August Ornamental Plant Tips

(More tips from HGIC)

  • Late August through September is usually a good time to transplant, divide and plant perennials such as daylily, liriope (photo above), and echinacea. (HG 99) Be sure to keep them well watered during dry periods.
  • Annuals and perennials, like yarrow and salvia, may have grown spindly and are not flowering well. Cut them back to encourage re-bloom. Deadhead the spent blooms of annuals like zinnias and marigolds. This will encourage them to continue blooming more vigorously.
  • Plant hardy mums for fall color this month so they will become well established prior to the winter.
  • Poison ivy (HG 34) foliage can be effectively controlled with a labeled herbicide applied to the foliage later this month. You will have to make multiple applications to significantly weaken and kill the plants. If you cannot reach the foliage to spray, cut the vine down to the ground. Treat the cut surface still attached to the roots, with glyphosate or triclopyr. Do not handle the hairy poison ivy vines with bare hands even after they have died back.
  • This is a good time to apply glyphosate to control bamboo shoots and other hard to control weeds because this is when the plants are transferring nutrients to the roots. (HG 28) View our Bamboo Barrier video.
  • Sawfly larvae have been observed feeding on hibiscus. Their damage causes extensive skeletonization of leaves. The larvae are small, green and resemble caterpillars. Light infestations may be controlled by manually removing and destroying sawfly larvae.
  • Irises with leaves that are flopping over may be infested with iris borer, the larva of a clear wing moth. The eggs are laid on the foliage in the spring and the larvae move down to the crown and bore into the rhizome. A smelly bacterial rot usually follows infestation by the borer. This is a very destructive pest. Dig up infected plants and cut out the larvae and damaged tissue and discard infected plants – do not compost infected material. Re-plant the healthy rhizomes. (Read about dividing iris)iris borer damage
  • In hot, dry weather, spider mite populations can be high on many landscape plants. Damage occurs on a wide variety of flowering plants. Spider mites are pinpoint size sucking pests that can be observed on leaf undersides with the unaided eye. Keep plants hosed down during hot dry weather. Spraying insecticidal soap or horticultural oil on mite damaged plants during hot weather can cause serious leaf burn. If you notice spider mite damage on different types of plants in your yard you might consider purchasing and releasing beneficial mites, which will hunt down and consume the pest mites. They are a good value and environmentally safe.
  • Slugs continue to be active through the summer. The three types of slugs found in Maryland are the spotted garden slug (3-5 inches), the tawny garden slug (2-3 inches) and the gray garden slug (2-3 inches). They cause damage (large holes in leaves) to a wide variety of annuals and perennials. Favorite plants include hosta, salvia and marigold. Set out shallow saucers of beer or yeast mixed in water to attract and drown the slugs. Safe and effective baits containing iron phosphate are widely available. Sprinkling used coffee grounds around plants can also be very helpful in repelling slugs. Read label directions on all products for safe and effective control of slugs. Note: some baits can be harmful to pets and other small animals.
  • Southern blight is a significant disease of annuals and perennials encouraged by hot, humid weather. It is a soil-borne fungal disease that attacks a wide range of perennials including thyme, coneflower, coreopsis and black-eyed susan. Affected lower stems turn brown or black, foliage wilts and plants will eventually dry up and die. Tan colored fungal fruiting bodies, the size of a small seed may be observed on affected stems. You may also notice a white fungal mat growing over the infected stems. In some cases, plants can be cut back to the ground and plants will re-grow. However, the disease may kill crown and roots and plants will need to be removed. Keep mulch away from plant stems and water in the morning to minimize problems with southern blight. Working organic matter into the soil may help lessen the problem. No chemical controls are available. Read more...
  • Rhizoctonia web blight is a problem on sweet woodruff and other perennials, which produces similar symptoms to southern blight. Foliage becomes matted and brown. Rhizoctonia usually does not kill roots or crowns. To reduce the incidence of disease, remove mulches, work organic matter into the soil and space plants out to improve air circulation. 
  • Powdery mildew may be seen now on many perennials, like roses, phlox, peonies, and Monarda. This fungal disease favors high humidity and unlike other fungi does not require wet weather to thrive.. Powdery mildew may be observed on plant foliage as a grayish, powdery coating on upper leaf surfaces. When possible select resistant cultivars.
  • Groundcovers - If turf grasses have failed due to poor location (i.e. too shady for turf), consider planting appropriate groundcovers this fall. Soil should be loosened and organic matter incorporated prior to planting. Select plants based on the amount of sun they require. Good choices include striped or spotted wintergreen, trailing arbutus, moss phlox, Epimedium, sweet woodruff, partridge berry and ferns. Groundcovers are also useful as a border around buildings and garden beds. However, do not plant rapid growers near property lines or woodlands where they can become invasive.
  • Periwinkle/Vinca are prone to another fungal disease called Phomopsis blight that causes plants to wilt and turn brown. Prune out infected plants. No chemical controls are available.
  • You may have hostas that are yellowing and becoming scorched. In many cases this was caused by a combination of hot, dry conditions and diseases like Alternaria and Anthracnose (Colletotrichum). Remove badly affected leaves to slow down the progression of this leaf disease.

Water Gardens

  • Water lilies are growing rapidly and should be fed with fertilizer tablets regularly according to label instructions. Periodically remove the older, yellow leaves and spent flower heads of tropical lilies.
  • The water lily leafminer is a common pest that produces serpentine mines in leaves. Simply remove and discard affected leaves. The china mark moth larva is another pest of lilies. The larvae live in small packets of leaf tissue and float on the pond eating holes in the edge of lily pads. Pick off and dispose of the affected foliage.
  • Floating plants such as water lettuce and water hyacinth are tropical plants that can help keep your pond water clean. Their roots act as filters and the plants shade the water thus reducing algal growth.  
  • Tropical lilies are at peak bloom and will continue flowering through frost. Keep them fertilized to encourage strong blooming. Trim off and remove ragged and dead plant parts. Weak or dying plant parts should be removed to help keep the water clean. Water lily aphid feeding has been observed. Remove heavily infested leaves or direct a strong water spray at the aphids.
  • Now is a good time to clean out excessive plant growth in your pond and remove dead or diseased leaves. If you have a circulator like a filter or a fountain, keep it running during hot weather to increase the amount of oxygen available for your fish. The heat will evaporate water and ponds will need topping off. If using public water do not add large amounts of water at one time. The chlorine added to public water systems may kill fish. Chlorine is not an issue if you are using well water.
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