The Ornamental Garden in September

The Ornamental Garden in September

  • By Susan Martin
  • /
  • September 2018 - Vol. 4 No.9
  • /

Although the ornamental garden is getting a little drowsy at this point, we can still enjoy end-of-season blooms in the rich hues of early fall. September is also a time to complete some necessary chores that will help make the most of our mature gardens, as well as ensuring a healthy re-emergence next spring.


In addition to the all-important tasks of watering and weeding, it’s important to keep garden beds clean of plant foliage that might carry fungus or other diseases from this season to the next. Bag up diseased plant material and discard; don’t add it to the compost pile. As perennials die back in the fall, be sure to cut off the dead foliage and discard since the foliage could encourage insects and disease. Don’t cut off the brown fronds of your ferns in fall since the old fronds help protect the center through the winter. Once you see new green fronds in spring, you can carefully remove the brown fronds.  Or, just let them fade off naturally. Re-edge your garden beds to make them look sharp and tidy.


Prevent deer damage by putting up plastic fencing around vulnerable shrubs and trees to protect them from both deer browsing and antler rubbing. You can also strategically place 4-5 tall metal fencing stakes around small trees to protect from deer.


Early fall is a good time to divide many kinds of perennials, but transplants need at least six weeks to get established before winter. Therefore, divide plants in our area six weeks before October 31, the average first hard frost (32 degrees). There’s a 50% chance of first hard frost by October 31; a 10% chance by October 13; and a 90% chance by November 18. (As many have noted, however, global warming has made these expected dates less reliable.) This would mean a target planting date of around the second-to-third week of September. Another guideline is soil temperature; root development stops once soil temperature drops below 40 degrees. As a general rule, spring-blooming plants are divided in fall, and summer-blooming plants are divided in spring. Many plants, however, can be divided in either spring or fall. Some perennials, including many with long tap roots such as Baptisia australis, do not respond to division. Check guidelines for specific plants before dividing. For detailed guidelines on division, see the Clemson Cooperative Extension publication on “Dividing Perennials.”

In addition to being the prime season for planting trees and shrubs, fall is also a good time to plant perennials that you might find on sale at the end of season. Just be aware of the same first-frost date and 6-week guideline as described above.


Some of our annuals in the garden and in containers are beginning to look a little spent. But when the hummingbirds and the bees are still daily visitors, it’s hard to callously discard popular sources of nectar such as annual salvia. Another alternative is to save the healthiest plants with the most complementary fall colors and add fresh plants. In the summer, I like to work with blue, lavender, pink and white. In the fall, I remove from the containers pink flowers as well as any plants that are spent, and add fresh plants in colors of gold, orange, burgundy and rust. It takes just a few weeks for the newcomers to fit in with their fuller, taller, established companions. Sometimes it doesn’t take much to create a new autumnal palette!

In Container 1, pictured below, I removed pink Angelonia augustifolia (summer snapdragon). At some point, I’ll probably also need to take out the annual blue salvia, but since it’s still a pollinator magnet, I’ll keep it a while longer. I also kept trailing white verbena and then added Bandana® Lantana camara, Lady Godiva™ yellow Calendula hybrid (English marigold), and Celosia argentia ‘Dragon’s Breath’.

Container 1 Fall

Container 1 Summer

Container 2 was a mass of purple Torenia fournieri (wishbone flower) and angelonia in purple and pale pink. I removed the pale pink angelonia and added Lady Godiva™ Calendula. Both planters will need some additional tinkering as September wears on, but for now, they look fresher and more seasonal.

Container 2 Fall

Container 2 Summer


Resist the urge to grab your pruning shears when you want to “tidy up a bit.” You can remove dead branches on your bushes but then put away the clippers. Bushes that bloom on new wood can be pruned in late winter. Bushes that bloom on old wood can be pruned right after they bloom in spring or summer. If you prune in fall, you might mistakenly prune off buds that have already set for spring bloom. Or, you could encourage a late season blush of growth just when a plant should be quieting down for winter.


This is the time of year when we notice large webs in many of our trees. Can we ignore them or should we opt for some kind of insecticide treatment?

The fall webworm, Hyphantria cunea, is a general feeder on nearly all trees except conifers. The insect makes webs at branch tips and is harmful mainly to the beauty of the host. Hosts are seldom seriously harmed because defoliation usually occurs later in summer rather than during a period of active growth and not enough terminal growth is consumed to affect tree growth. In addition, more than 75 natural enemies parasitize and prey on the fall webworm. Branches that have active webs (“nests”) may be cut out and destroyed. Webs are always on branch ends and are easier to remove when they are small. Pole pruners are helpful for reaching into trees. It is considered to be more of a nuisance than a threat to the health of the tree.

Another tent-forming caterpillar is the eastern tent caterpillar, Malacosoma americanum. While fall webworm caterpillars nest at branch tips and feed inside the webbing, eastern tent caterpillars make webbed silk nests in a fork of a branch or tree trunk and leave the nest to feed. Young caterpillars feed during the day and remain in the tent at night; older (and larger) caterpillars feed at night and remain in the tent during the day. As with the fall webworm, it is not necessary to spray insecticides to control the eastern tent caterpillar. Healthy defoliated trees will grow new leaves. Infested trees can be unsightly and are less vigorous than attacked trees, but they are seldom killed. Typical natural controls include birds, predaceous and parasitic insects (especially wasps), and disease organisms. Small tents can be pruned off or wrapped around the end of a broomstick or pole that has a small brush or nails mounted on the end. Caterpillars can also be handpicked and dropped into soapy water.

For more information on identification, life cycle, and treatment options, see the VCE publications, Eastern Tent Caterpillar and Fall Webworm.  Refer also to the Missouri Botanical Garden online site on Fall Webworm and Eastern Tent Caterpillar.


Spring-blooming bulbs can be planted throughout the fall until the ground is frozen. They do best if planted about a month before the ground freezes. A good rule of thumb is to plant bulbs when the average nighttime temperatures are in the 40- to 50-degree range. If bulbs are bought before planting time, store them in a cool, dry, dark place at a temperature of 60˚ to 65˚F. See the September 2015 Tasks and Tips article in The Garden Shed for recommendations of deer-resistant spring-blooming bulbs in addition to daffodils.


Many plants provide seeds that are easy to collect and easy to sow. Zinnias and marigolds are two of the easiest. Just be aware that the seeds of cultivars will not grow true to the parent plant. But that doesn’t have to be a bad thing. Our cultivar marigolds have self-seeded and have also been collected and sowed. This year, all the plants had orange blooms, except for one plant which had yellow flowers. It was a fun surprise! The blooms, however, are now noticeably smaller than those of the original plants. Cleome hassleriana, or spider flower, is a very tall, large-blossomed annual with obvious seed pods that are easy to collect and easy to grow. Rather than collecting and storing seed in the fall and planting in the spring, you can sow fresh seed from your favorite annuals in the fall just by shaking out the dried seed heads. Make a note of where the seeds have been shaken so that you don’t mistakenly weed out the seedlings next spring. For more information on collecting and saving seed, see The Garden Shed article, “Growing Plants From Seed You Collect.”

In the meantime, enjoy fall “in the moment”— the only way to fully appreciate its beauty!


For more September garden tips, see past issues of The Garden Shed, 2016 and 2017, as well as the Virginia Cooperative Extension September Tips.


Frost Dates,

“Dividing Perennials,”

“Perennials: Culture, Maintenance and Propagation,”

“The Ornamental Garden in September, 2015,”

“The Ornamental Garden in September, 2016,”

“The Ornamental Garden in September, 2017,”

“Growing Plants From Seed You Collect,”

Horticulture and Natural Resources, September Tips,

Eastern Tent Caterpillar,

Fall Webworm,

Fall Webworm,

Eastern Tent Caterpillar,

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