The End of Summer Perennial Garden

August 21, 2023 | C.L. Fornari | Perennials

Adding color, dividing/transplanting, and soil health.

Spending a little time in the perennial garden now can give you months of pleasure ahead. Even if you don’t want to do a fall cleanup, there are some plants that can be cut back now for renewed growth into the fall. Here are some guidelines for creating a perennial garden that will have you smiling through November.

Perennials You Can Cut Down Now

There are some plants that don’t bring much to the party in late August, but that will be refreshed with new growth into the fall if you cut them down now. Daylilies (Hemerocallis), cat mint (Nepeta), and Rozanne geraniums are four such plants. These are not plants that have the thick, pithy stems that some insects overwinter in, so you don’t have to worry that this cleanup of these plants is hurting pollinators. These perennials will grow back fairly quickly with new foliage for the fall, and the Rozanne geranium will even produce more flowers.

Leave any perennial that has thick, pithy stems in place until next spring. Hardy Hibiscus are examples of such plants. Perennials that have foliage that is still photosynthesizing, or woody plants such as lavender, should also be left through the winter. Other perennials such as Hosta will completely die to the ground in the first hard frost, so you can let Mother Nature take care of those plants later in the season.

Geranium Rozanne is one of the longest flowering perennials, but at the end of the summer it can look a bit “stemmy.” If you cut it to the ground, it rebounds with fresh growth and fall flowers. If you don’t have Rozanne in your gardens yet, you’ll find it in the perennial section right now.

Add Late-Season Bloomers

Sometimes we will look at our perennial gardens in August and realize that most of our plants flower earlier in the summer. If you don’t have fall flowering plants in your gardens, this is the ideal time to plant them. Fall asters and Montauk daisies are two favorites on Cape Cod. The asters typically flower in September and October, and the Montauks (aka Nippon daisy) bloom in October.

The Woods Blue and Woods Pink asters are low-growing, and they attract bees and butterflies.

Keep Montauk daisies full and round by trimming them in early May. (Later trimming is not advised.) Grow these plants in full sun, and water deeply but not frequently. When given too much water, shade, or excess fertilizer these plants get top-heavy and fall over. Note that as you see on these plants, it’s common for the inner leaves on this plant to turn yellow and drop.

A few small pots of perennials sneak into the front annuals section at this time of year. These can be an economical way to add fall color in your garden. Here are two varieties of sedum (drought tolerant and the bees love them) and there are pots of Euphorbia Ascot Rainbow as well.

Add Annuals For Flower Power

From marigolds and zinnias to the traditional fall mums, our annuals section is filled with plants for late-season color. These can be tucked in between the peonies, iris and other early-flowering perennials to add color to your garden well into October.

The marigolds and zinnias shine in the fall and are perfect companions for perennial plants. Orange is especially attractive when combined with purple.

Annuals are especially attractive in fall gardens when planted in groups of 3, 5 or more. The human eye likes to see odd numbers, and groups of flowers show up better. Tip for mum success: for the best chance that fall-planted mums will return next year, keep them watered regularly even after the flowers have browned. Leave the stems and leaves in the garden all winter, and only cut the plants back in late April or May.

Divide Perennials or Transplant Now

Late August and early September are the ideal time to move perennial plants because they will have at three or four months to grow new roots and get established before winter arrives. After replanting clumps of perennials, water them in well, even if rain is expected. Spreading a layer of compost around those recently moved plants will not only enrich the soil, but the dark color of the compost absorbs the heat of the sun, and warm soil encourages root growth.

Spread a Layer of Compost

One of the best things you can do for a perennial garden in the fall is to spread an inch of compost over the ground, in between your plants. This can be put right over any mulch that remains in place, unless that mulch was applied too thickly and is over 3 inches deep. In those cases, rake some of that mulch away, spread the compost, and then replace the mulch on top. Do not add more mulch next spring, but let the entire area be for a year or two.

Compost can go right on top of mulch that is an inch or two thick. Next spring, only apply an inch of mulch over the compost. The combination of compost and mulch will feed the soil from the top down, which is how nature improves growing conditions.

Got an Open Space? Decorate for Fall!

I often cut my peonies down in late August because the leaves have mildew and are turning brown. That can leave some open areas in my perennial bed, and often I’ll use that space for an autumnal display.

If you have two flower pots that are the same, you can use one upside-down with the other on top to create a display for a stack of pumpkins. In this garden, I planted small Heuchera around the edge of the pot on top, and piled the pumpkins in the center.

Fall Rabbit Alert!

Whether you’re putting new annuals or perennials into your garden, or moving things around, it’s smart to “train the bunnies” from day one that those plants are not on their menu. Use Plantskydd liquid animal repellent, spraying the plant the same day you’ve put them into the garden. This repellent is also the best fall deer protection for Hydrangeas and Rhododendrons.

Plantskydd is a blood-based repellant that is highly effective for keeping ornamental plants safe from rabbits and deer. Although you can see and smell it the day you apply it to plants, you won’t detect it the next day. Animals will smell it for months, however.

Posted in Perennials