Maybe April is the cruelest month (especially this year) but early May can be tough on vegetable gardeners who are raring to go. You’ve got your spring crops in the ground and growing; maybe if you got an early start you’re even harvesting. But what about all those delicious summer veggies? If you’re lucky, you have some tomato plants, maybe some peppers or eggplant; you’ve got bean seeds and squash seeds and more. And you have well-prepared soil to plant them in. But when is it safe?
When people ask me this, which they do a lot around this time of year, I usually sound a note of caution. But really, there’s no one clear answer. It depends on factors we have no control over, and it depends on how risk-averse you are. Many of us prefer to put a planting date on the calendar; even better if it’s an easy one to remember. St. Patrick’s Day: plant your peas and potatoes. Mother’s Day: time for the tomatoes to go in. But it’s not that simple.
Here’s what you should consider before planting your summer crops.
Just like humans, plants have temperature preferences. And since they can’t put on a coat when it’s cold or retreat into an air-conditioned building when it’s hot, they function well within a narrower range of temperature than we do. Air temperature is important (see section below) but soil temperature is equally if not more critical. Seeds will not germinate and transplants will not thrive if the soil is too cold or hot. And yes, this response varies depending on the plant.
Those spring crops humming along in your garden now do fine in a chilly soil; in fact, if you tried to germinate those seeds in July, they might not come up at all, and certainly the plants would struggle and suffer an early death. The same is true in reverse for plants that love warmth. You can find lots of germination temperature charts on the internet: here’s one. And another. They don’t always agree exactly, but will give you a general idea of minimum, maximum, and ideal soil temperatures for seed germination. These apply whether you are starting your seeds inside or directly in the garden, but in the latter case you have very little control over when Mother Nature says “go.” (It is possible to warm the soil by using plastic mulch, or cool it with shade cloth, but let’s not worry about that right now.)
If you have a soil thermometer (or a compost thermometer; they’re basically the same thing) you can stick it a few inches into the ground and find the result.
As a general rule, for heat-loving crops you want the temperature to be at least 60° F. before sowing seeds; 5-10 degrees warmer is better. In Germantown, Maryland on April 29 I measured ground temperature between 55-57°. Raised beds are usually warmer; one of mine measured 65°, which was warm enough to plant beans. The same is true for transplants: wait until the soil is 60° before you put in your tomatoes, whether it’s Mother’s Day or not. More sensitive crops, such as peppers, eggplants, or melons, would prefer a soil temperature closer to 70°. Plants struggling to survive in cold soil may be stunted, may not be able to take up the nutrients they need, and may be more subject to pests and diseases, even later in life.
What if you don’t have a soil thermometer? Well, there’s a lovely old farmer’s guideline that advises sitting on the ground with a bare butt and judging whether it’s time for beans to go in depending on how long you can stand it. Even with social distancing, I don’t necessarily advise this technique in densely populated suburbs. But you could try sticking your finger in the ground; if that’s unpleasantly cold after a minute or so, it’s probably not time to plant yet.
Air Temperature and Weather
Okay, the soil’s warmed up: what next? Look at the weather forecast.
In most of this area we are past the average last frost date, or it’s coming up soon. But “average” means just that; it doesn’t mean we can’t still have a frost. In fact, weather patterns in recent years have tended toward warm early springs followed by a chill in April or May; we’ve had several post-Mother’s Day frosts. Choose a weather source that will give you forecasts of at least a week ahead, and check it frequently. This will give you warning of cold weather and also of storms.
Look at the nighttime temperatures, not just the daytime highs. Drops down into the 40s are not great for plants that love warmth; even if those temperatures won’t actually kill the plants, their growth may be slowed or even stunted, and you may see the results later in the season when fruits are distorted.
Get your transplants ready to go in the ground by hardening them off. This means taking them outside into a sheltered spot and letting them get used to the outdoors gradually. Start by putting them in the shade and out of the wind, and shift their position to more sunny and windy over the course of a week. Bring them inside for the first couple of nights, and then if it’s warm enough you can leave them out full-time.
If conditions seem right for transplanting, make sure we’re not expecting a storm in the next couple days. Fierce winds and rains are not good for plants that haven’t established their roots or strengthened their stems.
Direct-seeded plants will also be affected by weather as they grow, but they won’t have that period of adjustment to new conditions. Just make sure the soil is warm enough and no frosts are in the forecast before you seed.
Gardener Effort and Risk Tolerance
If you’re willing to take a chance on putting in your plants early, I can’t stop you. Just be prepared for some extra effort. If a cold snap appears in the forecast, be ready to run out and cover your cold-sensitive plants: you can use a heavy row cover, an old bedsheet, whatever you have on hand. Temperatures in the low 30s will require blankets or plastic sheets. You may want to use some of the many tomato-protection devices on the market when you put plants in (they don’t have to be tomatoes, just whatever you want to keep safe), but remember that when the temperature soars you’ll need to give those plants some air or they may get too hot. And make sure that your plants have enough support in case it’s windy.
Jumping the season can be fun, but it can also lead to disappointment and the search for more seeds or transplants, so plant with care.
Here are some plants that can be direct-seeded when the soil temperature is at least 60°, preferably 65°, and we’re not expecting a frost:
- Corn (but better warmer)
- Cucumbers (but better warmer)
- Greens such as amaranth and New Zealand spinach that prefer heat
If you missed seeding beets or Swiss chard earlier, you can still start them in May; they are fairly heat-tolerant. Radishes and turnips that will mature in under 40 days can also be started now. Lettuces and Asian greens that claim heat tolerance might prefer partial shade but can be grown until the hottest part of summer.
You should wait to direct-seed these plants until the soil has warmed to 70°:
- Malabar spinach
Tomato transplants will tolerate slightly cooler temperatures than tomatillos, peppers, eggplant, sweet potatoes, or any of the list immediately above.
Pretty soon we’ll be complaining about the heat, so enjoy early May even if it’s a time of frustration for eager gardeners.
By Erica Smith, Montgomery County Master Gardener. Read more by Erica.
Thanks for a great post. I have given up on trying to rush my tomatoes out into the garden. They seem to do better if I wait and let the weather settle down and warm up. Your advice on soil temp for various seeds and seedlings was particularly helpful. Do you know of a reliable chart for a broader range of plants?
They really do perform better if they’re not challenged by cold soil and weather! There are so many charts out there; you could try searching on “germination temperature” and a particular plant. You can also make some deductions based on similar plants that are on a chart.
I am bewildered that you would provide links to other states’ planting guides, bypassing our own publications GE007 (the color coded calendar) and HG16 (alpha listing).which conform to central MD temps and available at extension.umd.edu/hgic. I do believe most people incorrectly put plants in only based on day temps and frost free forecast, instead of soil and night temps.
UMD doesn’t have a guide for germination temperatures, which are the subject of those charts. That’s different from a planting calendar, which is more specific to our location, although still subject to the whims of each year’s weather. It’s very useful to have general guidelines, but if you’re trying to decide where within that range to put your plants in the ground, it’s important to also look at your own very local conditions, including soil temperature.
Thanks for the info. I’ve been hardening off my tomato plants for a week and was thinking of planting tomorrow but with next week’s forecasted nighttime lows in the 40s, I guess I’ll wait until mid month to plant them.
I’m in the same place, Chris! Good luck!
Thanks for the this info! Is there a time of day that is optimal for taking the soil temp? It would seem that there might be spring days when the soil temperature reaches 60degrees, for example, but only for a short time – would that be sufficient for germination? Does soil temperature fluctuate much less than air?