year obviously), plant the fall garden to the north of a stand of corn, or even one row of corn, or between widely spaced rows of corn. I did that serendipitously this year and the lettuces, spinach, and scallions are happy in spite of the late heat wave.
Comment by Jackie Fri Sep 9 17:00:34 2011

Anonymous --- I like that idea a lot. I'm not sure if row cover fabric would keep the soil cool enough though. Not sure what would be better....

Jackie --- That's an awesome idea! The okra in front of it was probably part of why my one bed of Asian greens sprouted so well.

Comment by anna Fri Sep 9 19:38:06 2011
  • Spread powdered chalk (might influence soil pH, though) or titanium dioxide powder (which is inert) over the grond to reflect the light
  • Vertically sink perforated pipes to under the ground water table and use cotton cords to wick up water to help cool the soil by evaporation
  • Run water through some horizontal pipes buried in/under the beds so you can regulate the soil temperature.
  • Use your knowledge of the path of the sun through the sky to put some (PV or hot water) solar panels up so they will shade the beds during the hottest time of the day (and meanwhile produce electricity or warm water, win-win)
Comment by Roland_Smith Sat Sep 10 07:52:47 2011

I haven't had great luck with row cover fabric, either directly on the ground or on hoops, cooling the soil enough for germination. But after having radish and lettuce seed languish ungerminated under it for three weeks this summer, I gave up and transplanted bok choys into the bed and mulched them with straw. I had thin row cover suspended on hoops above them (not extending all the way to the ground so as to allow air movement). The seed unexpected germinated where the soil had not been disturbed and had no trouble pushing through the one to two inches of straw between the transplants. I've subsequently experimented and found that straw from a native wheat grass that grows in a meadow here in northern New Mexico works better than wheat straw from bales. It's thinner stemmed and sort of mats together a bit, so a thinner layer will do, though I'm experimenting with thicker layers also. I've planted lettuce, spinach, radishes, beets, chard, peas, arugula, and various bok choys several times since and gotten 60% (peas, spinach) to 90% (arugula, bok choys) germination.

I was also able to get decent germination with a mix of cow peas and vetch broadcast in early July over an area next to my garden where I scratched the dry as a bone soil with a garden rake. I'd already whacked down the native yellow clover that grows sporadically there, and the summer rains hadn't come yet--no precipitation since a snow/rain on May 2--and the native grasses and weeds, that grow in patches there were still dormant. After broadcasting I threw down a couple of inches of my native straw. The next day we got almost an inch of rain and have gotten enough more sporadically since--three inches total--that I got a decent stand that competes well with the perennials but I've weeded the vigorous annuals that sprouted in that little oasis of damp soil. About ten days ago I similarly broadcast another pea/vetch mixture and included daikon radish and a variety of cool weather veggies. Some peas and choys are popping up and it's rained sporadically the last couple of days so I expect more surprises daily.

Mulch is great. Necessary here to preserve soil moisture. I take the soil temps sporadically and mulch can keep the ground ten degrees or more cooler (I don't have air temps as high as most, but the solar radiation is intense at almost 8000 feet elevation). I was pleasantly surprised that little seedling can punch through as well as what I've seen so far.

Comment by Walter Sat Sep 10 08:54:12 2011

Roland --- I'm looking for something that's both modular and reusable, so that I can use the same shade setup on different beds in different years. That nixes your first three suggestions. The fourth suggestion is a great permaculture use of the sunny space, but probably more effort that I have at that time of the year. Quick and dirty is the way to go with summer projects. :-)

Walter --- I'm so glad you shared your experience with seedlings under mulch. The truth is that the few things I've had great luck sprouting at that time of year were cover crops and I started them all under light layers of straw. So maybe I should just experiment with doing the same with my fall vegetables --- sure, the seedlings are smaller, but maybe I'm not giving them enough credit.

Comment by anna Sat Sep 10 15:21:12 2011
We have real trouble with this same problem; fall planted spinach seed sits until late winter to sprout (Arkansas, zone 6). So I'm trying pre-sprouting spinach seeds in the refrigerator, which was recommended by market gardeners Paul and Alison Wiediger of Au Natural Farm as the technique they now always use.
Comment by sweetgum Sun Sep 25 23:11:57 2011
It's funny you should comment on this post today since I was just talking to my father yesterday about how we both have exactly this spinach problem. Sprouting them in the fridge is a method I'll have to try --- I had been thinking I'd just give up on spinach and stick to the greens we can sprout (the crucifers and swiss chard.)
Comment by anna Mon Sep 26 07:49:00 2011
Anna--- I just wanted to let you know the spinach sprouting REALLY worked. When I last commented I wasn't sure if my seedlings were up, but now I have more spinach than I've ever had germinate. What worked here: at beginning of September I soaked seeds in water overnight, then drained and put the damp seeds in the fridge in a jar (exactly like for growing alfalfa or bean sprouts to eat), rinsed them every day and planted on about day four. They seem to not have missed a beat and are happily covering their bed outside. Just a nice trick to please those in the family who prefer spinach. Good luck!
Comment by sweetgum Fri Sep 30 15:56:33 2011
I appreciate you posting a followup comment! It sounds like the fridge-sprouting method really works --- that will have to be the way I start spinach next fall. (Too late now, unfortunately.)
Comment by anna Sat Oct 1 10:31:03 2011

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