The Ornamental Garden in August

The Ornamental Garden in August

  • By Susan Martin
  • /
  • August 2018 - Vol. 4 No. 8
  • /

Don’t let this beautiful month of summer slip too soon into fall, even though the stores are stocking plastic pumpkins and the candy aisles are screaming “Boo!” It’s still summer, and our Zone 7  gardens are still producing new and beautiful blooms. Late-summer-blooming perennials, and perennials that continue to bloom through late summer, offer both the brushstrokes of vivid July and the early tones of more autumnal hues:

  • purple and pink native aster
  • orange and burgundy chrysanthemum
  • purple passionflower (Passiflora incarnata)
  • bluish/lavender Russian sage (Perovskia atriplicifolia);
  • blue mist shrub or bluebeard (Caryopteris x clandonensis);
  • lavender-blue giant hyssop (Agastache ‘Blue Fortune’)
  • lavender-to-purple anise hyssop (Agastache foeniculum)
  • orange, pink, yellow, purple and magenta Agastache hybrids; sunset hyssops (Agastache rupestris) in bronze, salmon, orange and yellow, with hints of purple and deep pink
  • Joe Pye weed (dusty rose Eutrochium fistulosum and mauve purple E. purpureum)
  • bright yellow sneezeweed (Helenium autumnale and Helenium virginicum)
  • yellow tickseed (Coreopsis tripteris)
  • golden, russet, or lemon yellow sunflowers (Helianthus annuus)
  • red, pink, and white cosmos (Cosmos bipinnatus)
  • red, white, purple, orange, and pink autumn sage (Salvia greggii)
  • flame red canna lilies, also orange, pink, yellow, cream, and bicolors
  • big-color, big-headed dahlias that last until first frost



To keep gardens looking their best, there are always the general tasks of weeding and watering; watering is especially important to keep plants healthy and strong before they go dormant. There are also the month-sensitive tasks that require special attention.


Continue to fertilize container plantings and annuals. Always water the bed after applying fertilizer, or, if feeling lucky, apply before a predicted rain. This will wash the fertilizer off the foliage and prevent burn. It will also make fertilizer available to the plants immediately.

DO NOT fertilizes trees and shrubs. A late summer fertilization could promote a spurt of tender growth that would make the plant vulnerable to the rigors of winter. Perennials should not be fertilized in late summer for the same reason.


Deadheading is not a month-specific chore, but there seems to be a lot of deadheading at summer’s end as we try to encourage that last colorful hurrah.

As flowers shed their petals and begin to form seed heads, energy is focused into the development of the seeds, rather than the flowers. As soon as they are allowed to set seed, chemical messages are sent back telling flower production to stop. Removing old flowers before they produce seed will keep plants blooming longer. The best time for deadheading plants is just before the blooms die back completely, i.e., as soon as the flowers begin to fade, wither, or turn brown. Pinch or snip the wilted bloom, along with the stem, down to the next leaf, stem or bloom.

Examples of August-blooming plants that benefit from deadheading include: Agastache (giant hyssop, anise hyssop, and agastache cultivars), asters, dahlias, marigolds, salvia greggii, Shasta daisies (Leucanthemum x superbum) and zinnias (Zinnia elegans). If you are pinching off spent canna blooms, be careful. New buds usually form right next to the spent flowers. Some canna lilies produce big black seed pods, while others are sterile. Watch to see if seed pods develop. If they don’t, deadheading the flowers isn’t necessary, except for aesthetics.


Salvia greggii ‘Red Swing’

Salvia hybrida® Arctic Blaze Purple

August is a busy month in the garden for birds happily searching for natural seed sources. As the flowering season approaches its end, leave the remaining seed heads in place. The most popular seed-bearing flowers for backyard birds include many of the late-summer blooming flowers listed above: asters, black-eyed Susan (Rudbeckia hirta), coneflowers (Echinacea purpurea), coreopsis, cosmos, daisies, giant hyssop, marigolds, sneezeweed, sunflowers, and zinnias. Hummingbirds are especially attracted to the bright red of autumn sage (Salvia greggii ‘Red Swing’, ‘Hot Lips’, and ‘Flame’). Salvia’s nectar-rich flowers attract a wide range of pollinators including honey and native bees, butterflies, and hummingbirds.


Cutting plants down to the ground for winter should be done later in the season in September/October, but maintenance trimming can be done now.

Some plants are particularly susceptible to powdery mildew. Trim back diseased parts now and bag; do not add to compost. Plants that are especially prone to powdery mildew include: phlox (Phlox paniculata), peony (Paeonia), beebalm (Monarda didyma), sneezeweed (Helenium autumnale and Helenium virginicum), and Zinnia elegans.

Certain plants do not like to be cut to the ground before winter because the foliage protects their crowns. If the leaves are looking spent or limp, or are riddled with insect or slug holes, trim the leaves but don’t cut to the crown. Plants that like some winter foliage protection include: butterfly weed (Asclepias tuberosa), chrysanthemums, coral bells (Heuchera sanguinea), Siberian bugloss (Brunnera macrophylla), Salvia x sylvestris, lungwort (Pulmonaria), bearded penstemon (Penstemon barbatus) and catmint (Nepeta). Clean up in spring.

Consider cutting the stems of Shasta daisies back to the basal leaves after flowering to preserve plant energies and perhaps extend the life of this somewhat short-lived plant.

Evergreen perennials such as certain ornamental sedges are not cut back in fall; remove dead foliage in spring and summer.


Irises and daylilies are best divided in summer, after they flower up to the end of August; keep moist throughout the fall.

Other spring- and summer-blooming perennials can be divided in September/October; allow at least 4-6 weeks before the first average frost. The first average frost date in Charlottesville is November 1-November 10.


When admiring your August garden, you should also make notes on what to improve for next year. Which plants did especially well, which did not? Are there any spaces that need to be filled? Remember to add some kind of marker to highlight those spaces next spring. Look at the overall color palette. Should you add more plants of the same color for a more concentrated presence, or should you add different colors for contrast? Does the garden design include plants of varied heights and textures? Review any notes you took from visiting other gardens and see where you could incorporate new ideas. Take photographs to illustrate your notes. What seems obvious now can be a hazy memory next spring!


It will soon be time to plant spring-blooming bulbs. Consider ordering now to assure the best selection. See the October 2015 issue of The Garden Shed, Spring-Flowering Bulbs, for ideas.

Lycoris squamigera, Naked Lady or Resurrection Lily

Fall-blooming bulbs from the Colchicum group, autumn crocus, should be planted in August for bloom in September to October. Autumn crocus is so named because most varieties bloom in fall (although some bloom in late summer). Plants send up somewhat unattractive foliage (4-6 dark green leaves) in spring that gradually yellows and dies by summer when the plants go dormant. Naked flower stems rise from the ground to 6-10″ tall bearing pink-to-lavender-pink star-shaped flowers. Spring crocus is in the Iridaceae (iris) family, but autumn crocus is in its own family, Colchicaceae.

Lycoris squamigera, resurrection lily or naked lady, is a fall-blooming bulb from the amaryllis family (Amaryllidaceae). The leaves sprout  in the spring, then die back during June; thick, naked flower scapes rise to 2’ tall in late July to August. Each stem bears 4-7 funnel-shaped fragrant flowers that are rose-pink tinged with lilac. Plant bulbs in the fall.


Broadleaf annual weeds: Seeds germinate from late summer through fall. Weeds over-winter and continue to grow in early spring. Control with a broadleaf postemergent herbicide applied when the weeds are actively growing in the spring. The exception is common chickweed (Stellaria media) which can also be controlled with a broadleaf preemergent herbicide applied in early to mid-September before it germinates.

Perennial weeds: The largest group of weeds, these are persistent from year to year. They reproduce by seed and also by vegetative means. They range from weeds that are easy to eliminate, to some of the most difficult to control. Treat with a broadleaf postemergent herbicide applied when the weed is actively growing.  For difficult-to-control weeds such as creeping Charlie (Glechoma hederacea), both a spring and fall application of a postemergent herbicide is often necessary.

Grassy winter annual weeds: Seeds germinate in late summer to early September. Control with a preemergent herbicide applied in early September before the seeds germinate. Annual blue grass can be treated this way. If herbicides are applied in the fall, you will not be able to sow grass seed in the fall.

Herbicides should be considered an aid, but not a cure for broadleaf weed problems in lawns. The presence of weeds indicate conditions that are not conducive to a healthy lawn. Such conditions include: close mowing, improper watering, poor drainage, compacted soil, too much or too little lime or fertilizer, insect and disease damage, and too much shade.  Spot-treating with a liquid herbicide appropriate to the type of weed is also effective. Do not apply insecticides on a breezy day, or during the day when insects are active. Hand pulling is an option when weeds are not too numerous or when gardeners wish to avoid using herbicides. Try to get all the roots and runners, bag, and discard. Covering larger weedy areas with heavy cardboard to block sunlight is another non-chemical option.

Invasive Japanese Stiltgrass (Microstegium vimineum): In August, mowing and weed-whacking can greatly reduce stiltgrass seed formation. Flowering begins any time from July into October, and seeds ripen and drop to the ground from August to December. Mowing is best done just before flowering in August and September, and needs to be done only once if you wait until then. Cut stiltgrass as low as possible, scalping the ground, to remove all flowers.

(Treat Japanese stiltgrass in early spring with a preemergent, without nitrogen fertilizer. Look for the active ingredient: Prodiamine (Barricade) or other preemergents labeled for crabgrass control. Stiltgrass seed germinates a couple of weeks before crabgrass seed germinates.)

Note: Follow the highlighted links for preemergent and postemergent herbicides to the University of Maryland Extension site for weed identification with photos as well as recommendations for weed-specific herbicides. Additional weed treatment sites are included in the Source list.


The invasive plant for this month is autumn olive (Elaeagnus umbellata). This nitrogen-fixing, deciduous shrub has multiple trunks and dense branching and can grow to 10-16′ tall and 20-30′ wide. In spring, it leafs out before most native vegetation, and its new leaves are bright silver. Leaves mature to olive-green with silvery undersides. The leaves are alternate on the stems, and vary from narrow-to-wide ovals. The stems are speckled, silver or golden brown, and often thorny. In spring, the shrub produces a profusion of small, fragrant, four-petaled, creamy-to-pale-yellow tubular-shaped flowers. Oval-shaped, single-seeded fruits ripen to a dull-to-bright red dotted with tiny silvery speckles in August and September, aiding identification.

Autumn Olive (Elaeagnus umbellata), Wikimedia

Autumn olive with fall berries: LazaregagnidzeOwn work

Autumn olive in spring flower:  KENPEI, Wikimedia

The primary eradication method is to prevent seed production and dispersal by killing or cutting back autumn olive shrubs by mid-July. However, autumn olive can be controlled at any time of year, except during spring growth, by cut-stumping. Cut or saw all stems to several inches from the ground and immediately spray cuts with a concentrated recommended herbicide or a ready-to-use stump killer. Watch for resprouts; cut and treat all new stems or apply a foliar herbicide spray to the new foliage. Cutting down autumn olive without applying herbicide only increases the number of stems that sprout from its crown and roots. Seedlings and young autumn olive shrubs can be hand-pulled or dug if the population is not extensive. Digging larger plants is problematic because they resprout from any roots left behind. See the  PRISM website (Partnership for Regional Invasive Species Management) for more information.

GENERAL INFORMATION TIP: There is an online site that searches by topic from Cooperative Extension Sites around the country. Just make sure that the information is appropriate to Zone 7 if climate affects your question.


There is still so much to enjoy in the final “school vacation” month of summer. The August palette is changing naturally over to autumn hues. In some respects, August is a month for “procrastination without guilt” for the ornamental gardener! It’s too early to divide most perennials; too early to mulch; too early to plant spring bulbs; too early to sow fall seeds; and too early to rake leaves! What we can do is water, weed, deadhead spent flowers, trim spent or diseased foliage, fertilize annuals and container plantings, divide irises and daylilies, and plant autumn crocus. We can also mow or weed-whack invasive Japanese stiltgrass low to the ground to remove the flowers, and cut down invasive autumn olive and treat with an herbicide. But, once the chores are done, make sure to spend some time in the hammock or on the patio, enjoying the beautiful late-summer blooms and the birds attracted to their seed heads.


“Avoid the Coneflower Blues,”

“Sunset Hyssop Information: How to Grow Sunset Hyssop Plants,”

“Fertilizing established perennial gardens – Feed’em and weep,”

“What Plants Should You Deadhead?”

“Off With Their Heads: Deadheading Perennials,”

“Lawn Weed Identification,”

“Broadleaf Weed Control in Established Lawns,”

“Weeds as Indicators,”

Partnership for Regional Invasive Species Management (PRISM),

“Perennials: Dividing,”

Plant Finder,

Topic Index for Cooperative Extension Sites,

Virginia Average First Frost Date Map,

“Spring-Flowering Bulbs,”

“Plant spring-flowering bulbs over the holidays!”




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