Soil Temperatures and Vegetable Seeds

 

Wiese Acres vegetables

It's hard not to get anxious to be out planting the vegetable garden.  I can just about taste the freshly picked lettuce with the sweet cream dressing that Mom always made, or the large, sweet radishes that Dad and I sliced to make radish sandwiches.  But, most seeds are pretty particular about the temperature they want the soil to be before they are planted.  If soil temperatures are too cool, the seeds will lie there and rot before germination has a chance to occur.

Cool season crops generally tolerate planting in soil temperatures between 35 and 40 degrees.  That is pretty early in most locations, but here's the rub with even these crops:  they might germinate and grow in cold soil, but the green part of the plant still will not tolerate air temperatures that are much below freezing, as can often occur in these northern climates well into May.

Lettuce, onions, and parsnips are some of the earliest crops that can be planted.  They will germinate in a minimum soil temperature of 35 degrees, but their optimum germination temperature is well above that at about 70 degrees.

Peas, radishes, spinach, swiss chard, turnips, beets, carrots, leeks,and all the cole crops (broccoli, cabbage, cauliflower, etc.) will germinate at a minimum soil temperature of 40 degrees, but prefer the much warmer 70 degree soils, as well.


Wiese Acres cabbage

The conventional wisdom that potatoes should be planted on Good Friday might work well in some places, but given that potatoes' minimum germination temperature is 45 degrees, most years Good Friday is going to be just a tad bit early.

Warm season crops need soil temperatures of 50 to 60 degrees to germinate before the seed rots.  These include beans, cantaloupe, corn, cucumbers, eggplant, peppers, tomatoes, summer and winter squash, pumpkins, and watermelons.

If you use treated seeds, soil temperatures are not as critical.  If they are anywhere close, the seeds will probably just lie dormant without rotting until the soil warms enough.  For untreated seeds, which is what I prefer, it is well worth the time to actually check the soil temperatures before planting, because untreated seeds can rot very quickly in cold, moist soil.

Soil thermometers can be purchased at any garden center pretty cheaply and they are probably worth the few bucks investment.  Theoretically, any thermometer that can be inserted into the soil to a depth of four to six inches is able to monitor soil temperatures. 

As a rule of thumb, I don't plant anything in my heavy clay soil until at least mid-May and seedlings don't get transplanted until Memorial Day weekend.  That is when I am pretty sure that soil temperatures are going to be adequate to support germination and the likelihood of a hard freeze killing all my baby plants and having to start over is greatly diminished.