May Gardening Tips

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Annuals and Perennials

  • Lily leaf beetles often show up first in spring on leaves of the crown imperial (Fritillaria). Check both sides of the leaves and down inside the center whorl of leaves. Also check the undersides of leaves for tiny orange eggs. The larvae have orange, brown, or greenish yellow bodies that are sometimes hidden under their excrement. Hand-picking the adults and the egg masses is the easiest control method.
  • Scout for black vine weevil adults feeding on the foliage of Rhododendron sp., Taxus sp., Euonymus sp. and Ilex crenata and use a foliar insecticide.
  • Remove spent blooms on tulips, daffodils and other spring flowering bulbs. The plant will focus its energy on growing new bulbs rather than producing seeds.
  • Plant dahlias, gladioli, cannas and other summer flowering bulbs.
  • Insert stakes or hoops now to prevent plants like peonies, asters and Baptisia from flopping over.
  • Divide and relocate daffodils if needed but keep the foliage intact until it has all died back.
  • Fertilize bulbs as soon as you see the foliage in the spring as they take in more nutrients when actively growing than when dying. Cut off the flower stalks but leave the foliage intact until it dies.
  • Continue to cultivate planting beds and carefully remove young weeds.
  • Lift, divide and replant late summer and fall-blooming perennials such as asters, mums, and sedum.
  • Plant caladium and tuberous begonias in shady spots.
  • Bring container plantings inside on cold nights.
  • Now is a good time to lay soaker hoses in flower and shrub gardens.
  • Clematis vines like cool roots so apply mulch or plant a low-growing ground cover to shade the ground.
  • Use fresh potting soil in your containers as old soil has fewer nutrients and may contain harmful bacteria and fungi.
  • Disinfect all pots and containers before refilling them for the season.
  • Lace bugs and aphids will appear soon. Spray with water or use a low-toxicity insecticide to control them.
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Education and Events

Houseplants

  • As night temperatures moderate into the 60's, move houseplants outdoors (avoid full sun and windy locations).
  • Check to see if your houseplants are root bound. Water them thoroughly and carefully remove them from their pots. If the roots have compacted around the outside of the root ball, it is time to repot.
  • Potted houseplants can be buried up to the rim in planting beds to add color and texture.
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In the Vegetable Garden

  • Herbs can hold their own in early spring containers. Snip leaves to season dishes and create salad dressings.
  • Harden off tomato, eggplant and pepper transplants before planting out at end of month.
  • Plant tomatoes, peppers, corn, cucurbits, and melons after all danger of frost is gone and the soil is warm. This is usually the last week in May.
  • Plant tomatoes in a different spot each year to reduce fungal disease problems.
  • Have frost protection fabric or cloches, such as you can buy at garden stores, ready for your transplants.
  • Continue to cultivate planting beds and carefully remove young weeds.
  • Plant a second crop of lettuce, arugula, spinach, and radishes for healthy, homegrown salads.
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Insects

  • Aphids and lace bugs will appear soon. Spray with water or use a low-toxicity insecticide to control them. Asian lady beetles are a beneficial insect that feed on aphids.
  • Lily leaf beetles often show up first in spring on leaves of the crown imperial (Fritillaria). Check both sides of the leaves and down inside the center whorl of leaves. Also check the undersides of leaves for tiny orange eggs. The larvae have orange, brown, or greenish yellow bodies that are sometimes hidden under their excrement. Hand-picking the adults and the egg masses is the easiest control method.
  • Scout for black vine weevil adults feeding on the foliage of Rhododendron sp., Taxus sp., Euonymus sp. and Ilex crenata and use a foliar insecticide.
  • Check family members and pets for ticks after being outside, especially when in tall grass or wooded areas. If necessary send ticks to the Connecticut Veterinary Medical Diagnostic Laboratory for testing.
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Landscape and Lawns

Fruits

  • Put nets over ripening strawberries to protect them from birds and other wildlife.
  • Clear grass and weeds from root zone areas around fruit tree trunks and remove any suckers growing from the base of the trees such as apples and crabapples.
  • Spread compost around fruit trees and top with 3 to 4” of bark mulch. Keep mulch an inch or two away from the trunks.
  • Remove any tree wraps or guards you placed on young trunks for winter protection.

Lawns

  • Thin or compacted turf will benefit from core aeration and overseeding.
  • Mow your lawn any time the grass is 1½ times the normal height, for example, if you mow at a 3-inch height, don’t let the grass get longer than 4 to 5”.
  • Ground covers such as vinca, ajuga, pachysandra, creeping foamflowers, and ivy can be divided and transplanted now to create new beds or enlarge existing ones. On a cloudy, cool day, use a sharp shovel or trowel to separate offshoots from mother plants and transplant them into a shady new location. Keep them well watered.

Trees and Shrubs

  • May is an excellent time to plant a shade tree or flowering tree in your yard.
  • Clear grass and weeds from root zone areas around tree trunks and remove any suckers growing from the base of the trees. Spread compost and top with 3 to 4” of bark mulch. Keep mulch an inch or two away from the trunks.
  • Remove any tree wraps or guards you placed on young trunks for winter protection.
  • Wait to prune evergreens, hedges and other shrubs until late spring into early summer.
  • Begin deadheading roses.
  • Fertilize roses and monitor them for insects and fungal diseases.
  • Twist off the wilting seed heads from rhododendrons and azaleas, so that the plants energy can go to foliage growth and next year’s flowers, rather than seeds.
  • Pines and other conifers can be kept to a compact size by pinching off one-half of the new growth 'candles'.
  • Prune lilacs after they finish flowering.
  • Now is a good time to lay soaker hoses in flower and shrub gardens.
  • Many summer-blooming tropical plants such as hibiscus and mandevilla bloom on new growth. Fertilize and prune carefully to encourage growth and flowers.

Wildlife

  • Hummingbirds and orioles return to northern states by mid-May. Clean and refill feeders to attract these colorful birds to your backyard or fill hanging baskets with flowers that will attract them such as petunias, salvia, and fuchsia.
  • Some perennials that attract hummingbirds are trumpet vine, honeysuckle, foxglove, and columbine.
  • Apply deer repellents if needed.
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Yard Accessories and Miscellanea

  • Aerate and moisten the compost pile to speed decomposition.
  • Add water lilies to your pond when the water temperature reaches 70° F.
  • Clean and store birdfeeders.
  • Disinfect bird baths with a 10 to 1 solution of water and bleach. Rinse well.
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Despite good cultural practices, pests and diseases at times may appear. Chemical control should be used only after all other methods have failed.

For pesticide information please call UConn Home and Garden Education Center weekdays, in Connecticut call toll free 877-486-6271. Out of state call 860-486-6271

Revised by the UConn Home and Garden Education Center 2017

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, the Dean of the College, Cooperative Extension System, University of Connecticut, Storrs. The Connecticut Cooperative Extension System is an equal opportunity employer and program provider. To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326-W, Whitten Building, Stop Code 9410, 1400 Independence Avenue, SW, Washington, DC 20250-9410 or call (202) 720-5964.