The average last frost date in central MO is mid-April, but the true last-frost date is mid-May. This is particularly true for those of us down in steep valleys, which trap cold air and extend the frost season far beyond the uplands. That was borne out Saturday night, as we got a light frost overnight.
This wasn’t unexpected on our part. We’ve had two very strong cold fronts come through in the past week, generating the severe weather that spawned tornadoes in Kirksville, large hail around the state, and very heavy rains on Friday. We got 3.5″ in two hours Friday evening, generating the highest streamflow we’ve seen yet and nearly blocking me in Saturday morning with a very heavy waterlogged log that I just managed to drag off the road without the aid of the tractor. The forecasts for Saturday and Sunday nights were around 41, which we know how to interpret.
Generally speaking, we can expect our valley-bottom temperatures to be 5-10 degrees cooler than the official NWS forecast at the airport. Thus, any time night-time temps are forecast in the low 40s, we know to expect a chance of frost. This is because our narrow valley really traps cold air and blocks wind, creating low, calm pockets of air that keep cooling beyond the relatively warm situation at the airport. When I got up at 5:30 Sunday morning, sure enough our house thermometer read 35 and there was light frost on the mulch and leaves in the lowest part of the market garden. I didn’t go out to the field, but I’m sure it was there too.
This is the reason we’ve never tried to start summer things too early, or beat people to market. It’s just not practical in our bottom-land. The trade-off is that we’re less susceptible to things like wind damage as compared to an open-land farm. In any case, we’ve been holding off on planting corn, beans, and more, while holding onto our tomato & pepper transplants, and we’re glad we did. Partly we also just haven’t gotten to those tasks yet, but we haven’t felt an urgency to because we know that late frosts are always possible here. And so this mid-May frost is little more than a weather note, rather than the disaster it could have been if we didn’t know our land and our hyper-local weather tendencies.
One of the best qualities a gardener/farmer can develop is an intimate knowledge of their land and weather/climate, however large or small. Plenty of gardening books offer advice like “talk to your neighbors; what works for them will probably work for you”. There’s some truth in that, but there’s also danger. It may work in a town, but our gardening neighbors (only a few hundred yards away) are also at least 80′ higher in elevation with a garden on an open ridge, and so their local weather conditions are thoroughly different from ours. If we planted corn or tomatoes on their schedule we’d likely lose it to late valley frosts. But our soil and wind patterns are different and allow us some benefits they don’t have. All part of being a responsible and observant farmer.