Gardeners go through the same questioning every spring: when can they sow or plant out vegetables? Of course, most plants won’t tolerate frost, so do check with a weather service about whether one is expected over the weeks just after you plan to start. However, when there appears to be no risk of frost, is that enough? Sometimes nights are still cold, but days are warm. Is that alright?
Here’s a guide.
Sowing seeds is a bit less stressful on the gardener than planting out vegetables started indoors. In most cases, cooler soil temperatures that actually required will simply slow germination down, not stop it. Plus it will take a week or so before the seedlings are tall enough to really be exposed to cold night air, giving you a bit of leeway. When the seeds do germinate, sign the soil is warm enough, usually nights have warmed up too and they’ll simply grow normally. So if you sow seeds a bit early, that doesn’t necessarily delay the harvest to come.
Still, it’s wise to know that some vegetables (cool season crops) germinate quite readily at fairly low temperatures (beets, carrots, lettuce, onions, parsley, parsnips, radishes, etc.) and you can consider it safe to sow them when the soil temperature has reached about 45˚ F (7˚ C), while 55˚ F (12˚ C) is safer for turnips, the various cabbages (broccoli, kale, cauliflower, etc.), Swiss chard and corn. Wait until they reach a balmy 70˚ F (21˚ C) before you sow warm season vegetables like beans, cucumbers, squash and melons.
You can find an inexpensive soil temperature thermometer at a garden center or online. Take the temperature at about a depth of 4 inches (10 cm).
Does the Moon Have an Influence on Frost?
In short, no. None whatsoever. Look it up on any serious gardening site and you’ll see. That it is safe to plant out after the full moon of May (or March, or April, or whatever the local legend says) is just another gardening myth. Like the one that says it is safe to plant out once oak leaves reach the size of mouse ears. Oaks do get frosted occasionally, even when they are leafing out or even in full leaf. You just can’t trust Mother Nature when it comes to frost!
This is where things become serious. You carefully sowed the seeds of many vegetables indoors to get a head start on the season and you’ve been caring for them for weeks. Or you bought them at great expense. You certainly don’t want to risk harming them or even slowing down their progress when you plant them out.
Since sprouted vegetables are immediately exposed to air temperatures as well as soil ones, you’ll need to take air temperatures into account. And by mid-spring, the soil, having absorbed the sun’s heat all day, is often warmer than the night air. Look most carefully at night air temperatures, cooler than day ones … often considerably so! And even when night temperatures warm up, you still have to consider the possibility of late frosts (see above).
Note that you need to acclimatize seedlings started indoors to outdoor conditions before you plant them out (a few days in the shade, then a few in partial shade before exposing them to sun) and you can usually start to do up to 10 days before you actually expect to plant them out, putting them out on balmy days. But do bring them in at night if night temperatures drop to any degree (as they often do early in the season).
There are a few cool season vegetables that are often started indoors, like leeks, lettuce, onions and again, the various cabbages. You can plant them out quite early, when night temperatures remain above 45˚ F (7˚ C)… assuming that, by there, there is no danger of frost in your area!
Warm season vegetables don’t usually die under cool night temperatures (unless there is frost), but instead go into shock and slow down, which delays the harvest. Plant out a tomato or cucumber plant too early and it will actually come into fruit later than one transplanted a week or two later, when temperatures are warmer.
Consider night temperatures of 55˚ F (12˚ C) as an absolute minimum for transplanting tomatoes (but even so, they prefer warmer temps). The other warm season vegetables are even less happy with cool nights. I suggest 65˚ F (18˚ C) for cucumbers and peppers and 70˚ F (21˚ C) for eggplants (aubergines), melons, okra and squash (including pumpkins and zucchinis).
I hope the above information will help you decided what to plant out when!