Ornamental Garden Calendar for May

Flowering Dogwood in Bloom.

May is one of the busiest and exciting times in the garden. The days are getting warmer and longer. Plants are quickly filling out, weeds are establishing, and insects are buzzing all around. The trees are breaking out in leaf and spring flowering trees and shrubs are putting on their show. Mid May is also when we here in southern Ontario typically begin planting out our annuals and vegetable gardens. Note: these times are approximate and can vary depending on the weather and on your specific hardiness zone. The zone I write from is a zone 6, adjust your dates accordingly.

Trees and Shrubs

  • Do not prune apples and other plants susceptible to fireblight at this time of year as you can spread the disease. This also applies to black knot, which affects plums and other members in the Prunus genus.
  • Junipers and eastern red cedars affected by Apple Cedar Rust are sporulating and these fungal spores are being carried on the wind to susceptible apple, crab-apple, hawthorn and quince. Apply protective sprays on apples, hawthorns and quince for rust disease if warranted.
  • Check pines and junipers for signs of tip blight. Spaeropsis tip blight and Kabatina blight.
  • Monitor pines for sawfly damage. 
  • Scale crawlers are active now. Infested pines and euonymus should be treated at this time using insecticidal soap spray or any other insecticide labeled for this use and legal in Ontario.
  • Monitor azaleas for early signs of lacebug damage.
  • Scout for eastern tent caterpillars and destroy webs if found.
  • Examine boxwood leaves for signs of boxwood leafminers. To obtain some control over the insect prune the plants back and dispose of the leaves before the insects exit the leaves in late May. Once the the adult flies are buzzing around (end of May- mid June) they can be sprayed with insecticidal soap.
  • Check yews for mealybugs and hand remove using a cue tip dipped in rubbing alcohol.
  • Pinch azaleas and rhododendron blossoms as they fade. Double flowered azaleas need no pinching.
  • Be on the lookout for holly leaf-miner. 
  • Spring flowering trees or shrubs that have finished blooming may be pruned at this time.
  • Honey locust plant bugs become active this month.
  • Treat for borers if found in dogwood, ash, lilac, apple, and peaches. 
  • Apply a preventative insecticide spray to the trunks of peach trees and other stone fruits in the first half of the month to control peach tree borers. Apply again in August.
  • Begin monitoring for crown galls on euonymus, lilac, willow, honeysuckle, weeping fig, poplar, apple, cherry, plum, apricot, chrysanthemums, asters, daisies, grape vines and brambles like raspberries. These are caused from a soil born bacteria that enter wounds near the soil line.
  • Also look for fungal galls on forsythia, viburnum, highbush blueberry, American elm, hickory, maple, oak, and privet.
  • As soon as young junipers begin actively growing begin protective fungicide treatments for Phomopsis blight, where warranted. Also, late May/June begin looking for any signs of tip dieback.
  • Around mid-May begin scouting for Box tree moth (BTM) larvae on the inner branches of boxwood and treat with BTK if infected. (This invasive pest is predominantly in the Toronto area, but spreading).
  • Spongy moth aka. Gypsy moth caterpillars become active this month begin scouting trees for them. They can be controlled with BTK (Bacillus thuringiensis kurstaki)


  • Begin hardening off indoor hydrangeas outside slowly increasing the amount of time they spend outside and the amount of sun they receive. Keep them in their pots, well-watered and free from frost.
  • Slugs and snails become very active, especially on hosta plants.
  • Begin planting gladiolus bulbs as the ground warms. 
  • Pinch back mums to promote bushy growth.
  • Pull the stems of any rhubarb plants damaged by frost as they may contain oxalic acid (a poisonous substance).
  • Garden beds may be top dressed with compost or composted manure. It is best to do this early before the plants get to big and it becomes difficult to apply.
  • Start watching for leaf tiers on hydrangeas particularly ‘Annabelle’ and ‘Incrediball’.
  • Begin checking roses for Roseslug Sawflies and remove them by hand and destroy them.
  • Examine Iris leaves for signs of the young larvae of Iris bores, (pinholes and discoloured lines, which may appear water soaked). Infected leaves can be cut off below the point of infection and destroyed.
Spring planting of annuals begins.


  • Begin hardening off annuals. They may be planted May long week or later as long as no frost is in the forecast.
  • Hardier annuals like alyssum, dianthus, viola, marigolds, bachelor buttons and sweet peas, geraniums and petunia may be planted earlier.
  • Hanging baskets may also be set out early May and brought into a frost-free space at night and on colder days.
  • Sow cornflowers, nasturtiums, poppies, calendula, flax, marigolds, morning glories, love-in-a-mist, cosmos and sunflowers directly in the garden this month.


  • Weeds really start taking a strong hold this month. Keep up to the weeding in May and June and you will be rewarded with fewer weeds during the summer months and more time to enjoy the gardens. For a list of common spring weeds here in Ontario.
  • Mulches may be applied after mid-May. At this time the soil has had a chance to warm up a bit allowing the roots to begin actively growing again


  • Watch for sod webworms towards the end of the month.
  • May is a good month for top dressing and over seeding.
  • Dandelions are in full bloom spray them or dig them out before they set seed.
  • The white grub larvae of Japanese beetles are feeding on the roots of turf the first half of this month, then they will pupate until the end of June at which time they are not feeding.


  • Last frost date in Toronto is typically May 11-20. To check your local frost date.
  • Early May is a good time to fertilize your trees, shrubs and perennials to support the flush of new growth they are putting out.
  • Turning the soil is a good way to control insect pests that have overwintered in your soil, such as hydrangea leaf tiers, cutworms and many more. Try to avoid getting too close to the root ball of your plants.
  • Cool season vegetables like arugula, beets, broccoli, carrots, cabbage, Kale, lettuce greens, peas, parsnip, radish, spinach, can be planted early this month. Warm season vegetables can be planted near the end of the month, watch your frost dates.

Photo Credits: all photos by the author.

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