Horticulture News: Frost-free days are now here; soil is warming

By Kim Cowherd Published:

The long anticipated frost-free dates for Central Kentucky are here!

Derby Day (yesterday, the first Saturday in May – hope your horse won!) to Mother’s Day (next Sunday! Don’t forget the mom in your life!) are the “rule of thumb” dates after which we generally don’t receive any more frost or sustained freezing temperatures.

We all know that with Mother Nature, there can be days that are very cold after those dates, but generally the day warms back up quickly and we usually have few plant problems.  

Phil Case, your Gardening by the Signs guru and Extension Master Gardener, and I warn home gardeners every year not to plant summer vegetables and tender annual flowers before those dates, even when there is spring in the air. Some of you do not heed our advice or just want to take that chance, just in case there isn’t the adverse weather we caution about.  

Often, people assume that when there are warm, sunny days in early spring (or even late winter) that this is good enough for the plants.  The 60, 70 and even occasional 80-degree days cause us to think that our vegetables and flowers will be perfectly fine in the ground before the critical frost-free date.  

One key piece of plant growth that is often overlooked by the casual gardener is soil temperature.  The air may be very warm and comfortable, but the soil where the roots live can be very chilly indeed.  Think about how you feel jumping into the river, lake or even the deep end of a swimming pool in mid-May! Brrrr!  That is what your plant or seed feels like when you plop it in the cold soil.  

Cool nights, rain

Soil temperatures in the spring can still be very cold because, in part, to cool nights and rain. Tuesday of last week I checked the soil temperature and it was around 58. I checked this week (as of Tuesday, May 1) and the soil temperature is around 70°.  You can check soil temperatures by logging onto the UK Ag Weather page, http://wwwagwx.ca.uky.edu/ldm-images/soil_temp.gif.

Or you can take the temperature yourself. Use a metal soil thermometer, which is sold in many garden and hardware stores. Take temperature 2.5 inches deep at about 10 to 11 a.m. Diurnal variations affect soil temperature, with lowest readings after dawn and warmest around mid-afternoon.

The late-morning reading gives a good average temperature. Soil temperatures should be consistent for several days before seeds are sown to ensure that the seeds are being exposed to optimal temperatures for germination.

Delay planting seeds and transplants until the soil temperature has reached the proper temperature for optimum germination and/or growth for that particular plant. Otherwise, seed and root rot disease and related disorders are likely. Also planting at lower temperature can delay seed germination and increase the chance of the seed rotting before germinating. 

Also you need to think about planting your fall garden and high soil temperatures. In the same way low soil temperatures can affect seed and plant growth, high temperatures can do the same. The upper limits of soil temperatures for vegetable seed survival range between 86° and 104°.

Happy veggie temps

Some common vegetables and their temperature ranges are listed here. You can begin to see when looking at germination and growing soil temperatures why some plants are recommended for planting in the early spring or very late summer (for fall harvest) and others are recommended for planting in late spring or early summer months. 

Warm season crops: tomato, 65-85; beans, 60-85; corn and cucumbers, 60-95; eggplant, 75-90; cantaloupe and watermelon, 75-95; peppers, 65-95.

Cool season crops: spinach, 45-75; radish- 45-90; lettuce, 40-80; peas, 40-75.  

For a chart of specific vegetables and seed germination temperatures, log on to: http://www.hort.purdue.edu/ext/ho-186.pdf .

For additional information on growing vegetables, consult UK Extension’s Home Vegetable Gardening in Kentucky. Log on to: http://www.ca.uky.edu/agc/pubs/id/id128/id128.pdf. 

For other horticultural questions or printed material, contact the Franklin County Extension Office, 101 Lakeview Ct., by phone at 695-9035 or email DL_CES_Franklin@Email.Edu.UKy.

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