September Gardening Tips and Tasks

• Save and share seeds from flowers that have finished blooming and have dry seed heads. See our three-part seed saving video series for seed collection and storage tips. Check with your local Master Gardener program for upcoming seed shares.
• Wait on pruning perennial plants until late next spring. Certain bees and other beneficial insects use plant stems as habitat to survive the cold winter months. Only remove unhealthy plant stems to manage disease.
• Cover bare soil to discourage weeds and erosion: use cover crops in a vegetable garden, groundcover plants, and/or mulch around new landscape plants until they get established and fill in.

• Avoid planting one type of tree or shrub (a monoculture) when planting a living screen. Diseases and insects are typically host-specific, so you run the risk of losing the entire screen if a pest or disease attacks the plants. Select several different species of trees and shrubs to prevent this from occurring.
• In most cases, healthy, mature trees and shrubs do not require fertilizers. Woody plants receive nutrients from lawn fertilization if their roots are adjacent to or growing under the turf. Their roots also take in nutrients from decaying mulches and leaves and from the minerals in the soil.
• Hold off on pruning trees and shrubs (except for dead branches, which can be pruned out at any time). Any desired pruning of summer-flowering trees and shrubs would be best done in the spring. Spring-blooming trees and shrubs can be pruned just after they finish flowering.

• Plant garlic late September (Western MD) through early November (Eastern Shore and Southern MD) for a late June/early July harvest next year. Plant the cloves root-end down; space them 4 to 6 inches apart, pointy-end up, and cover with 1 to 2 inches of soil. Mulch the garlic bed with fallen tree leaves.
• If tomato plants are rapidly declining due due to disease, you can pick fruits as soon as they start changing color and finish ripening them indoors.
• Harvest pumpkins and winter squash when the rind hardens (fruits can tolerate a light frost). Cut them from the vine leaving a 4- to 5-inch stalk for a handle. Handle them gently and store in a cool, dry location until ready to use.
• Time to plant cover crops to protect and improve the soil in your vegetable beds.
• Harvest figs when they soften slightly. Ants may enter the small pore or "eye" at the bottom of the fruit as the fig ripens. Figs that are nearly ripe (fully-colored but firm) can be picked and finish ripening indoors.
• Cut down the dead raspberry and blackberry canes that fruited this past summer (canes that overwintered). Fall-fruiting raspberries and blackberries can be mowed to the ground in late winter.

• Enjoy goldenrods and asters this month, which are important fall flowers for pollinators. 
• Purchase spring-flowering bulbs like Allium, Crocus, and Anemone. Keep bulbs cool (60 to 65°F) until planting time, usually in October.

• Do you have areas of your lawn that are growing poorly? Consider using other plants that are better adapted to those areas. Refer to The Challenge of Growing a Lawn in Maryland and Lawn Alternatives. For inspiration, case studies and photos show how Maryland residents are adding more plant diversity in place of turfgrass.
• If you have a lawn consisting of cool-season grasses (tall fescue, fine fescue, Kentucky bluegrass), apply fertilizer in the fall and fertilize according to the fertilizer schedule. Make sure to sweep any fertilizer that lands on sidewalks or driveways back onto grassy areas.
• If you are renovating a lawn or repairing bare spots, use recommended cultivars of grass seed.

Beneficial

• Ambush bug adults are active and feeding on insect pests in our gardens. Support these predators by incorporating fall-blooming plants, reducing pesticide use, leaving their black barrel-shaped eggs laid on stems and leaves, and leaving non-diseased plant debris as winter habitat.
• Wheel bug and other assassin bug adults are active and feeding on a wide range of insect pests. Many species will begin laying egg masses. Support these predators by leaving these insects be (as they may poke you with their straw-like mouthpart), recognizing and leaving their egg masses be, and planting a diversity of native plants.
• Scoliid wasp adults (also known as digger wasps) are active, pollinating flowers and parasitizing scarab beetle grubs found underground. Support these important pest controllers by planting fall blooming flowers (adult wasps feed on nectar), leaving flying adults (not aggressive) and their ground nests be, and reducing pesticide use.
• Praying mantis adults are active and feeding on a wide range of insect pests. Around late September they will begin laying egg cases (ootheca). All mantid species in our area are considered beneficial predators (do not target and kill non-native mantid species). Support these predators by leaving their egg cases be, outdoors. Do not bring egg cases indoors during the fall and winter months, as the warm temperatures inside your home could cause an early emergence of immature mantids (allow them to naturally emerge in the spring).
• Bess beetle immatures (grubs) and adults are active. They feed on and break down rotting wood, helping to return nutrients back to our soils. Support these natural recyclers by leaving fallen logs where/when you can in conservation areas.
• Common buckeye butterfly adults are active and pollinating flowers (typically preferring yellow flowers). Support these late-season pollinators by planting fall-blooming plants with yellow flowers to fuel their migration south for the winter.

Pests

• Various types of caterpillars may be found feeding on the leaves of trees at this time of year. Research has shown that this late-season damage is minor to overall tree health and management of caterpillars is usually not necessary.
• You may notice European hornets stripping the bark off shrubs (especially lilac) and trees. This stripping of the bark is usually minor and does no real harm to a shrub or tree. The European hornet is a large yellow and reddish-brown wasp that nests in cavities in trees, stumps, wood piles, sheds, etc., and feeds on insects.
• Spruce spider mites are active again on evergreen trees. Monitor for this pest by tapping branches while holding a piece of white paper underneath; look for moving specks. They can be controlled with ultra-fine horticultural oil (do not use on blue spruce, or the bluish needle wax will be removed). Follow label instructions.
• Spotted lanternfly adults may be found feeding on many host plants, especially tree-of-heaven (Ailanthus altissima). If you observe spotted lanternflies, inform the Maryland Department of Agriculture (MDA) by reporting the sighting on their website. These insects are mainly nuisance pests in residential gardens and landscapes. They have not been shown to significantly damage otherwise healthy ornamental plants. Consider sustainable management options if you want to take action.
• Fall webworm caterpillars are becoming more abundant and are creating tent-like webbing on the ends of woody plant branches. Caterpillar feeding damage is usually minimal and control is not needed; caterpillars are typically only active through October. Tents can be pruned out or left for natural predators like birds and parasitoids. Do not try to burn any webbed tents. Contact a professional if you would like to prune out a tent you cannot reach safely.
• Mexican bean beetle adults are active and feed on the underside of legume leaves (lima beans, snap beans, etc.). Adult beetles will spend the winter months in plant debris. Pull up and bag infested legume plants after harvest. Watch this short tutorial on how to harvest and bag bean plants.
• Striped and spotted cucumber beetle adults are active and chewing holes in leaves, flowers, and fruit. Common host plants include cucurbits, legumes, and tomatoes. Adults spend the winter months in plant debris and soil. Remove infested debris after harvest. In the fall or spring, lightly till the soil to disrupt soil-living life stages.
• Many different cabbage-feeding caterpillars and moths are active. Check host plant leaves (mainly crucifers/brassicas) for signs of feeding damage and for the presence of caterpillars and droppings (frass). Found caterpillars can be removed and destroyed. Remove infested plant debris after harvest to remove pest overwintering sites.
• Boxelder bug nymphs and adults are active and may congregate on boxelder trees, mulch, or in sunny areas of buildings. Adults will seek shelter for the winter, possibly finding their way indoors. This insect cannot harm humans, pets, or buildings and does not significantly damage plants. Control is generally not needed. If found indoors, vacuum or sweep up the insects.
• Many different social wasp species, like yellowjackets, will seek out sources of sugar this time of year. Keep outdoor food and beverages covered, rake up and discard fallen tree fruit, and equip trash cans with tight-fitting lids. Certain wasps may sting if they feel threatened. Avoid swatting or handling any wasps.
• Blister beetle adults are active and commonly found on goldenrod flowers or flying around lights at night. Learn to recognize and avoid these beetles, as they can release an oily substance called cantharidin when handled or squished and can cause temporary skin blisters.

Powdery mildew is common this season on many species of trees, shrubs, and perennials, like pin oak, roses, lilac, phlox, peonies, and monarda. This fungal disease favors high humidity, but unlike other fungi, does not require wet weather to thrive. It usually appears on foliage as a grayish, powdery coating on upper leaf surfaces. When possible, select resistant cultivars. No chemical control (fungicide) is necessary.

• Poison ivy leaves will begin to develop fall color this month, but they still contain the skin-irritating sap. Never burn poison ivy debris, and sensitive individuals should not handle or shred the leaves (fresh or fallen and dried).
• Avoid storing pesticides over the winter in sheds and garages. Cold temperatures can cause these materials to become ineffective. If you have questions about the efficacy of your pesticides call the manufacturer, using the phone number listed on the label. For additional pesticide storage information visit the National Pesticide Information Center website.

• Before moving houseplants back indoors, check plants for ants, earwigs, pillbugs, and other nuisance arthropods. Wash off insect pests or apply a low-toxicity, labeled houseplant insecticide like horticultural oil or insecticidal soap to reduce populations of aphids, scale, thrips, spider mites, and mealybugs.
• Avoid wetting the leaves of fuzzy-leaved plants like African violets. Wet foliage can cause leaf spotting.
• Be careful not to overwater houseplants while they are acclimating to indoor growing conditions, a process that can take several weeks. Due to reduced light, growth will also slow or stop for most plants while they are indoors for the autumn and winter.
• As houseplants that were outdoors for the summer adjust to less light indoors, leaf yellowing and leaf drop is possible. Using grow lights to supplement the light plants receive can minimize this coping mechanism.

• Crickets, multicolored Asian lady beetles, boxelder bugs, stink bugs, cluster flies, and other innocuous insects may attempt to enter your home this fall to overwinter. Caulk, weatherstrip, and seal all cracks and entry points around the home's foundation, vent openings or pipe/wire entry points, windows and window screening, and doorways to prevent these insects from coming indoors.
• House mice (and sometimes field mice) may be more noticeable around and inside homes with the onset of cool weather. Keep grass and weeds mowed around the house and seal all cracks.