May

Indoors

  • Cut back poinsettias to two eyes above ground. Grow them outdoors for the summer and bring them indoors again before cold weather arrives.
  • Divide and repot old ferns.
  • Callas are heavy feeders but they will soon want to rest. Reduce watering and  feeding.
  • Move African Violets out of the strong sun and to the north or other protected window – not under trees.
  • Christmas cactus likes to be pot-bound to encourage blooming. Do not repot unless really crowded.

Outdoors

  • Harden off indoor or greenhouse plants about 2 weeks before planting into the garden. Gradually condition the plants to outdoors a little each day, starting with shade moving to full sun over the course of 2 weeks. Return plants to protected cold frame, garage or house overnight.
  • Transplant tomatoes, peppers, eggplants, melons, cucumbers, and sweet potatoes after the soil has warmed to 60 degrees F and danger of frost is over. If frost is predicted, protect transplants by covering overnight.
  • Plant seeds in the garden of squash, beans, corn, and okra around mid-month.
  • Sow more leafy greens, radishes, and carrots for succession of crops.
  • It is important to water and weed the garden. Transplants and seeds need water to grow. Weeds pulled before they flower result in fewer weeds throughout the season.
  • Sow sunflowers, zinnias, marigolds, cosmos, and other flower seeds. Plant dahlia tubers about a week after the last frost is expected.
  • Attract pollinators by planting herbs and flowers such as rosemary, marjoram, mint, sage, basil, thyme, zinnias, sunflowers, cosmos, and bee balm.
  • Certify your pollinator friendly garden with Penn State Master Gardeners.
  • Inspect plants and garden for pests and disease on a regular basis, identify and treat if necessary.
  • Mulch roses with a 1-inch layer of compost.
  • Fertilize asparagus beds when the season’s harvest is spent (when no stems are fatter than ½ inch in diameter). Use a fertilizer with 5-10-5 or 10-10-10 immediately after the harvest and again in mid-July and mid-August.
  • Prepare window boxes and containers using a rich soil. Consider using houseplants such as begonias.
  • Provide support for sweet peas before they are too tall and fall over.
  • Tender plants such as cannas, caladiums, and all summer bulbs can be set out later in the month.
  • Feed all bulbs before or while in bloom with a prepared bulb food or any good vegetable fertilizer. When the flowers have passed, do not cut off or braid the foliage. The plants need all their leaves exposed to the sun, to manufacture food for the next year.
  • Remove faded flowers.
  • Continue watering newly transplanted shrubs and trees.
  • Flowering cherries, magnolias, and flowering almonds should be cut back very sparingly after the blossoms fall.
  • Flowering hedges of spiraea, deutzia, or lilac should be cut back when the flowering season has passed to keep hedges in good condition.
  • Shape evergreens like arborvitae, yew, hemlock, spruce, and euonymus.
  • Mow lawns considering correct height and frequency for your type of grass. See “Mowing Turf Grasses”.