In case this helps, I planted cold weather crops – brassicas, peas, and fava beans – outside in early January and February.
Most of the peas and fava beans lived, but didn’t sprout until early March. Some of them sprouted in January, which was awesome, but then they stayed tiny little sprouts that didn’t get any taller until March. (They did live through all the massive quantities of snow, though!)
Most of the brassicas sprouted in late January, but died after being buried under six inches of snow for two weeks. I had to replant brassicas three times, and they’re still just small seedlings.
Overall, I don’t feel like planting them in January gave me that much of a head start. Maybe it will someday – I would love to select for crops that can grow through the winter and not just stall and survive – but for right now, I suspect that planting before February won’t make any difference for me.
The one exception is the ones I sowed under a mini greenhouse in January – they’ve grown a bit more than the ones I just left out to the elements! It’s not much – maybe two months’ worth of growth in winter equaled a week’s worth of growth in summer – but a week’s head start is a week’s head start, and I’ll take it.
So I probably planted too early, but I’m not sure if that would be true in a normal year. January may have been right for the fava beans and peas in a normal year. They did (almost) all survive!
The brassicas may prefer to be planted a month after the peas and fava beans. I don’t think anything lived unless it was sowed in late February to mid March.
I will say that this year was unusual. We usually get our last snows in March, and they’re light. This year, it snowed heavily all through March, and we had a huge blizzard on April 4th. Now it’s an early, blazingly hot summer. Joy.
I suspect in a normal year, sowing in January would have given me a much more significant head start, because there would have been far less snow covering the winter crops and blocking sunlight.
Now, for the opposite – when is too late?
Well, last year, my first time trying cool weather crops, March was much too late to plant brassicas, because it was already too hot and dry for them to germinate. So I’m starting to think early to mid February is the best time for me to plant cool weather crops here. Sometime in that range when there’s not snow on the ground, and there’ll be a stretch of warm-ish weather for several days straight.
Meanwhile, with warm weather crops, “too early” is probably before May 15th if you want to grow divas that can’t handle a few very light frosts that may or may not happen after April 15th (our average last frost date). I want to select for warm weather crops that can deal with a light frost or two. That’s a month of valuable growing time, the only time of year with warm temperatures and abundant water left in the soil, so I want to use it. So I plan to plant them then, whether or not I “should.” Most of my neighbors wait until after May 15th.
With warm weather crops, “too late” is probably late August / early September. Our average first frost is October 15th. Last year, I planted several cucurbits in early August, and they just barely grew me mature fruit that I harvested just before our first frost on October 15th.
With overwintering cold weather crops, I’m starting to suspect “too late” is around mid September. They need to have time to get big enough to survive. Or so I suspect, based on the brassicas that I planted in early October all surviving frosts until December, but not growing at all, and then dying when gigantic piles of snow started flumping on them. They may have done better if I had put an unheated greenhouse on top of them, though. I suspect it wasn’t the cold, but rather the weight of the snow and the way it blocked sunlight, that kept killing my brassica seedlings.
As for eco-signs that may be helpful as reference to people in other places:
My daffodils have been flowering for about a week and a half. My echinecea seeds sprouted a month ago.
The peach trees in our neighborhood are starting to show little flower buds that haven’t opened up yet.
As for invasive weeds, the Siberian elms are starting to bud, the dandelions aren’t flowering yet, and the stars of Bethlehem are three times as tall as the grass, with no flowers yet. No signs of bindweed or purslane yet.
Hopefully those are all helpful references.