Ornamental Garden Calendar For August

Photo by Kimberley Pacholko

Trees and Shrubs

  • Continue to watch your lilacs, flowering crabapples, ninebark and currant bushes for signs of powdery mildew and continue your protective treatments. Maintain even soil moisture and try to avoid overhead watering. 
  • Gypsy moths are still in flight and laying their eggs. Pheromone traps may be used to trap some of the moths. Seeking out and destroy their egg masses. Egg clusters are usually ¾” and oval in shape. They look like a piece of felt or velour and are buff colored this time of year. Gypsy moths lay their eggs everywhere some common hiding places include the underside of branches, tree trunks, fences, firewood, outdoor furniture, swing sets, boats, and trailers and under the eaves of buildings. When an egg mass is observed it should be scraped off with a knife and dropped into a bucket. Destroy the eggs by burning, crushing or by pouring scalding hot water over them. 
  • Continue to monitor purple-leaved sand cherries and fruit trees for European fruit lecanium scale, Eggs may still be hatching. Remove any found and/or spray crawlers with insecticidal soap.
  • Watch for  fall webworms.
  • Viburnum leaf beetles are active on viburnum shrubs this month and mating. Hand pick them by tapping them into a dish of warm soapy water. Like Japanese beetles they drop to the ground when disturbed so hold your bowl under them.
  • For junipers and eastern red cedar spray a protective fungicide every 10-14 days until the end of August to protect against apple cedar rust, especially if there are apple trees in the area with rust.
  • If your linden tree has spindle galls there is a naturally occurring predatory mite that feeds on Eriophyid tiliae (the insect causing the gall formations) you may be able to attract them to the area by soaking a couple of cotton balls in wintergreen essential oil, placing them in a small container then hanging that container in the affected tree.
  • Examine magnolia trees for signs of magnolia scale and if found begin spraying summer oil sprays or insecticidal soap sprays the end of August through September to suffocate the crawler stage.
  • Continue to prune out the cupped leaves in boxwoods caused by psyllid.
  • Keep trees and shrubs well watered (at least 1″ of water per week) to help prevent leaf scorch, as common problem in August.
  • 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.
  • Continue monitoring for crown gall and galls on susceptible trees and shrubs.
  • Continue pruning out any dead or diseased twigs from junipers infected with either phomopsis blight or kabatina blight. Be sure the foliage is dry and will remain dry for some time. Clean your blades between cuts and burn or black bad the clippings.
  • Keep your dogwood shrubs adequately hydrated this month and monitor them for insect infestations such as scale, bores, sawflies, aphids and club-gall midge and control any found. Both efforts will help to reduce the risk of Botryosphaeria canker.
  • Galls are common on many plants throughout the season. They can be pruned off.
  • If wisteria vines were not pruned in the back half of July give them a very hard pruning in the the beginning of August. Prune to maintain desired size, shape and to thin out overcrowded stems completely. 
  • Warm season mites still active. Watch for stippled leaves and fine webbing, a sign of their presence. Use a summer oil spray or chemical miticide. 
  • Try to wrap up all (especially evergreen) pruning and fertilizing this month to give any new growth formed a chance to harden off before winter.
  • Wrap up fertilizing of shrubs and trees this month to allow new growth a chance to harden off before winter.
  • Watch for for Fall webworms. They build their messy, potentially fairly large, tent like webs near the tips of branches. Prune it out then bag it and destroy it.
  • If you live in an affected area, examine your boxwoods for box tree moth larvae mid-July to mid-August. Treat with BTK if infected. The end of August is also peak flight time for the adult moths. Set up pheromone traps to catch the adult moths.


  • Continue to monitor for powdery mildew on susceptible plants like phlox, roses, asters, coreopsis, delphinium, lungwort, monarda, centaurea and peonies. Avoid overhead watering where possible and maintain even soil moisture.
  • Continue to monitor roses for Bristly rose slug sawflies and large rose sawflies.
  • Continue dead heading flowers to keep them flowering. Some of the plants that require dead heading are: Coreopsis, Coral bells, Daylilies, Blanket flowers, Maltese cross, Daisies, Pinks (Dianthus), Columbines, Butterfly weed, Delphiniums, Globe flowers, Buttercups, Pincushions, Gas plants, Golden marguerites, Campanula Bellflowers, Salvia, Yarrow, Spider wort, Centaurea, Lady’s mantle, chives, Foxgloves and repeat blooming roses.
  • Aphids can be a real nuisance this month if there is ample moisture and moderate temperatures. Scout the growing tips, especially flowers, for their presence and squish them, blast them with water, prune them off or spray with an insecticidal soap. Ease up on fertilizers to reduce the succulent new growth that attracts them.
  • Divide oriental poppies now.
  • Bearded iris can continue to be divided this month. Examine the rhizomes for bore damage and discard them along with older rhizomes.
  • Wrap up fertilizing this month to allow growth to slow in preparation for dormant season.
  • Begin monitoring daylilies for rust. If it is a problem in your area apply protective fungicide sprays. If they are already infected but are not yet producing powdery spore masses, systemic fungicides may be able to cure the infections and prevent pustule development.
  • Continue to remove columbine leaves infected with leaf miners and black bag them.


  • Continue to monitor for powdery mildew on zinnias, begonias, snapdragons and verbena. Avoid overhead watering where possible and maintain even soil moisture.
  • Deadhead Geraniums, Dahlias, Heliotrope, Zinnias, Marigolds, Pansies, Snap dragons, Verbena, Lantana, Petunias and Cosmos to keep them blooming.
  • Continue to pinch back Basil, Tarragon and Coleus.
  • Continue to cut back petunias by 1/4 to 1/2 of it’s growth to keep them from getting leggy. 
  • For hanging baskets that have been allowed to dry out place the entire container in a tub of water or deep saucer of water, for about 15 minutes to rehydrate. 
  • Continue to monitor impatiens for signs of downy mildew. Remove and black bag if infected.
  • Continue fertilizing your potted plants to keep them blooming.


  • Weeds traditionally begin to slow down during summer heat. Where they are well watered tough they will still be abundant. Keep up to the weeding and do not let anything go to seed.
  • Weeds in bloom this month include: ragweed, goldenrod, tansy ragwort, spotted knapweed, garden loosestrife, Joe Pye weed, Queen Anne’s lace, wild asters, chicory, burdock, white clover, black medic, carpet weed, lambsquarter, pigweed, prostrate knotweed, spurge, purslane, vervain.


  • Continue to pull back on fertilizers until temperatures begin to drop.
  • Continue to watch for cinch bugs they remain active until the temperatures begin to drop around mid September.
  • White grubs begin hatching.
  • Crane flies begin emerging from the lawn late this month, if you have a problem with these reduce watering as they require moist conditions to lay their eggs.


  • Japanese beetles continue to be a problem until about mid August. Pheromone traps can be set well away from your vulnerable plants to lure them away. Hand picking (dropping them into a bowl of warm soapy water) helps for smaller populations.
  • Rake wood mulches to help control birds nest fungi.

Photo credits: all photos taken by the author.

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