August in the Ornamental Garden

• Deadhead annuals and perennials such as zinnias, cosmos, garden phlox, or salvia to improve the appearance of plants and encourage them to continue blooming. For Echinacea, Gaillardia, and other seed-bearing plants, stop deadheading at this point so that birds can eat the seeds this winter. Also, keep in mind that some dried flower heads on plants such as tall sedum, globe thistle, astilbe, and coneflower can look attractive throughout fall and winter.
• Stake or cage perennials that flop over due to heavy seedheads, weak stems, late summer storms, or other reasons.
• Cut back dried flower stalks all the way to the crown of the plant and remove any browned or yellowed foliage or tattered leaves (think hostas) that have been damaged by deer, rabbits or slugs.
• Cut back and fertilize overgrown, leggy annuals in beds, containers, and hanging baskets to produce another flush of growth before the plants succumb to frost this fall. Do not fertilize perennials, trees, or shrubs at this time of year.  Late summer fertilizing produces tender new growth that will probably be damaged by cold weather.
• Monitor moisture levels and provide supplemental water to plants as needed. In general, 1” of water applied at the soil level per week is sufficient.
• Do not prune shrubs or trees this late in the growing season. Pruning now will stimulate new growth which may not have time to harden off before winter sets in.  So, unless you’re removing a damaged or diseased limb or stem, hold off on pruning until late winter or early spring when the plant is dormant.

Don’t let the weeds get out of control

While all our ornamental plants are gasping for water and respite from the heat, weeds perversely thrive under such stressful conditions.  Every weed that produces seed this year means more weeds and more work for you next year. Some particularly prolific weeds include the following:

• Common Ragweed (Ambrosia artemisiifolia).  This broad-leaf summer annual is the source of pollen that causes late-season allergies for many of us.  Ragweed produces an amazing quantity of seeds in late August through September.   One mature plant can bear between 30,000 and 62,000 seeds.  Should any of the seeds become buried in the soil, they can remain viable for decades. See Virginia Tech’s Weed ID website for photos and additional information on Common Ragweed.

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  Spiny Amaranth or Spiny Pigweed (Amaranthus spinosus). This summer annual is capable of producing about 235,000 seeds per mature plant according to the University of Tennessee Extension’s fact sheet on Spiny Amaranth.  Obviously prolific, this weed also has a taproot, which is difficult to pull when the soil is dry.  Wearing gloves when pulling this weed is highly recommended due to the quarter-inch long spines at the base of the leaves.

• Common Beggarticks (Bidens frondosa). This weed sports small, golden-yellow blossoms in late summer through early fall followed by seeds that have two tiny barbs designed to latch onto animal fur and clothing.  If you spot this weed in your garden, be sure to remove it before the seeds mature.  Otherwise, you may never get rid of it.  See the University of Missouri Weed ID website for several good photos of Common Beggarticks.

Stay alert for plant pests and diseases

Just as weeds appear to thrive in sweltering heat, insect pests and diseases don’t seem to be impeded by it either.  Stay on the alert for such problems as:

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  Rust – This fungal disease occurs when relative humidity is high and moisture stands on leaf surfaces for extended periods of time. Rust fungi produce masses of yellow, orange, brown, or rust-colored spores as part of their life cycle.  Like powdery mildew, rust is an unsightly disease, but it rarely kills a plant outright.  It will, however, stunt the plant and reduce its vigor.  Rust is particularly common on ornamental plants such as asters, daylilies, dianthus, irises, hollyhocks, and phlox. For mild infections, remove infected leaves to contain the disease.  The University of Massachusetts publication on Rust Diseases of Ornamental Crops is a good source for information on controlling this disease.

• Black spot – As its name suggests, this fungus appears as round black spots on the upper sides of rose foliage. The spots are often surrounded by yellow halos.  As the disease progresses, the leaves turn yellow and fall from the plant. If you leave the leaves where they fall on the soil or mulch, the fungal spores will overwinter and infect next year’s roses.  To contain black spot, remove all fallen rose foliage and dispose of it in the trash.  Do not put it in your compost pile. The Missouri Botanical Garden website provides a detailed explanation of black spot of rose and recommends several integrated pest management strategies for containing it.
• Fall Webworms – A widely distributed native pest of shade trees and shrubs, fall webworms appear in mid- to late summer through early fall. They skeletonize and consume leaves inside the protection of a tent-like silken web, which they spin over the foliage they are consuming. See Virginia Cooperative Extension Publication 2808-1013, Fall Webworm, for additional information on the life cycle of this pest and methods for its control.
• Spider Mites — These tiny pests can inflict serious damage to flowers, shrubs and both evergreen and deciduous trees during hot, dry weather. According to the Virginia Tech publication on spider mites, they use their needle-like mouthparts to pierce the leaves of host plants and suck out the fluids from individual plant cells. This results in a stippled or flecked appearance on leaves.  It may be too late to eradicate spider mites this year but note their symptoms so that you can use safe and effective controls in the future.
• Scale Insects – Many species of armored (hard) and soft-bodied scale species are difficult to detect unless you know what to look for. These immobile insects use their piercing-sucking mouthparts to extract fluid, causing loss of vigor, yellowing of foliage, and branch dieback to a range of trees, shrubs, and other ornamental plants. At the newly hatched or juvenile stage, scale insects are called crawlers.  Depending on the species, more than one generation may be born per growing season resulting in crawlers during May and June and then again in August and September.  Ladybugs, lacewings, and parasitic wasps are natural predators of scale insects.  At the crawler stage, scale insects are also vulnerable to insecticides.  VCE Publication 2808-1012, Scale Insects, provides additional information.

For more information on a variety of plant pest and disease treatments, see the Virginia Tech Home Grounds and Animals Pest Management Guide 2023.

Evaluate your perennials and annuals for heat and drought tolerance

Using plants that are naturally heat and drought tolerant help keep the late summer garden looking fresh and inviting.  The following characteristics generally indicate good drought tolerance in plants:

• Gray or silver-hued foliage — The silvery color helps cool the plant and reduce water loss through transpiration.
• Fuzzy or woolly-looking foliage – The leaves of many gray or silver-leaved plants may also be covered with tiny hairs, giving the plant a fuzzy, woolly, or hairy look. The hairs reflect solar radiation, which helps to cool the leaf surface. In addition to slowing evaporation, they also capture moisture on the leaf surface and help offset the effects of drying winds.

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  Small Leaves – Many drought-tolerant plants have fine or lace-like foliage. The smaller leaf surface area offsets the loss of water through the leaves.

• Thick, fleshy leaves – The cells within the thick, fleshy leaves of some plants, particularly succulents, evolved to store water as a survival strategy during periods of sparse or no rainfall. Many of these plants are also able to store water in their stems and roots.
• Waxy-coated leaves – All plants have a thin waxy coating called a cuticle on their leaves, but those plants that have evolved with a thicker waxy coating are better equipped to retain water by limiting transpiration.
• Taproots – Plants with long taproots rather than fibrous roots are able to penetrate well below the surface of the soil in search of water.

Another important strategy is to use plant species that are native to central Virginia.  These species have evolved using a variety of strategies for coping with the environmental conditions specific to this geographical area.  Check out this list of Native Drought-Resistant Plants for the Charlottesville area.

Watch for signs of drought stress in woody ornamental plants

Lack of water affects plants in various ways, depending on the species of plant and the level of stress to which they are subjected.  In addition to wilting, which is frequently the first sign of drought stress, look for the following signs:

• Premature fall color on trees – This indicates that the leaves have stopped producing chlorophyll and is a signal that the tree may be in trouble. Drought stress may be the problem but not necessarily.  Repeated defoliation by Japanese beetles or other insects, for example, can stress a tree.  Also, the problem could be caused by damage from weed eaters and lawn mowers or from lack of oxygen at the root level if the tree was planted too deeply.  Some detective work may be required to properly diagnose the problem.
• Leaf Scorch – This condition appears as a browning of leaf margins and tips. It is commonly observed on deciduous trees, such as maples, oaks, lindens, horse chestnuts, dogwoods and Japanese maples, and on broad-leaved evergreen plants such as magnolias, rhododendrons, hollies, and Japanese Andromeda (Pieris japonica). Leaf scorch may be the result of a combination of factors, including high temperatures, drying winds, and low soil moisture.
• Shedding of leaves – In an effort to conserve water, some trees sacrifice their older leaves in order to divert moisture to new growth and buds.
• Shoot dieback – Drought may make some woody plants (trees and shrubs) more susceptible to canker diseases. These are localized fungal infections that can cause the dieback of twigs and branches. Typically, a canker appears on a tree branch or twig as a sunken, slightly discolored lesion.   Prune the twig or branch several inches behind the lesion.  Do not cut into the lesion.  Otherwise, you may renew or spread the fungal activity.  Sterilize pruners after each cut using rubbing alcohol or a 10% bleach solution.

A drought this year may affect the health and vigor of next year’s plants.  Water-stressed shrubs that are forming flower buds for next year’s display may produce fewer buds and smaller leaves. Fruiting shrubs, such as winterberries, may drop their berries.

Also, pay attention to evergreen species.  Although water stressed, they often do not provide any clues to their condition and may stay green until it’s too late.

Be alert to invasive species

Japanese stiltgrass (Microstegium vimineum) is an annual invasive that germinates a couple of weeks before crabgrass and flowers beginning in July into October.  A single mature plant can produce about 1,000 seeds. The seeds ripen and drop to the ground between August and December and can remain viable in the soil for up to 7 years.   Mowing and weed whacking it in August greatly reduces seed formation. For more information on this invasive species and how to control it, see the Blue Ridge Partnership for Regional Invasive Species Management (PRISM) fact sheet on Japanese Stiltgrass and the Invasive Plant Control Calendar in the May 2022 issue of The Garden Shed.

Plan ahead

Fall is the best time to divide spring or early summer-blooming plants. Make a list now of perennials to move or divide this fall and decide where to plant the new divisions.  A little planning now will save you lots of time later when you get ready to perform this task.  Plan to finish dividing plants about 6 to 8 weeks before the first hard freeze so that the roots have plenty of time to recover before the onset of winter weather.

In addition to perennials, anticipate planting trees and shrubs later this month or in early fall so that they have time to settle in before winter.  Prepare the hole in advance so that the plant can be transferred from its container and into its permanent home as quickly as possible.  Water immediately and monitor closely while the plant is getting established.  Plan on watering until the soil freezes this winter.

Sow seeds in late summer for cool-weather annuals such as calendulas, Iceland poppies, primrose, pansies, violas, snapdragons, stock, or forget-me-nots.  Next spring, you’ll be glad you planned ahead and sowed the seeds now.

Order spring-flowering bulbs now while selections are good.  If you’re buying bulbs directly from garden centers or nurseries, choose the largest bulbs available.  Be wary of so-called “bargain” bulbs.  If the bulbs are small or of inferior quality, they may not be much of a bargain.