Guided by soil temperature rather than the calendar or phase of the moon, here is a bit of gardening advice for the month of May. These ideas, plus many more, can be found in my upcoming 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,” to be published later this year by Cadent Publishing.

Plant by soil temperature

Soil temperature is the most important factor in deciding when to direct sow main-season crops and when to transplant seedlings. Many of our summer vegetables are tropical in origin and will not grow well until the soil has sufficiently warmed. Here are some useful guidelines:

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.

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.

Gardeners who plant by soil temperature have one or two soil thermometers in the tool shed. These thermometers often have a large round dial and a probe more than long enough. You may prefer the pocket-size soil thermometer with a smaller dial and holder.

Use row covers to thwart leaf miners

Leaf miners are fly larvae that tunnel between the upper and lower leaf surfaces leaving distinctive blotchy trails or “mines” in the leaves. Use lightweight row covers to protect spinach, beet and chard leaves from damage by these herbivores.

Harvest asparagus spears

You should not harvest asparagus spears during the planting year, but plants in their second year and older can be harvested in May, snapping off the spears at ground level. Do not leave above-ground stubs as they can attract asparagus beetles and provide entry points for disease organisms.

Plant a Three Sisters Garden

According to Native American legend, corn, squash and beans are three inseparable sisters who only grow and thrive together. Planting these three crops together in the same garden space is a widespread tradition among many Native American farming societies, a sustainable system that provides both long-term soil fertility and a healthful diet.

The corn stalks provide a natural pole for the bean vines to climb while the beans fix atmospheric nitrogen on their roots, improving the fertility of the entire plot. The beans also help to stabilize the corn plants against root and stem lodging. The squash vines form a living mulch over the soil, slowing weed emergence and preventing evaporation of soil moisture, and the spiny squash leaves and stems discourage predators, including raccoons.

Sweet corn grows best in loose, rich soil with a pH between 6.0 and 6.5; heavy soil inhibits rooting. After incorporating decomposed stable manure or compost into the soil, make mounds spaced about three feet apart, each 12 inches high and 24 inches across. If possible, group the mounds in a block, rather than in one long row, to enhance wind pollination.

Flatten the tops of the mounds, then plant five to six corn seeds in a small circle within the center of each mound. Plant the kernels an inch deep in heavy soil, two inches deep in light sandy soil.

When the corn seedlings are about five inches high, plant seven or eight pole bean seeds in a circle about six inches away from the corn seeds. Wait another week, then sow seven or eight squash seeds around the edge of each mound, about 12 inches from the beans.

As the plants grow, thin the corn in each mound to the sturdiest two or three seedlings. Also thin the beans and squash, removing weak seedlings and keeping three vigorous seedlings of each evenly spaced around the mound. Help the bean seedlings to start climbing the corn stalks by gently propping their growing points against the stalks.

Grow a bumper crop of onions

For producing the largest onion bulbs, weed control and regular watering are the most critical factors. Onions survive periods of drought by using water stored in the developing bulbs, thereby reducing bulb growth, and weeds are serious competitors for available water.

Irrigation during dry periods is essential for production of large bulbs. Use the knuckle rule to determine when to water during the season. If you can feel moisture when you stick your finger in the ground up to your first knuckle, then the onions are wet enough. Use drip irrigation, if possible, rather than an overhead sprinkler system, which may promote the spread of disease. We often let the hose trickle over the soil surface, moving it as we work in the garden, a makeshift form of drip irrigation.

Mulching helps reduce competition from weeds while maintaining uniform soil moisture levels. Weeds that manage to grow through the mulch should be pulled by hand; a hoe will nick the young bulbs.

Buying a tree for the garden

Purchase a small tree, 1 or 2 inches in trunk diameter, at most. Research has shown that the establishment time for a tree, the length of time required for the tree to replace roots lost during harvest and handling and thus regain a normal root:shoot ratio, averages one year for each inch of trunk caliper. During this time, the tree is especially sensitive to damage from drought and heat stress. Small trees establish adequate root systems more rapidly than large trees and ultimately catch up in trunk diameter with initially larger trees.

Avoid purchasing a container-grown tree that does not show an obvious trunk flare, a widening of the trunk, as it approaches the soil surface. It may have been planted too deep in its pot, a situation that can lead to tree decline in the future.

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