A local truck farmer, in business for going on 35 years, waits to put out thousands of transplants—tomatoes, peppers, eggplant—until mid-June, a full month after the area’s “last frost date” (a nebulous term, considering we’ve seen frosts on the fourth of July). Perhaps he got caught transplanting early in his farming career, was struck by an unexpected cold spell that devastated his crop, so quickly became gun shy about the weather. Because he plants late, though, he faces weather concerns that, to me, create more havoc than ill-timed frosts, however hard: excessive heat.
Even tender annuals, babied in a greenhouse for two months or more, usually survive a few hours of frost—albeit somewhat damaged. We always plant at least a week before that last frost date, and about half the time we get leaf damage to the more tender annuals—which we don’t harden off, by the way, preferring the risk of frost to the risk of me forgetting to water them in their trays. The only serious damage to ever occur happened when the transplants put out in the shade of our thirty foot tall house spent not just the night getting frosted but most of the morning—down the row, beyond the shadow of the house, the same species survived just fine because the plants were warmed by the morning sun. But not even hardened-off plants survive excessive, sudden dryness and heat. At some point they simply cannot recover, despite efforts to shrink their surface area (wilting) to lessen water loss,
The truck farmer never lost his plants to heat, despite frequent bursts of temperatures in the nineties at that time of year, because he flood irrigated, saturating the soil so the transplants rested in mud, the roots able to pull sufficient moisture to keep the stressed plants alive. (As you might imagine, the weed seed present enjoyed this irrigation style, providing summer long jobs to many) But drip irrigators can’t send nearly as much water into the soil as a flood irrigator or even a sprinkler, so heat immediately becomes the enemy.
Choosing when to plant is a lot like investing your retirement. You have to ask yourself what level of risk you’re comfortable with and act accordingly. The “last frost date” is just a guideline, basically a heads or tails moment that becomes more risky for frost as you move the date earlier and less risky as you move it later, though you never really reach certainty unless you plant WAY too early or WAY too late, both of which have unsuitable outcomes. There are plenty of farming-as-gambling jokes for a reason: farming is a series of decisions based on risk assessment.
We have our plugs come a week or two earlier than the last frost date, since cold temperatures may delay growth but only hard frosts will damage the plants. We pay close attention to the weather forecast and if it says the low will be mid thirties for the next three days, we plant out immediately, but if it predicts 30 or lower we wait until that time period passes. It takes but a couple nights in near damaging temperatures for babied seedlings to harden off enough to survive much harder frosts.
I sold alfalfa seed for a few years prior to getting involved in cutflowers, Alfalfa is a small seed that germinates at 60 degrees, and should be planted shallowly. It’s susceptible to frost at early stages, which made more cautious farmers plant later in the season (the soil here rarely hits 60 until June 1 or so), while risk-takers didn’t worry about early planting—some growers had no problem planting in the dead of winter if a spell occurred when they could get into the field, because they knew that the seed wouldn’t germinate but would be ready to when the first warmth hit. Early planters also knew that surface temperatures in a hot spell rise quickly, so deep thermometer readings might suggest the soil to be too cool for germination while the surface said otherwise. Short seedlings also benefit from the warm temperatures of the soil on cold nights, the heat rising just enough to give a shallow but sufficient layer of warmth strong enough to fight off the cold air. The frightful frost of early spring, once looked at this way, becomes far less scary—unlike that first fall frost that pretty much decimates most crops.
In all the years I sold seed, not a single customer lost a crop to frost, but many suffered loss to heat if they planted late and couldn’t keep the surface wet long enough to keep the alfalfa seedlings alive. Generally, even in the best scenarios local farmers can’t get their sprinklers across a field faster than every three days, too slow for seedlings that get baked from the sun. Seeing the farmers’ successes and failures informed my own decisions, just as the truck farmer’s early experiences must have informed his—and we each ended up in different logical cul-de-sacs.