Planning planting with the first frost in mind

By Grant McCarty
Local Foods and Small Farms Educator, University of Illinois Extension

As a continuation of last week’s introduction of cool season vegetables, this week we need to talking about planting dates. If you are going to be planting cool season vegetables, it is important to know when the first frost in the area will be as it will determine good growth or not. To begin with, we need to know what a frost is. It is defined as the first time air temperatures are 32 degrees F. There are frosts in both spring and fall. The last spring frost means we can plant after this date. The first fall frost date means we cannot plant after this date. Both of these dates bookend what is known as the growing period. In our region, the growing period is around 160 days. Southern Illinois has a growing period between 185-190 days.  As you might imagine, a month’s difference can have a considerable effect on the growth of your plants.

The first fall frost date in our area is around October 7. This is an average. The first frost this year could be earlier or later than this date. I recommend that you should expect an earlier frost date than the one given. This means to expect the first frost the end of September.

This date doesn’t mean that you should be planting a week before the first frost in the area though. You want direct seeded and transplants to have a good 6 weeks or more to grow, mature, and then be able to tolerate the sudden colder temperatures. When the first frost comes, it can mean a slowdown in the growth of the plants you have planted.

Most cool-season vegetables are what we call cold hardy. This means they can tolerate temperatures that reach 28 F, a temperature known as a hard freeze. It is possible that some of these cold hardy vegetables will tolerate colder temperatures than the hard freeze. The average first hard freeze in our area is around mid-October. Like with the first frost, you should plan accordingly for an earlier hard freeze than the average.

With these dates in mind, you are now in a period of time where you should be already direct seeding or transplanting your cool season vegetables.

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