NB: If you are not a member of my Facebook gardening group, you can find it here. That and this post deal mostly with the northern Philly ‘burbs. But all are welcome, and folks from outside the area might find a few nuggets of garden wisdom and/or childish humor.
Welcome to TweenerLand! No, not an obscure, ancient Disney movie — this is what I call the second half of April into early May. It’s a fraught time for gardeners in these parts, toying with emotions, preying on impatience, waiting to cackle that cynical last laugh as you stare at the corpse of a tomato seedling transplanted way too early. My goal every year is to unite my local gardening world in grand resistance, denying TweenerLand such evil pleasures.
The temptations loom large some years. In some sense, I feed them with weather prognostication micromanagement — which, of course, is not my intent. My intent is to keep it real. Weather forecasting has come a long way since I was a lad, particularly long term forecasting. No, we can’t predict what it will be like thirty days in advance. But the tools are now good enough to let us venture educated guesses about the ensuing two weeks. The truth is, as gardeners, we don’t need a lot of details this time of year; we need the big picture. In the northern Philly suburbs, we can reasonably assume that it will be safe to plant sensitive seedlings by late May. That’s kind of a given, The burning question every year is whether we can cheat the calendar. The challenge with answering that question is that it only takes one cold night to damage young tomato seedlings — if not kill them, at least to alter their growth trajectory, erasing any advantage of starting early.
Determining whether we can cheat in any given year often reduces to figuring out when what weather geeks call “the northern branch of the jet stream” will recede for the summer. The “jet stream” is a global flow of air at high altitudes — higher than, say, twenty thousand feet. This flow of air paints the broad strokes of our weather picture. Let’s look at an example:
Figure 1 is a graphic taken from a popular weather model that shows us what is going on in the jet stream as of this writing, on April 20, 2024. And the picture is stark. I drew the magenta line to accent the stark boundary between colder Canadian air, brought to us courtesy of the northern jet, vs. warmer air suppressed to the south. The low this evening will be in the upper thirties around here mostly because the northern jet is still doing stuff like this. Which means that it is still way too early to plant your tomato seedlings. But we knew that.
The cool thing about modern weather models is that we can fast forward the model to any day, up to about sixteen days into the future. Of course, the further out in time we go, the less reliable the portrait (deciding when the model might be out to lunch can be challenging, but experience helps). But I get ahead of myself. Let’s fast forward the model into next week, say April 24:
Arrrrghhhh! If anything, the situation has gotten worse! This is why current five-day forecasts show overnight lows in the mid to upper thirties on Wednesday and Thursday nights. Again, this is not the week to put tomato plants in the ground. Or start beans or cucumbers or squash outdoors.
Okay, let’s move the model ahead a few days and look at April 29 — almost to May:
Whoa! Now that looks different! Now the southern jet is flexing its muscles, giving us a nice flow of spring air. And this is what we begin to see around this time of year — this is how we get those occasional, beautiful seventy-five degree days mixed in with those brutal thirty-seven degree overnights. Sure we’ll take it. But the story isn’t over. This is a struggle between heavyweights, after all. Sure, we know who wins — but when???
Also, keep in mind that this prediction is nine days out; we’re getting into a time frame when we do need to think about whether the model has fallen off the deep end for whatever reason. But, for now, let’s move things forward one last time:
To quote a famous politician who flubbed a line at a debate: Oooops. That southern jet muscle flex was short-lived. This is for May 4. According to the model, the northern jet returns with a vengeance, for a time. Will this happen? I can’t say for sure. But I do know it happened last year. And it happens frequently in early May. And that’s enough for me to hold off on any cheating for at least a couple weeks. To cheat Mother Nature this much, I will need to see the classic Bermuda High developing much earlier than normal. And there is no sign of that. So far, the model is telling me that this is a typical year as far as the model can reasonably see. Translated: business as usual for gardeners.
I will revisit this in seven to ten days. A lot can change in that time frame. Until then, I would hold off on moving any warm weather seedlings into the great outdoors.