The middle of September marks the start of the fall gardening season but you also need to keep spring blooms in mind. Rhododendrons and azaleas are making flower bud this month so keep them watered for maximum spring bloom. Hydrangeas are also making flowers for next summer so avoid cutting back your hydrangeas in the fall.

September is also the perfect time of year to dig and divide all types of iris. Don’t be afraid to lift the clumps right out of the ground and snip off the foliage leaving just a few inches of leaf. Use a sharp knife to separate the rhizomes and toss out the oldest, middle section of the roots. Replant these iris roots but don’t bury the rhizomes too deep – they should be just below the surface of the soil.

Any iris plants that have failed to bloom will have a fresh outlook on life after division and will reward you with renewed vigor in the spring. Dividing in September is good for all types of iris, the bearded, the species and the Pacific Coast iris.

Here are the top three shrubs stopping traffic in local gardens – you can find them at local nurseries this month and once planted they will give you years of color each autumn when the rest of your plants are growing faded and summer weary:

Rose of Sharon (Hardy Hibiscus)

There are a bunch of new varieties of this old fashioned shrub but the best behaved with eye-catching blooms in my garden is a reliable Rose of Sharon variety called ‘Helene’ with snow white  blooms accented with a deep red eye. Drought and cold resistant this shrub gives a tropical look to any garden and can be grown in a large pot as a patio plant.

The naked truth is that the Hibiscus syriacua varieties lose their foliage early in the fall and don’t grow new leaves until late spring. This means you are left with a leafless shrub at least six months of the year.

My solution is to use the woody frame as a support for spring flowering vines. Clematis makes a great partner for many shrubs but is an especially useful vine for draping through a naked hibiscus. Look for hardy hibiscus in shades of purple and lavender blue as well as hybrids with double and huge single blooms.

Don’t prune the Rose of Sharon hibiscus shrub in the fall – it is a bit cold sensitive and prefers never to be pruned. If you must shape it up just remove any awkward branches in late spring after all danger of frost has passed.

Smoke Trees (Cotinus coggygria)

The ‘Royal Purple’ smoke tree is the most common variety but the lime green foliage of the ‘Golden Spirit’ smoke tree will also add a pop of color to a sunny garden bed. The name smoke tree refers to the dry panicles or seed heads that arise from the plant in late summer and look a bit like puffs of smoke emerging from the leaves.

This is the perfect plant for dry soil, rocky soil or areas where you do not want to irrigate. If left unpruned the smoke tree will grow into a small but rather boxy looking tree. You can also keep this shrub compact by cutting it to the ground every spring.

In England the smoke tree is used in perennial borders as a back drop for summer flowers especially in the back of the border where it might never get water.

Add more drama without more drinking to your smoke tree by underplanting it with silver foliage plants such as lamb’s ear, dusty miller or lavender. Another tidy option is to pile gray stones around the base of this sun loving plant as a weed block.

Ornamental Purple-Leaf Grape (Vitis vinivera ‘Purpurea’)

This magnificent vine is worth searching for at local nurseries as the eye-popping color from the deep purple leaves begins in August and continues until frost. The small grapes themselves are not edible but the foliage and good manners of this vine makes up for the weak and random production of sour tasting fruit.

Train this vine to grow up the trunk of a white birch tree or to festoon an old stump or garden shed. The rich color comes when you most need it in the early autumn garden.

In my garden I grow the ornamental grape vine in a mostly shaded site but it really prefers full sun for the most intense leaf color. You can cut this vine almost to the ground in early spring or let it reach for the sky and scramble through your entire garden.

Ornamental purple leaf grape vines are breathtakingly beautiful when planted with purple asters and the dusky purple shades of ornamental cabbage and kale.

Marianne Binetti has a degree in horticulture from WSU, is the author of a dozen garden books and the host of Dig In Seattle as TV show about gardening and cooking. She can be reached at www.binettigarden.com.