So, when do we plant?

I hear that question so often. The obvious answer is another question; what do you want

to plant? Things can get complicated.

Shrubs and trees can be planted pretty much any time the ground isn’t frozen. Dormant

perennials can usually be put in that category too. Flowers and vegetables are a little

more complicated.

In general, the calendar is a bad guide for figuring planting time. This year certainly

bears that out. Soil temperature is a much better indicator. Soil moisture to a large

extent influences soil temperature. Therefore, in spring when soils are wet, they will be

cold.

The reason for this is simple. Water has a high specific heat. That means it takes a lot of

energy to change its temperature. Dry soils heat up rapidly, so usually sandy soils can be

planted earlier.

Raised beds should drain well because of gravity alone. Amending them with coarse

sand or porous materials like those found in potting soil helps too. These beds will be

ready to plant earlier.

In general, soil moisture should be measured at a two to three-inch depth. This will

encompass the seeding zone and the bulk of the feeder roots for young plants. If the soil

temperature is too cold you should delay planting. Soil temperature is very important,

because nothing is harder to overcome than a poor stand.

So, what is too cold? That’s a loaded question. Peas can tolerate very cold soils. Onion

sets can too. Forty degrees is warm enough.

That’s fine for potatoes, too. However, potato foliage can be more sensitive to hard

spring frosts and wet soils, since they’re planted deeper.

Cabbage, broccoli, cauliflower, kale and other crucifers perform better at slightly higher

soil temperatures. I like to shoot for 45 degrees. Carrots and lettuce can tolerate 45-

degree soils.

Beets, spinach, Swiss chard and seeded onions should have slightly warmer soil

temperatures before planting. I suggest somewhere between 45 and 50 degrees. Warmer

is better.

I like to see soil temperatures up above 50 degrees for sweet corn and beans, and maybe

a little warmer for most beans. For example, lima beans are more susceptible to cold

temperatures. Field corn is a little tougher and can usually withstand slightly cooler

soils. As with all crops, cultivar is an important consideration, too.

Soil temperatures should be near 60 degrees before planting tomatoes. Soils should be

even warmer before setting out peppers.

Vine crops, like all the cucurbits, are even more warm-natured. Watermelons and

cantaloupes shouldn’t be set out until the soil has warmed well into the 60s.

In Maine we rarely could grow these. Summer temperatures were adequate, but soils

would never warm sufficiently unless we used plastic mulches or cloched the area.

Sandy soils on south-facing slopes were slight exceptions.

Eggplant, okra, southern peas and sweet potatoes were a waste of time up there too.

Soils weren’t warm enough and the season was too short. Surprisingly, we always could

grow the heck out of pumpkins and winter squash.

This is just my opinion on garden vegetables. In another blog I’ll profile all the flowers

we like to plant in our beds and how they handle extreme temperatures.