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.
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 in 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. We’re not there yet. 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 soils have 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. I wish I had column space to profile all the flowers we like to plant in our beds. That’s a story for next week.
Ted Manzer teaches agriculture at Northeastern High School (firstname.lastname@example.org).
A garden column does not help as much as you might think. Space is limited. Information must be simplified. Your explanation is better than I would do. I conform to our local climate, but lack space to address much else outside of our region. That is why I talk about planting tomatoes out in the garden while most everyone else is sowing their seed in basements.
You’re right. There are too many variables. Guidelines can be a slippery slope, but I’ve found over the years that most people seek them. It’s a balancing act I never would have attempted when I worked in research.
The guidelines you cited are accurate for all climate zones, though. They are not about the precise time to plant, which I write about. They are more about how to know ‘when’ to plant. That works here as well as there, and everywhere.