Ornamental Garden Calendar For September

Trees and Shrubs 

  • Wrap up any pruning the first half of the month then stop pruning so you do not create a flush of new growth that will not have a chance to harden off before winter.
  • Do not prune blighted junipers this month wait until the hot dry weather returns next season.
  • Stop fertilizing to prevent new growth that will not have a chance to harden off before winter.
  • September is a great month for planting new trees and shrubs and relocating current ones. Exceptions are butterfly bush, dogwood, rose of Sharon, black gum (Nyssa), vitex, red bud, magnolia, tulip poplar, birch, ginkgo, hawthorn and most oaks which are better planted in spring.
  • Continue monitoring for crown gall and galls on susceptible trees and shrubs especially on new plants you wish to newly introduce to the landscape.
  • Continue to watch your lilacs, flowering crabapples, ninebark and currant bushes for signs of powdery mildew and continue your protective treatments.
  • Continue to watch for for Fall webworms. They build their messy, potentially fairly large, tent like webs near the tips of branches. Prune them out then bag it and destroy it. 
  • Continue to prune out and destroy the cupped leaves of boxwood to control boxwood psyllid.
  • Continue searching boxwood plants for leaves infected with boxwood leaf miners and remove and destroy any found.
  • Bristly Rose Slug Sawflies and Large Rose Sawflies are still actively feeding on roses.
  • Viburnum leaf beetles continue to lay their eggs until killing frost. Tap the beetles into a container of soapy water and prune out their egg laying sites.
  • Examine your boxwoods for box tree moth larvae the first 2 weeks of Sept. Treat with BTK if infected.

Perennials

  • 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. 
  • Ease up on fertilizers and watering to slow growth helping them to begin preparing for winter.
  • You can begin planting spring bulbs but hold off on planting tulips until late October.
  • Divide peonies now.
  • Give wisteria vines their final pruning in mid-month (or when growth has ceased). Don’t prune it quite as hard this time, leave four or five nodes or buds per branch; these will form next year’s flowers and branches. This is a good time to also train in any new shoots for training along your growing support. 
  • Continue monitoring daylilies for rust. If it is a problem in your area continue protective fungicide. 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. 
  • Daylilies can appear quite ragged this time of year. Remove dead and unattractive foliage to improve appearance or cut the entire plant back. It will re-sprout new growth. September is a good time to split and or relocate daylilies.
  • Echinacea flowers are fading, they can be dead headed to tidy appearances or they can be left for the birds like yellow finch to eat the seed.

Annuals

  • Continue dead heading flowers to keep them flowering.
  • Continue to monitor for powdery mildew on zinnias, begonias, snapdragons and verbena. Avoid overhead watering where possible and maintain even soil moisture. 
  • 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

  • Weeds populations will begin increasing as the temperatures decline, keep them pulled and from going to seed.
  • Rag weed is in full bloom this month
  • The winter annual weeds (weeds that germinate in the fall and over winter to flower in spring) are going to start germinating this month and next. Apply corn gluten meal to reduce germination. Some winter annual weeds are: Hairy bittercress, yellow rocket, whitlow-grass, Henbit, purple dead nettle, common chick weed, corn speedwell, Persian speedwell, annual bluegrass.
  • A few perennial weeds that are germinating now dandelions, clover and plantain.

Lawns

  • Crane flies continue emerging from the lawn until around mid Sept. if you have a problem with these reduce watering as they require moist conditions to lay their eggs. Nematode sprays can begin.
  • Monitor for white grubs. They can do a great deal of damage in September and October. Begin nematode treatments.
  • By mid month resume fertilizing switching to low nitrogen fertilizers for fall.
  • September and the first half of October is lawn repair time. Over seeding, sodding and over all lawn repairs can be carried out at this time.
  • In areas not over seeded, corn gluten meal can be applied to control winter annual weeds and a few perennials weeds that germinate in fall. (see above under weeds for a list of those weeds).

Miscellaneous

  • Wasps can be a real problem this month, especially the first half of the month. Watch for signs of their activity and try to avoid sugary drinks and treats in the garden at this time. 
  • Japanese beetles continue to skeletonize a wide variety plants. Their numbers should begin diminishing as the month goes on. Continue tapping them into a container of soapy water.
  • Warm season mites are still active but should be winding down by the end of the month. Watch for stippled leaves and fine webbing, a sign of their presence. Use a summer oil spray or chemical  miticide.

Photo credits: all photos taken by the author.

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