With the seed sowing season upon us, it can be useful to take a careful look at just when you should sow your seeds of vegetables, annuals, herbs, etc. Most seed packs and seed charts suggest a date X number of weeks before the last frost. For example, tomato seeds may well be show “Sow 6 to 8 weeks before the last frost” on their pack. But what is the last frost date for your locality?
If you go to most government or gardening sites, you can usually find the date of the “average last frost” for your locality. But that’s not the date you want!
About half the time, there will be frost after the average last frost date. That’s what an average means. You’ll rarely if ever want to plant out tender seedlings when there is still a 50% risk of frost. Even putting aside the frost risk, that early in the season the ground will still likely be cold, not something fragile seedlings are going to appreciate.
If you use the average last frost date as the basis for calculating when to start your seedlings, in most years, they’ll end up spending a longer time indoors than they really should. And seedlings are best transplanted when they are still young and bursting with energy, not overgrown and stressed out from being indoors too long.
A Reasonable Last Frost Date
You’ll need instead to calculate at what date there is very likely to be no risk of frost. Let’s call it the “reasonable last frost date”. And that’s easy enough to do. In many climates, the risk of frost drops by half – to 25% – a week after the average last frost date. Add another week (so 14 days after the average last frost date) and the risk decreases to 10%. That’s much more acceptablee.
I suggest using 14 days after the average last frost date as the date you really should use for starting your seedlings. (If your climate is particularly subject to late frosts, make that 21 days instead of 14.) This should give you young, healthy seedlings ready to pop into the garden at just about the right time.
But Watch the Weather
Of course, no matter when you start your seedlings, you still have to pay attention to weather reports as planting out time approaches. It still might be too cold some years at the “reasonable last frost date” to plant your seedlings out: look for nights above 50˚F (10˚C) before planting especially fragile seedlings, like tomatoes, peppers, begonias or impatiens out. And if the weather reports suggest there is still an upcoming risk of frost (at the “reasonable last frost date”, there is still a likelihood of frost about 1 year out of 10, after all), believe it.
The Old Farmer’s Almanac is one place you can go to find average last frost dates: take a look for your locality… but make sure you add at least 14 days!