Plant by Soil Temperature, Not by the Calendar or Phase of the Moon

(Author’s Note: The following essay appears in The New England Gardener’s Year by Reeser Manley and Marjorie Peronto.  This month-by-month guide to gardening in New England is now on sale at local bookstores and online.  Reeser will be speaking and signing copies of the book at the Blue Hill Library on Monday evening, April 22. His talk, part of the Spring Environmental Lecture Series, is titled “Gardening with Insects: The Little Things That Run the World”. This event, co-sponsored by the Blue Hill Heritage Trust and Downeast Audubon, begins at 6:30 pm in the library’s Howard Room.)

May will soon be upon us and as temperatures rise the urge to plant may overwhelm a gardener’s good sense.  This week’s column is a reminder to temper that enthusiasm in favor of a more reasonable planting schedule.

For gardeners in USDA Zone 5, Memorial Day weekend at the end of May is a popular time to plant tomatoes and other summer vegetables.  Gardeners along New England’s southern coastal zone, USDA Zone 7, reserve Mother’s Day weekend in the middle of May for planting these summer crops.  There seems to be a wide consensus on these dates among a lot of Calendar Planters in both zones, although the reasoning is not clear.

Other gardeners believe that summer vegetables grow faster and larger when planted by a full moon.  They hang onto this believe 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.

When the full moon comes in mid-May, and it is obviously too damp and cold to plant summer crops, Moonlight Planters have to wait until early June.  This may be the best of choices, not because the moon will be full, but because soil temperatures may have finally settled above 60 F by then.

Soil temperature is the most 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, as indicated in the following tables.

MINIMUM soil temperatures at a 4-inch depth for transplants:

60 F – tomatoes, tomatillos, cucumbers
70 F – peppers, squash (both summer and winter)
75 F – cantaloupe, sweet potatoes

MINIMUM soil temperatures (at a 2-inch depth) for direct-sown vegetables:

50 F – onions

Wait for soil temperatures to reach at least 60 F before planting transplants or sowing seeds of warm-season crops.

50 F – beets, Swiss chard
60 F – snap beans and dry beans
60 F – sweet corn
70 F – lima beans

When I consider that the average end-of-May soil temperature in Maine’s Zone 5 at a 6-inch depth for the period 1997 to 2010 was 55 F –and that is some years, according to University of Maine records, we lag about 10 F below that average, I understand why the survival of transplants and germination rate of direct-sown vegetables is higher for gardeners who exercise a little patience.

For direct-sown crops, such as beans, we can ask not only what is the minimum soil temperature for germination, but also what is the optimum daytime soil temperature (at a 2-inch depth) for maximum seedling production in the shortest time.  For the vast majority of summer vegetable crops, the answer is 77 F.  For cucumber and eggplant, however, it’s even higher, 86 F.  At soil temperatures of 60 F and below, seedling production drops dramatically.  Clearly, direct sowing of summer vegetables when soil temperatures are still in the mid-40s to mid-50s makes no sense.

Where soil temperatures will be by Mother’s Day or Memorial Day in any given year is anyone’s guess and will depend entirely on the weather and soil type.  It may make sense to hold off until mid-June to sow beans or plant peppers.  On the other hand, home-grown tomato seedlings that are getting leggy under the fluorescent lights need to go in the ground.  Better to plant these sooner than later, 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, or in a garden bed.  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.  Over the years, from Georgia to Washington State, I’ve been a Calendar Planter and a Moonlight Planter, but Maine springs are something else.  My money is on the soil thermometer.

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About Reeser Manley

I was born in Laramie Wyoming but moved to the southeast at an early age. I was educated through my B.S. in Biology in the Columbus, Georgia area, then crossed the Chattahoochee River to earn my M.S. in Botany at Auburn University. For the next ten years, I worked as Horticultural Manager for the George W. Park Seed Co. in Greenwood, S.C. At 40 years of age, I decided to return to graduate school and in 1994, I earned a Ph.D. in Horticultural Science from Washington State University, Pullman, Washington. Fast forward through 10 years at university (7 at UMaine, Orono) and you find me teaching high school science in Eastport, Maine, the edge of the world, and writing a weekly garden column for the Bangor Daily News. My new book, The New England Gardener's Year, a Month-by-Month Guide for Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, Connecticut, and Upstate New York”, will be published later this year by Cadent Publishing. You can learn more about the book by visiting its Facebook page: