Last Saturday was a shocker for many Eastern Canadian gardeners. Frost hit in the early hours of the morning in many regions and, even where it didn’t, the nights were terribly cold… enough to send early plantings of tomatoes, peppers and annuals into shock. People reacted as if a frost at the end of May was something totally unheard of. And yet, it is far from unusual, not just in Eastern Canada, but in fact pretty much anywhere north of the Mason-Dixon line.
When is it really safe to sow our garden seeds and plant out our warmth-loving vegetables? Well, first of all, don’t trust the “last frost date” you may have seen for your municipality: Veseys seed catalog, for example, notes May 9 for Toronto, May 20 for Fredericton, etc. But those are average dates for the last frost. In other words, you can expect frost after that date one year out of two. Not really what you want for safe planting!
In the past I used to have all sorts of methods that never really worked for determining when to sow or transplant: it was supposed to be safe after the last full moon of May, when oak leaves were greater than the size of mouse ears, etc. If I followed them, my plants would still get frosted occasionally. But now I have my own on-line crystal ball… and it really seems to work.
I now use the 14-day trend forecasts of on-line meteorological services to decide when to move forward. I confess to using The Weather Network, but there are many others just as reliable.
Of course, 14-day trends are not 100% sure, but the “trend” does tend to be. If you see the line on the graph suddenly dip 10 days ahead, it may not happen on just that day or exactly to that degree, but there will almost certainly be a dip. And that’s what I look for.
What I’m seeking is not just no frost at all, but night temperatures that remain above 10˚C/50˚F. When night temperatures consistently stay at that level or above, that usually means cold nights are a thing of the past. This is my way of determining when to plant and sow tender plants (I include in this group tomatoes, peppers, eggplants, cucumbers, squash, melons and the vast majority of annuals). Note that I chose 10˚C/50˚F as my limit to play it safe. It’s not just frost that causes damage: at temperatures of 5˚C/40˚F and below, many tender plants start to shut down. And when temperatures do warm up, they can take days or weeks to start growing again. Therefore, by making 10˚C/50˚F my “safe planting temperature”, not only should be plants be free of cold, but the soil ought to be warming up and germination and growth can therefore “proceed as usual”.
I really stick to my guns when it comes to my minimum planting temperature: if there is even one night where I see temperatures a single degree below 10˚C/50˚F, especially when it is supposed to occur during the less trustworthy second week of the trend, I hold off. By the time that week rolls around, the weather service may well have decided it could be even colder. And that I don’t want.
So, here is my recommendation: when planting season seems to be on the horizon, check your municipality’s 14-day trend looking for nights consistnly above 10˚C/50˚F. If it is not yet time for you to plant (be forewarned, this year there is still a worrying drop in temperature on May 31st and June 1st in many regions!), it ought to probably come very quickly! You can start acclimating your seedlings to outdoor conditions (after all, you can still bring them in if it gets cold), but it would be wise not to plant them just yet. And for those who can now safely start mucking about in the garden, happy planting!