REESER MANLEY

Plant by soil temperature, not by the calendar or phase of the moon

Posted May 20, 2011, at 3:02 p.m.

Memorial Day weekend, soon upon us, is a popular time to plant tomatoes and other summer vegetables. There seems to be a consensus on this date, a lot of Calendar Planters, although the reasoning is not clear. If you have a large garden, the extra day may be necessary.

Other gardeners believe that summer vegetables grow faster and larger when planted by a full moon. They hang on to this belief because it has produced admirable results, not because it has any support from plant scientists, many of whom believe it to be pure lunacy.

I think that Maine gardeners came up with the idea of planting by moonlight because black flies feed only during daylight hours. I still have bumps on my scalp from planting potatoes a week ago.

This year, with May 17 obviously too damp and cold, moonlight planters will have to wait until June 15, a Wednesday. And this may be the best of choices, not because the moon will be full, but because soil temperatures may have finally settled above 60 degrees.

Soil temperature is an important factor in transplanting seedlings of tomatoes, peppers, cucumbers, squash and other summer crops. Many of these vegetables are tropical in origin and will not grow well until the soil has sufficiently warmed. For example, here are some MINIMUM soil temperatures at a 4-inch depth for transplants:

  • 60 degrees: tomatoes, tomatillos, cucumbers
  • 70 degrees: peppers, squash (both summer and winter)
  • 75 degrees: cantaloupe, sweet potatoes

Soil temperature is also critical for success with direct-sown crops.  Here are the MINIMUM soil temperatures (at a 2-inch depth) for direct-sown vegetables:

  • 50 degrees: onions
  • 50 degrees: beets, Swiss chard
  • 60 degrees: snap beans and dry beans
  • 60 degrees: sweet corn
  • 70 degrees: lima beans

Now, consider that the average end-of-May soil temperature (6-inch depth) for the period 1997 to 2010 was 55 degrees. So far we are lagging about 10 degrees below this average, according to University of Maine records.

This agrees with recent measurements of soil temperature in Marjorie’s Garden. When we planted potatoes on May 15, the soil temperature (4-inch depth) was 47 degrees. On May 18, after several days of rain and no sun, it was 45 degrees (considered the absolute minimum for planting potatoes).

For direct-sown crops, such as beans, you could ask: What is the OPTIMUM daytime soil temperature (2-inch depth) for maximum seedling production in the shortest time? For the vast majority of summer vegetable crops, the answer is 77 degrees. For cucumber and eggplant, this temperature is 86 degrees. At soil temperatures of 60 degrees and 50 degrees, seedling production drops dramatically. Clearly, direct sowing of summer vegetables at the end of May, when soil temperatures are still in the mid-40s to mid-50s, makes no sense.

Where soil temperatures will be by Memorial Day is anyone’s guess and will depend entirely on the weather. It may make sense to hold off until mid-June to sow beans or plant peppers. On the other hand, the tomato seedlings under the lights are getting leggy and need to go in the ground. We will probably plant ours sooner than later, perhaps on Memorial Day weekend, holding off on the mulch so that the sun can warm the soil as quickly as possible.

Those of us who plant by soil temperature have one or two soil thermometers in the tool shed. These thermometers have a large round dial, much like some cooking thermometers, but with a long probe (at least 8 inches) that can be pushed deep into the soil. A combined soil-compost thermometer with a 19-inch probe can be used to monitor both compost pile temperature and soil temperature.

There is an old saying that fits: “You pays your money and you takes your choice.” You live with the consequences. I’ve been a Calendar Planter and a Moonlight Planter, but this spring is something else.  My money is on the soil thermometer.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@shead.org. Include name, address and telephone number.

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