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From The Professor's Greenhouse: September (Part 2), Fall Fertilizing and Preparing Ornamentals

Season-relevant tips on planting, growing, and maintaining thriving gardens and landscapes from Professor Kevin Jones, Botanist & Biology Professor at Charleston Southern University.

There is Harmony in Autumn and a luster in its sky.
-Percey Bysshe Shelly

Fall Fertilizing

With the approach of the cooler months, stimulating the leafy growth of perennials

should be discouraged. September and maybe early October are the best time for the

last fertilizing of the year, but with a shift from one that will stimulate top growth – leaves

– to one that will stimulate a robust root system in perennials and lawns.

Fertilizers have three numbers, which represent the ratio of nitrogen (N), phosphorus

(P), and potassium (K). A high ‘N’ number will stimulate green, leafy growth that may

not harden off and thus be damaged by a frost in November. Higher ‘P’ and ‘K’ will

stimulate root growth and overall health and disease resistance of the plant.

Choosing a fertilizer to apply to plants and lawns higher in ‘P’ and ‘K’ will stimulate the

growth of a healthy root system without stimulating green growth that may be damaged

by the frosts ahead.


Bougainvillea in greenhouse
PC: Kevin Jones, Bougainvillea in his own greenhouse.

An ornamental often seen and treated as an annual is the tropical vine, bougainvillea.

The flowers come in many colors – red, orange, pink – and there are green and variegated leaf forms. In the tropics, they cover hillsides and arbors but will not survive the occasional severe frosts here. Fortunately, this plant is adapted to dry periods of a

monsoon climate. We can take advantage of this by allowing the plant to dry in its pot at the approach of colder months. As it dries, it will lose its leaves. Before the first frost, bring it into a cool garage or unused space, water it every 3 weeks or so, and let it rest there until after the last frost. In the spring, you will be rewarded with a spectacular show of growth followed by the bursting colors of the flowers. Bougainvillea blooms on new growth. It loves a little neglect and severe pruning. Plant

in full sun. It can get big but is easily pruned to keep in check.

Divide Perennials

September and early October are the best time to divide those perennials that are starting to be a little crowded in their clumps, such as daylilies, rudbeckias, and many more. They are starting to slow their growth and getting ready for dormancy.

To divide them:

With a sharp spade, dig them up and divide the clump into smaller clumps. I have found that a sharp knife works ok, but a folding saw is excellent for this and replant. Dig a new hole wider than the new plants and amend the soil with a slow-release

fertilizer. Place the new plant in the hole and backfill halfway. Make sure the orientation is to your liking, and then water thoroughly. Fill the hole the rest of the way with soil, tamp it down, and water again. Adding a mulch now is an excellent idea, but don’t let the mulch rest directly against the stems of the new plant.

Compost Pile

Leaves and garden debris will start to accumulate as fall progresses, and then there are the grass clippings. Consider establishing a compost bin or pile. Some advantages of a composting arrangement are less waste in the landfill and a ready source of some of the best soil amendment material you can find. There are models available that are easy on the eyes but can also be as simple as a pile in an unused corner of the yard. My yard is small, so I use a two-compartment bin arrangement. Taking five shipping pallets (free from many sources), I made a simple structure behind the tool shed. In one compartment, I add the garden debris, leaves, and kitchen scraps (veggies, egg shells only, no meat or dairy), I alternate 5ish-inch layers of this organic material with 1ish-inch layers of soil and water it as I would the garden. Every month or so, I turn it by moving this pile to the second bin. By spring, I have a pile of decomposed organic material that is garden gold.

Here are a few tips on the compost material:

  • Don’t add diseased or pest-infected plant material.

  • Don’t add meat scraps from the kitchen.

  • Don’t add pet waste or cat sand.

  • Grass clippings are good, and you may consider shredding plant material such as garden waste and leaves. (Running over it with a lawnmower is my preferred method of doing this.)

two compartment compost bin
PC: Kevin Jones, his two compartment compost bin

Some advantages of a composting arrangement are less waste in the landfill and a ready source of some of the best soil amendment material you can find.

Spring Bulbs

Now is the time to order or buy those spring bulbs that are planted in the fall. Daffodils, crocuses, snowdrops, and many more need to be planted in the fall so they can form astrong root system and overwinter in the cooler soil to give spectacular displays of spring and early summer flowers. Miss Karen will have a selection of these bulbs by September. Get your bulbs early for

the best selection. Buying locally helps ensure the bulbs are suitable for our area. BUT do not plant until late October and into the end of November. Until then, keep them in a mesh or paper bag (not plastic) and put them in the veggie drawer of the refrigerator. These plants need this cool spell and will reward you in the spring.

Ornamental September Chores (Veggie Chores are in Part I)

  • Some bulbs, such as caladiums, freesia, and dahlias (well, dahlias can sometimes with a thick mulch) cannot overwinter in the ground and may start fading if the month turns cool. They can be dug up to keep for spring planting.

A trick to keep them organized is to use egg cartons. The 18-egg size is ideal to keep them separated to prevent mold, and the cartons are easily labeled.
  • As warm-weather annuals fade, replace them with cool-weather ones; good choices are pansies, bachelor’s buttons, calendulas, snapdragons, and flowering kale.

  • Some cool weather annuals that made it through summer from the spring can be rejuvenated for a fall show. Cut back petunias, dianthus, and other annuals that were taking a break in the heat of summer back by a third for a new flush of fall color.

  • Sow seeds of cool weather grasses. Sod for warm-weather lawns can be set out now to avoid the intense heat of summer but to establish roots before winter.

  • Some perennials will be going dormant toward the end of the month. Label their locations to avoid planting too close to (or on top of) them next spring – or am I the only one that forgets where they are in the excitement of planting?

  • Keep up with the weeding. Every little weed pulled now is a big weed with a family that does not need to be dealt with in the spring!!


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