The Ornamental Garden in August

The Ornamental Garden in August

  • By Susan Martin
  • /
  • August 2019-Vol.5 No. 8
  • /

Weather in the first half of August often remains July-hot and dry; the month begins with an average high of 87°  and drops to an average high of 84° by month’s end. The probability of precipitation drops from 39% on August 1 to 29% by August 31. Plants are starting to look fatigued and brown around the edges. The beautiful expanse in our front yard of lily of the valley (Convallaria majalis) and dwarf crested iris (Iris cristata) has started to yellow, furrowed by paths from our terrier running up and down and across. The May Night salvia (Salvia x sylvestris ‘May Night’) has slug damage the size of quarters; the hostas are either eaten by deer or swiss-holed by slugs and snails. And then there’s the lacy damage of Japanese beetles and the webs of spider mites that pop up overnight. Tent caterpillars have moved in en masse. How did all this happen seemingly overnight? Was I too remiss? Too hot? Too summer-lethargic? In rightful fear of chiggers and ticks, I make myself put on combat garb when it’s time to weed or trim. This means wearing long-sleeved shirts and long pants with boots when it’s in the 90s. And so I avoid going out to do battle. The enemies take advantage as I watch from inside. I make a list of supplies; I lay out a plan that is realistic in the face of soaring temperatures and drought.


  • Weeding still needs to be tackled. I like to think that my very persistent weeding throughout spring and early summer has reduced the weed seed bank, which is why things seem to be under fairly good control. I also had set out some additional mulch in June as an optimistic replacement for hand-pulling weeds.
  • Keep deadheading annuals such as marigolds to invigorate blooming. I toss the seedheads into the bed and seedlings usually appear by early fall for a fresh look. Towards the end of the summer, you could save some seedheads to dry out, but as cultivars, they won’t grow true to color or blossom size next year.
  • Deadhead perennials unless you want to leave seedheads for birds and aren’t worried about encouraging seedlings.
  • Cut back slug-holed leaves on hostas and salvias.
  • If catmint (Nepeta racemosa) hasn’t been cut back in July, it is probably looking rather “tatty” at this point. You can freshen it up by either shearing across the top, or if time permits, snip out brown stems down to the ground. Leave the greener stems. Do NOT cut catmint to the ground for winter. Leave the basal leaves for protection.
  • Cut back the yellow foliage and the spent flower stalks of daylilies.
  • Watch for signs of drought stress: wilting that does not correct by early evening; leaf scorch; shedding of leaves. Water in early morning for a total (with rain) of about 1” per week, preferably with a soaker hose.
  • Fertilize annual plants and plants in containers.
  • Do not fertilize trees, shrubs or perennials at this point.
  • Divide irises and daylilies up to the end of August.
  • Plant fall-blooming bulbs such as autumn crocus (Colchicum autumnale) for bloom in September and October.
  • Freshen up containers for cooler weather with annuals such as snapdragons (Antirrhinum majus), violas, pansies, and ornamental kale (Brassica oleracea acephala). See tips on how to transition summertime containers to more autumnal colors in the September 2018 issue of The Garden Shed.


Check for powdery mildew on roses, garden phlox (Phlox paniculata), bee balm (Monarda fistulata), and other susceptible plants. Cut out mildewed areas, bag, and dispose.

Powdery mildew symptoms on rose leaves.
Jody Fetzer, New York Botanical Garden,

Warm, dry days and cool nights are most conducive to infection. If mildew is a problem this year, compare your garden to recommended good cultural practices: space plants adequately to allow good air movement throughout the foliage; water plants early in the day so leaves dry quickly; do not over fertilize (especially with nitrogen); remove diseased plant debris at the end of the season to minimize survival of the fungus over the winter. If these practices seem to be in place, make a note to seek out cultivars with resistance to powdery mildew.

In addition to good cultural practices, a number of fungicides have proven effective for control of powdery mildew diseases. See the VCE publication Powdery Mildew on Ornamental Plants for information on fungicides.


There are many signs of insect damage at this time of year from spider mites, scale, rust and fall webworms. See the Ornamental Garden in August, 2016 for a summary of these pests including signs of damage and treatment.

Spider mites have been a problem in our garden this season. Red spider mites are also a very common problem on houseplants. Spider mites feed on the leaves through their piercing-sucking mouthparts. They remove contents from individual plant cells, leaving behind the cell wall, which makes the emptied cells appear silvery. The most noticeable symptom of infestation is white stippling on the leaves. Heavily infested plants take on a faded, yellowish or greyish cast. Severely infested plants are covered by a thin layer of webbing created by the large numbers of spider mites.

Spider Mites Photo: Mokkie, Wikimedia Commons

As with treating aphids, spider mites can be rinsed off plant leaves. Rinsing treatments must be done frequently enough to ensure the mites will not climb back up the plants. Mid-season washing of the leaves can help reduce the potential for spider mite population booms. Unfortunately, rinsing is not effective on high populations, so be on the lookout for early signs of infestation. If mites are established, an effective choice would be either an insecticidal soap or horticultural oil. These miticidal products are designed to coat the mite’s exoskeleton and cause suffocation. Although these products can also kill beneficial mites and insects upon contact, they do not have residual activity. NOTE: Most insecticides are not effective on mites and some, especially carbaryl (Sevin), result in increased mite damage by killing their natural enemies. See publications from Virginia Cooperative Extension and Clemson Cooperative Extension for more information.


If you want help with deterring insect pests in the garden, Piedmont Master Gardner Ralph Hall recommends that you welcome the assassin bug (Arillus cristatus), also known as the wheel bug. This beneficial insect feeds on aphids, caterpillars, Colorado potato beetles, Japanese beetles, leafhoppers, and Mexican bean beetles. It also feeds on its cousin, the brown marmorated stink bug!  But appreciate from afar—although the assassin bug is not aggressive to humans, a stray hand could be the victim of a very painful bite that can take two weeks to disappear.

Assassin bug identification

  • An adult wheel bug is easy to identify because of the very prominent semi-circular structure found on the dorsal side of its body.
  • A second distinguishing feature is its odor.
  • A third feature is its mouthpart, which forms a long, narrow beak that extends posteriorly beneath the body. This deadly proboscis is both sword and siphon. It is used to inject a paralyzing toxin into the prey’s body, and to suck out the prey’s bodily fluids. The process of paralyzing and killing the target takes about 30 seconds.

These photos, provided by BJ Taylor, show an assassin bug trying to catch prey while visiting black-eyed Susans (Rudbeckia). In the top photo, the assassin bug is covering a little bee that winds up escaping. In the lower photo, the assassin bug is eyeing the bee and a moth.




When I look out at our August landscape, I see an army of Japanese stiltgrass. It creeps beyond the border of our neighbor’s woods each night, seemingly determined to ram its way through our front door. Japanese stiltgrass is a formidable invasive that many of us are combating.

Japanese stiltgrass on the march. Photo: Susan Martin

Japanese stiltgrass Photo: Susan Martin

Flowering begins any time from July into October, and seeds ripen and drop to the ground from August to December. Mowing and weed-whacking can greatly reduce seed formation. Mowing is feasible only in open areas, not in forest settings. 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. Japanese stiltgrass is easily killed with low concentrations of herbicides. Researchers at Virginia Tech showed that a grass-selective herbicide is the most effective control method. When a grass-selective herbicide is used, more native plants return than when a non-selective type of herbicide is used. The recommended time for spraying is from July into early September before stiltgrass flowers and sets seed. See the June 2018 Garden Shed for more information. Also, check the Blue Ridge PRISM (Partnership for Regional Invasive Species Management) website for a factsheet on Japanese stiltgrass.


I seem to have focused this month on all work and no play. When I wander around our yard, I don’t just see Japanese stiltgrass, spider mite webs, powdery mildew, and weeds! I enjoy the deepening colors of summer’s end: the purple of aromatic aster (Symphyotrichum oblongifoliumaster), the dusty pink of stonecrop (Hylotelephium ‘Herbstfreude’ AUTUMN JOY), the vibrant orange of butterfly weed (Asclepias tuberosa), the golden yellow of sweet goldenrod (Solidago odora). I see and hear the constant hum of pollinators, the pileated woodpecker flashing through the trees, the whir of hummingbirds fortifying themselves for the migration. I know that the days will be getting shorter and I remind myself to treasure each one. It’s beautiful out there — in August, as in every month.


See past issues of The Ornamental Garden in August: 2018, 2017, 2016, 2015.

Average Weather in August in Charlottesville,

Assassin Bug, Blogs @ VT, Eric Day, Virginia Tech,

Brown Marmorated Stink Bug, Rutgers,

Brassica oleracea var. acephala,

“Powdery Mildew of Ornamental Plants,” Virginia Cooperative Extension,

“Powdery Mildew on Phlox,” Iowa State University Extension and Outreach,

“Integrated Pest Management for Spider Mites, Clemson Cooperative Extension,

Factsheets, Blue Ridge PRISM,

Photos source,




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