• Sow or Fertilize warm-season grasses like Bermuda, Zoysia, and St. Augustine.
• Do not fertilize cool-season grasses like tall fescue, Kentucky bluegrass, and perennial ryegrass.
• Tall fescue and bluegrass should be mowed at a height of 2 ½ to 3 ½ inches.
• Long season vegetables should be side-dressed 6 to 8 weeks after germination.
• Fertilize summer flowers as well.
• Feed hybrid rhododendrons and azaleas with an acid fertilizer.
• Vegetable to be planted in May: snap beans, pole beans, bush lima beans, cantaloupe, sweet corn, cucumbers, eggplant, okra, southern peas, sweet peppers, hot peppers, pumpkins, squash, tomatoes, and watermelons.
• Gladioli bulbs can be planted.
• Summer annuals like begonia and geranium can also be planted.
• Hybrid rhododendrons and azaleas can be pruned as soon as they finish blooming.
• Outgrown hedges can be pruned.
• Pinch off garden mums till mid-July.
• Pick off any azalea leaf galls that may form.
• Spring bulb foliage should not be cut back till yellowing of foliage occurs.
• Rhododendrons should be monitored for borers and sprayed if present.
• Spray for iris borers in iris beds
• Scout and spray for bagworms on shrubs and trees.
• Use recommended herbicide to control poison ivy and honeysuckle if desired.
• Start spraying squash vines to prevent borers.
• Spray blueberry plants for blueberry borers.
• Start fungicide treatment on tomatoes, which show signs of blight.
• Continue with rose spraying program.
• Continue fungicide program for fruit trees and bunch grapes.
(Pesticides should be used sparingly! Use only when needed and always follow the label)
• Observe landscape shrubs for azalea-lace bug, boxwood-leaf miner, euonymus-scale, hemlock, juniper-spruce mites, and hybrid rhododendron-borer.
• Observe broccoli, cabbage, and cauliflower, for worms and spray if needed.
• Consider putting down mulch in beds to help with weed control.
See also: http://durhammastergardeners.com/2013/05/04/what-to-do-in-may/, http://durhammastergardeners.com/2012/04/26/what-to-do-in-the-garden-in-may/