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September Garden Tips

In September the weather here in Zone 5 Pittsburgh starts to moderate a little, and with the cooler temperatures comes a renewed desire to get out in the garden. Here are some of the things to do this month.


  • Fall is for planting- trees, shrubs, bulbs, grass seed, mums, asters, pansies and the list goes on. The cooler temperatures, and more plentiful rainfall makes fall a wonderful time to plant. An added benefit to fall planting is that it gives you a head start for next spring. Plants that are planted in the fall will be all settled in and ready to grow when the ground thaws and temperatures warm up next spring.
  • Late summer and fall weed control is much more critical than most of us believe. Perennial Weeds.  This is the best time to eliminate perennial and vine type weeds such a Canadian Thistle, Field Bind Weed, Poison Ivy and many others. During this season plants are moving food from the leaves to the roots for winter storage. Sprays applied to weeds and brush now are more easily translocated from the leaves to the roots insuring the best possible control.

Here are some recommendations:

  • Thistles-Thistle Down, by Monterey, is the best product on the market today for thistle and many other hard to control weeds. It is expensive, but it works.
  • Poison Ivy –Poison Oak & Ivy Killer, by Bonide will usually eradicate this vine with one application.
  • Grass in flower and shrub beds – Grass Beater kills the grass without harming broad leaf plants. 
  • General Weed Control –Kleen Up, kills both annual and broadleaf weeds. It works like Roundup, but better.

Winter Germinating Annual Weeds – These are the weeds that are such a problem in our gardens during April and May. They germinate in late summer to fall remaining unnoticed during the winter and ready to create havoc in spring. . The solution is to apply Amaze during September, which is a granular pre-emergence herbicide, which puts an end to weeds as they germinate. Take care not to use Amaze around plants you desire to reseed.

  • Start fall clean-up in the flower beds, cutting back anything that has finished blooming and is looking tattered. Leave plants such as sedum, ornamental grass, and others that offer winter interest.
  • Dispose of diseased or pest-ridden plant material in the trash, although these materials can be composted, this should only be done if you’re certain that your compost pile reaches hot enough temperatures to kill any pathogens or over-wintering insects/eggs.


  • Fall is a great time to plant trees and shrubs.  The new plants will have several months to grow new roots and will be ready to grow early next spring.  Select accent plants for your landscape that will provide autumn colors. Trees that have red fall color are Flowering Dogwood, Maples, Oak, Serviceberry, Smoke Tree, Ginkgo, Sour /Black Gum, Ornamental Pear, and some Crab Apples. Shrubs with stunning fall foliage include sumac, viburnum, winged euonymus and Barberry, Spiraea, Fothergilla, Oak Leaf Hydrangea, Itea, Chokeberry, Clethra, and Blueberry.
  • Allow plants to finish the summer growth cycle in a normal manner. Never encourage growth with heavy applications of fertilizer or excessive pruning at this time. Plants will delay their dormancy process that has already begun in anticipation of winter in the months ahead. New growth can be injured by an early freeze.
  • Water newly planted trees and shrubs to provide sufficient moisture and prevent winter damage. Add a three inch layer of organic mulch such as shredded bark around the base of plants to retain soil moisture and regulate soil temperature. Keep the mulch a few inches away from stems and trunks.
  • Fall is a great time to plant trees and shrubs.  The new plants will have several months to grow new roots and will be ready to grow early next spring.
  • Trees that bleed or are susceptible to disease if pruned in the spring may be pruned now.  This includes maples, birch, black walnut, oaks, honey locust and mountain ash. Prune young trees to a single central leader; remove broken, crossed or rubbing branches; and gradually remove lower branches.  Always make proper pruning cuts just beyond the branch collar but not leaving stubs.
  • Give evergreen hedges a final trim to ensure they are neat for the winter. 


  • It is especially important to mulch after planting if bulbs are planted very late in fall. Mulch insulates the soil, keeping it warmer longer so root growth can occur.
  • Spring flowering bulbs planted this month will provide welcome color early next spring. Drifts of a dozen or more bulbs of one variety make the most impact. Planting depth is normally 3 times the height of the bulb; space the bulbs according to the package directions. Espoma Bulb Tone should be incorporated in the soil below the bulbs.
  • The larger the bulb, the better the flower. Look for large bulbs, free of bruising and handling injuries to guarantee success.
  • Winter pansies, ornamental kale, flowering cabbage, and fall mums may be planted now, to give color to the garden now that summer flowers have faded away. The sooner they are planted the better so a strong root system can develop before the onset of winter. Flowering kale and cabbage will turn a beautiful color with the cold and will last until covered with snow.
  • Try planting bulbs in containers, and when doing so it’s worth placing them in two layers, one slightly staggered above the other. Plant small bulbs on the top layer and larger ones lower down. This will ensure a spectacular spring display. Consider planting hardy spring bedding plants, such as winter pansies, primroses, or violas on top.
  • Some annuals such as Coleus and many tropical plants can be taken indoors and grown as houseplants during cold winter months.
  • Pot up some spring flowering bulbs for indoor color during the winter. Store the pots in a cool, dark place, until new growth emerges from the soil, and then move them to a bright window.


  • This is a good time to evaluate your autumn landscape to see where new perennial plants can be added to provide fall color.  Asters, perennial blue salvias, and some of the sedums (including the popular 'Autumn Joy' and stunning new 'Drazzel Berry'.) are good choices for long-lasting color at this time of the year.   If your soil is moist and the spot is sunny, try Helen's Flower (Helenium) cultivars with their fall colors of reds, oranges, and yellows. Goldenrod is another possibility.  There are many nice cultivars, including ‘Fire Works (3-4’), 'Golden Fleece,' (2’), and Little Lemon (1’) Contrary to popular belief, goldenrod doesn't cause allergies. 
  • You may notice a dust or talcum-like powder on your, phlox, monarda or lilacs this month. That's powdery mildew.  It's a fungal disease, that while unsightly, it often causes little damage to ornamentals and garden plants.  Applying a fungicide now will keep the disease from spreading. Use Bonide Liquid Copper Fungicide (for copper) or Safer 3-in-1 Garden Spray which is natural sulfur based fungicide. Another alternative would be to include Bonide Fung-Onil, which contains a powerful chemical fungicide, as the third element into your spray rotation. One spraying will not, however get rid of the problem, so be prepared for subsequent applications.
  • If you prefer not to use chemicals, thinning the plants to improve air circulation will help, as will pulling mulch away from around the base of stems.
  • If you have perennial grasses invading your perennial garden you know that it is almost impossible to remove. The most effective way to remove it is to kill it with Bonide Grass Beater. This herbicide will kill the grass without harming broad leaf perennials. 
  • Dig and divide perennials that are growing beyond their assigned spaces – or ones that are dying out in the middle. Discard the dead centers and replant divisions from around the perimeter. Fist-sized pieces are fine. Late summer through early fall is the time to divide some of your plants that are over grown, don’t wait; they need time to establish themselves before cold weather arrives. Examples of perennials that can be divided include:  Astilbe, Bearded Iris, Black-eyed Susan, Bleeding Heart,  Daylily, Echinacea (cone flower), Foxglove, Peony, Phlox, and Shasta Daisies.
  • If you want to try to over winter your mums, cut the stems to the ground once plant tops die back. Apply a thick layer of straw or bark mulch in Mid November, removing it in the spring as the frost leaves the ground. 


  • For tomato and potato Late Blight protection, spray them with a copper based fungicide, 10 days later spray with a sulfur based fungicide, and then for the next application switch back to the copper. We recommend using Bonide Liquid Copper Fungicide (for copper) and Safer 3-in-1 Garden Spray for the sulfur based fungicide. Another alternative would be to include Bonide Fung-Onil if you are not an organic gardener. Fung Onil contains the very effective chemical fungicide ‘Daconil’, and it can be used as the third element of your spray rotation. 
  • If you notice any symptoms of late blight on your tomatoes or potatoes, pull the plants out immediately and put them in a plastic trash bag and discard them in the trash.
  • Do not put them in your compost pile, as compost can stay warm enough for the disease to over-winter and affect your plants again the next season. 
  • If frost threatens, cover your tender tomatoes, and peppers with cloth (bath towels or sheets). Paper and plastic sheeting will not provide sufficient protection. Don't worry about your carrots, cole crops, turnips, and parsnips. They'll actually taste sweeter after being exposed to temperatures between 28 and 34 degrees F. For harvest in early winter, cover these root crops with 18 inches of straw, hay, or dry leaves. This will keep the ground from freezing so deeply and make it easier to dig them up. 
  • Harvest remaining vegetables, including green tomatoes. (Ripen by wrapping each in a sheet of newspaper and storing in a cool (55 - 60 degrees F.) dark spot.
  • Spray tomatoes if stink bug damage is noted. Bonide Fruit and Nut Orchard spray provides very good control. Read the label before using.


  • If you have not already done so, remove the old canes from black raspberries and June bearing red raspberries now that the fruiting season is over. Removal of the old canes of ever bearing/fall red raspberries should be delayed until after mid October.


  • September is the best month of the entire year to seed your lawn. This includes both seeding a new lawn and reseeding (over seeding) an established lawn to make it thicker and healthier. Test your soil, and if you needed, apply lime, fertilizer.
  • September is also a good time to apply a grub control to your lawn if problems were experienced this summer. We recommend St. Gabriel Organics' Milky Spore Grub Control.
  • With the cooler days of early fall, grass growth speeds up (and slows down later in fall with colder temperatures), so keep mowing as long as it is growing.  Set the blades to cut grass at least two inches high.


  • Do not wait for frost warnings to move your plants indoors. Temperatures of 45 degrees Fahrenheit or lower can damage many tropical house plants.  If needed, spray them first with one of the following products: Bonide Eight, or use Bonide Systemic Houseplant Insect Control.
  • Repot pot bound plants with Fafard potting mixes.
  • Fertilize your houseplants with Bonide Liquid Plant Food, or Schultz All Purpose Plant Food Plus through November, and then stop feeding them until next spring.
  • September is the best month to plant Amaryllis bulbs. Remember not to plant them too deep.


  • If you have a problem with either dogs or cats disturbing your garden, spread Havahart Dog & Cat Get Away granules over the problem areas.
  • Fall Webworms can now be seen in tree branches and shrubs. They are the big, silky tents that are usually spun towards the tip of the branch. Inside the webbing there are many hungry caterpillars that will eat the plants foliage if not controlled. Although they usually cause no permanent damage, I like to remove them when they are found. My organic method of control is removal of the web mass with a stick, followed with a smashing of the worms.
  • We have had a several calls recently about bagworms. Bagworms are almost completely developed now – making it to late too treat for them. Bagworms are best treated in June with Bonide Fruit and Nut Orchard Spray. They can be identified as little gray / brown elongated bags 1-2” long hanging from defoliated evergreens. Open a few, and if you find larva within, try to pick off as many as you can to help control next year’s population.
  • Stink Bugs problems continue to grow, and so we are on the lookout for solutions. St. Gabriel Organics’ Stink Bug Killer is an effective organic insecticide that knocks this pest dead in an instant. It can be used indoors, but on the downside it can also damage plants. Bonide Fruit and Nut Orchard Spray contains the organic insecticide Pyrethrum is safe to use on plants and deadly on Stink Bugs. This product comes in a concentrated form and so a portable sprayer is required. Another new product is the Rescue Stink Bug Trap. An advantage of this trap is that it attracts the target insect from only at a distance of about 30 feet. It thus catches stink bugs located on your property, and does not draw them in from around the neighborhood.

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