Sweet Autumn Clematis is a species separate from the large-flowered types, and equally famous in its own right. This is the large, strong-growing vine that covers fences and arbors and produces a striking, billowing cloud of fragrant flowers in late summer and fall. If you need screening along a high fence, or have an arbor as gateway to your garden, this is the plant you need.
Sweet Autumn is a type 3 Clematis. Clematis in this group are later season bloomers and flower from buds developed in the current growing season. They should be cut back hard in late winter or early spring to stimulate a large flush or new growth.
Growing Clematis: If you're already a clematis grower, you know all this. But if you're not, here's all you need to know.
First of all, there are several kinds of clematis, but most people want to grow the large-flowered types. For some info on the others, go to the bottom of this page.
Basic Requirements: First of all, clematis vines always want their roots shaded, and the plant growing up into full sun. That means you can put some shallow-rooted groundcover around the roots, or simply some mulch--just something to keep the hot sun off the root run, and promote moisture retention in that spot. But be sure your vine grows into plenty of sunlight, which promotes heavy flowering.
Soil is important. Clematis do best in neutral or slightly alkaline soils, but they are somewhat adaptable. If you have very acid soil, try to add some calcium when you plant. Also, be sure to dig the hole deep. Remember you're planning to have this plant in place for decades.
Where to plant is important. These incredible vines are some of the most beautiful flowering plants, and we've all seen them blooming lavishly on fences, porches and trellises. They're not really hard to grow, and they get larger and stronger every year. Sometimes they take their time getting going, so be patient. It usually takes about two years for a newly-planted vine to come into its own. The large-flowered types are hardy into the very cold north, so almost everyone can use them. Be sure to place yours so it has something to climb---fence, trellis, or post.
Winter and spring care are important. In very cold places (like Vermont, where I grew them for years), the winter kills the whole vine right down to the dirt. They actually disappear. Then in the spring, they are somewhat slow to emerge, so you must protect the spot, and watch for the shoots. Once they pop up, they grow fast, but beware--they are brittle! If you happen to break off the young spring shoots, it sets the vine back terribly, so its important to watch and care for the new shoots until they really get going up your post or trellis.
Once that happens, it helps to gently guide the vining shoots as they find their way upward. You can actually arrange your vine as it grows, but again, be gentle; the stems remain brittle. Soon you'll see buds, and then suddenly one day, they begin to open. Most popular clematis varieties open incredibly large flowers, often as large as 5 to 8 across. They face the sunniest side of your trellis, and well, just take a look at the photo (That's Clematis Nelly Moser vining up a lamp post.). Nothing makes a lovelier display.
Bloom Season: In Vermont, most large-flowered clematis bloom from early July all summer long into September. But the varieties vary; check the individual information on each clematis page. Always deadhead the flowers as they fade, and you'll have a magnificent display for months.
Other types: The wild North American clematis, commonly called Virgins Bower has small white flowers that cover the large vine. The Montana types have smaller flowers than the large-flowered ones, but they create a much larger mass of vine. Montana clematis are wonderful for covering a roof or large area of fence. Unfortunately, both Virgins Bower and Montana clematis are limited to central and southern zones, not hardy in the far north like the large-flowered favorites.
The Viticella Group, sometimes called the Italian Clematis, has vines similar to the Large-Flowered group, but usually forms a larger mass and has very heavy bloom of somewhat smaller flowers.
For more info: take a look at the fantastic All-about-Clematis site, Clematis.com.