The Ornamental Garden in September

  • By Pat Chadwick
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  • September 2016-Vol.2 No.9
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If you’re tired of hot, humid weather, relief is in sight!  September 22 marks the autumnal equinox, the official start of autumn. The word “equinox” is derived from the Latin term meaning equal.  It denotes the time of year when days and nights are of equal length.  From this time forward, the days will grow shorter than the nights as the earth’s northern hemisphere tilts away from the sun and temperatures begin to cool. This is an ideal time of year to plant perennials, cool-season annuals, shrubs, and trees.  It’s also the right time to divide perennials and buy bulbs for next year’s garden.  With so much to do, September can be one of the busiest months of the year for a gardener.


Some gardeners view September as an “in between” month in the garden.  The summer annuals and perennials are pretty much done for the season but the autumn leaves are not quite ready to start changing color. The result is a landscape that may look tired and uninspiring. But there are ways to solve that problem:

  • A general sprucing up can make a big difference in your garden’s appearance. Clean up and remove all dead or diseased foliage.   Re-edge the borders, if they need it, to provide a nice sharp line of demarcation between lawn and garden.
    Author's Ornamental Garden in September

    Author’s Ornamental Garden in September

  • Cut back and fertilize leggy annuals for one more flush of blossoms.  Or, if the plants appear to be beyond hope, replace them with cool season bedding plants, such as mums or ornamental cabbage and kale.
  • For future reference, make a note to shear back mounding perennials, such as hardy geraniums, catmint, spiderwort, and salvia, after they finish blooming in the summer.   This encourages the plants to send out fresh new growth.  Some perennials, depending on the species, may even reward you with another round of blossoms before frost.
  • Visit some public gardens for inspiration.  Notice the plants that are in bloom at this time of year and try to visualize plantings that would make your garden more appealing. Don’t limit yourself to just flowers.  Consider plants that have interesting foliage or texture.  Some of the most fascinating gardens rely on foliage, contrasting shapes, hardscape features, and even light and shadow, rather than flowers, to carry the show.
  • Consider plants that have interesting fruits, berries or bark.  Rose hips, for example, make a fine substitute for flowers.  The clusters of colorful berries on beautyberry shrubs (Callicarpa Americana), winterberries (Ilex verticillata) and some viburnum species lend a great deal of excitement to the landscape. For a pleasing textural element, consider trees with exfoliating bark, such as paperbark maple (Acer griseum), river birch (Betula nigra), and some crape myrtles (Lagerstroemia).


This is the best time of year to plant trees and shrubs.  Without the stress of hot summer weather, plants can focus on developing good root systems before the onset of winter. Root development stops once soil temperatures drop below 40 degrees. Newly installed plants are happiest when the soil temperatures range between 55° and 75°F.   Water both evergreen and deciduous trees until freezing weather takes hold.  For suggestions of shrubs and shade trees  to plant, check out the Virginia Cooperative Extension’s publication 450-236, Problem-Free Shrubs for Virginia Landscapes, and publication 426-610, Selecting Landscape Plants:  Shade Trees.

Wait until spring before fertilizing newly planted trees or shrubs at this time of year.   If you fertilize at this time of year, the plant may try to push out new growth instead of preparing itself for dormancy.

Inspect all trees and shrubs for fall webworms, tent caterpillars, and bagworms.  Remove and destroy any that you find.   Try not to prune branches at this time of year.

Don’t become alarmed if the needles on white pines (Pinus strobus) start to show some yellowing around mid to late September.  It’s perfectly normal for the older, interior needles to shed.


For early blooms next spring, direct sow seeds this fall for cool-season annuals, such as Calendula, California poppy, Shirley poppies, larkspur, love-in-a-mist, and sweet alyssum.

Root cuttings from annuals to overwinter indoors.  It’s easier to overwinter rooted cuttings than to dig up large, often leggy, older plant specimens from the garden.  Geraniums, begonias, coleus, fuschias, annual salvias, pentas, lantana, heliotrope, and impatiens can be easily rooted and will thrive indoors if grown in good lighting.  Take the cuttings and bring them indoors for potting before the weather starts to cool down.

Save seeds from your favorite non-hybridized annual and perennial species for planting next year. Keep in mind that seeds from hybridized plants are often sterile and, even if they are not, the plants that grow from those seeds are unlikely to resemble the parent plant.

  • Gather seeds when they are fully ripe, but leave some for the birds to eat over winter.
  • If seeds aren’t already fully dry, spread them out on newspapers or leave them in an open paper bag to dry.
  • Place the dried seeds in envelopes or glass jars labeled with the seed’s name and the date.
  • Store the packaged seeds in a cool place. Some gardeners like to store their seeds in the refrigerator.
  • TIP:  Seeds from your garden make nice hostess gifts to give over the holidays.

Make a list of perennials that need to be divided. Decide which ones to divide this fall and which ones to divide in spring.  While spring is an excellent time to divide perennials, September and early October are ideal months, due to the combination of warm soil, cooler temperatures, and a greater chance of rain.   Peonies and hostas are two perennials that respond well to being divided at this time of year:

  • Peonies – As peony clumps become overgrown, they produce fewer blossoms.  This is a signal that the clump should be divided.  September or October is the best time to perform this task after the plant begins to go dormant. Carefully remove the soil from around the roots and try not to damage the “eyes,” which produce the flowering stems for next year’s blossoms.  Using hand pruners, carefully divide the clump into divisions that have three or more eyes each and a good root system.  Dip the peony roots in a 10% bleach solution (1 part bleach to 9 parts water) to kill any fungal spores.  After the roots dry, plant them in a prepared sunny location.  Peonies will not bloom if they are planted too deeply, so it’s important to plant them with the eyes positioned no deeper than 1 or 2 inches below the soil surface.  Be aware that peonies often don’t bloom the first spring after they have been divided.  So just be patient.
  • Hostas – It’s time to divide hostas when they become crowded or the center of a clump starts to die out.  While they may be divided anytime between spring and fall, late summer (August or early September) is generally the ideal time to divide them so that they have time to become established before the onset of cold weather.   Plant the divisions the same depth at which they were growing originally and water well.


As you select spring bulbs for planting this fall, keep in mind that a mass planting of one color is generally more visually pleasing than a mixture of colors.  Also, the display is likely to be more effective if planted against a contrasting background.  For example, a mass planting of pale lavender-blue Spanish hyacinths emerging through a bed of dark green vinca can be unbelievably stunning!   When it comes to gardening, sometimes less is more.


Weeds never take a break, which means that you must continue to remove them from your landscape until a hard freeze.   Here are a couple of weedy thugs to pay attention to:

  • Ragweed


    Ragweed (Ambrosia artemisiifolia) – This broad-leafed weed has a notorious reputation for causing seasonal allergic rhinitis (hay fever) in millions of Americans every year.  It typically blooms beginning in August and produces a fine pollen that peaks around mid-September.  A broadleaf weed killer may kill it but if you prefer to garden organically, pull it up by hand or use a hoe to remove it from cultivated beds.

  • Ailanthus – Although technically a tree and not a weed, this fast-growing plant competes with native species in the landscape.  The National Park Service reports that Ailanthus is invasive in natural areas in 30 states in the continental United States and Hawaii.
    Ailanthus altissima

    Ailanthus altissima

    The large clusters of yellowish-green flowers are quite showy during summer.  However, a single tree can produce about 325,000 seeds per year.  Because it seeds so prolifically, it is able to establish dense stands that crowd out native plants. Do not confuse this tree with staghorn sumac (Rhus typhina), which is a native species.  For more information on how to control Ailanthus, see VCE Pub. 420-322, Invasive Exotic Plant Species:  Ailanthus.


Autumn may be upon us, but garden pests are still out in full force.  Here’s a couple to look out for:

  • Spider Mites – Small and barely visible to the naked eye, azalea mites (Eotetranychus clitus) and southern red mites (Oliogonychus ilicis) are commonly found on azaleas, rhododendrons, and hollies.
    Spider Mite Damage

    Spider Mite Damage

    In general, they may be found on the underside of leaves where they suck plant sap causing the leaves to change color from dark green to stippled yellow or gray-green.  The leaves may also be covered with fine webbing.  Mites are not actually insects but are more closely related to spiders.  They are “cool season” pests, meaning that they are more active in spring and fall and mostly inactive during summer and winter. A stream of water directed at the leaves helps to dislodge the mites.

  • Iris Borers – Irises are susceptible to a variety of pests and diseases.  One of the most annoying is the iris borer, which is the larvae of the brownish-looking nocturnal Miller Moth (Macronoctua onusta).  The moth lays its eggs on old iris leaves and flower stalks in autumn.  The larvae hatch in late spring and tunnel into the leaves on their way down to the rhizome where they feed in summer and early fall.  The damage they cause makes the rhizome susceptible to bacterial soft rot.  To break the life cycle of this pest, remove dead leaves from rhizomes in autumn to prevent any eggs from surviving over the winter months.
  • Holly Leaf MinersPhytomyza ilicicola is a small black fly that is a common pest of American hollies.  It causes damage at the larval (maggot) stage of its development.   In early spring, the adult female fly punctures small holes in holly leaves and deposits eggs.   When the eggs hatch, the young maggots feed inside the leaves, causing light green serpentine tunnels.  Most of the damage is done in autumn, so September is a good time to examine hollies for signs of leaf miner damage.  If not controlled, the larvae will overwinter in the leaves and emerge from mid-May to late June to begin a new life cycle.  To control a light infestation, pick off damaged leaves and destroy them before the insect can complete its life cycle.  A heavily infested plant may drop many or most of its leaves and may require a systemic insecticide application.


If you moved your houseplants outdoors this spring, it’s time to start preparing them for the transition back indoors before night temperatures fall below 55°F.  Move them to a porch or other shady spot for about two weeks so that they become acclimated to less light.  Before moving them indoors, inspect plants for insects (don’t forget to check under pot rims for spiders).  Wipe down all pots and saucers to remove dirt and debris.







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