A question about clematis vines came via e-mail the other day from Pam Ferrell in St. Albans. She has a vigorous clematis vine in a sheltered spot on the east side of her house that is eight to ten years old. It’s had only two blooms in all that time. They were purple and were supposed to be white. Her question: “Why is it that the plant is such a strong grower but doesn’t bloom?”
There are a few possible reasons why the vine isn’t blooming:
1. The plant is not getting enough sunlight. This doesn’t always mean direct sunlight. Filtered sunlight often works as well. However, a dark spot produces good foliage and few or no flowers.
2. The plant is not getting enough phosphorus. It’s the middle number in fertilizer that helps make strong flower bud growth. Clematis are heavy feeders. A good slow release fertilizer in early spring each year can work wonders. You can follow that with doses of liquid fertilizer, diluted to half strength, at weekly intervals when flower buds are forming and opening. Clematis prefer regular watering during dry spells. Be sure your soil drains well, so you don’t drown your plants.
3. The plant is being trimmed incorrectly at the wrong time of year. This is the most confusing part of growing clematis, because different vines flower at different times.
It is really important to know the varietal name of your vine. Then you can look it up in a good clematis book like The Gardener’s Guide to Growing Clematis by Raymond J. Evison, or find information on the variety on-line.
Clematis flower on either old or new wood, that is, stems that grew last year, or stems that will grow this year. If your vine flowers on old wood, and you cut all the old wood off, you’ve cut off all of this year’s flowers. Most of the clematis that bloom early in the season bloom on old wood.
The summer flowering clematis, like the deep purple ‘Jackmanii’ and the tiny flowered sweet autumn clematis, bloom on new wood. You can cut these to the ground in early spring and you will still get plenty of summer bloom.
Then there’s a whole confusing group of clematis that bloom more than once during the season on old wood and on new. However, the new wood grows from the old, so these vines shouldn’t be cut back to the ground.
That’s why it’s good to know what variety you have. If you’ve bought a plant simply labeled pink clematis, good luck figuring out how to trim it! Clematis expert Evison has a tip for those who have lost their plant label, or never got one at all: “If it flowers before the end of June, don’t prune.” This means, don’t prune the vine until right after it blooms. Like spring flowering shrubs and trees, it will set next year’s buds then. If you cut it back in late summer or fall, you cut off the flower buds your vine just produced and won’t get any bloom next year.
Another reason to know what variety of clematis you’re buying is that they don’t all grow to the same height. The beautiful and popular variety ‘Nelly Moser’ with its white petals with broad pink stripes on them, will grow eight to ten feet tall. If you put it on a four foot tall trellis, you will have to prune it a lot, so you’ll probably cut off a lot, if not all, of the flower buds.
Evison says that ‘Nelly Moser’ fades badly in strong sunlight and produces its best coloring when it gets some shade. Even clematis varieties that perform very well in strong sunlight like to have their roots shaded, so mulch well or plant some flowers around the base of your vine.
One more comment about clematis vines. If you want to be sure about the flower color of the vine, buy it in bloom. Plants are tagged incorrectly more often that we can imagine, whether it happens at the nursery or the garden center. People pull tags out of pots to read them and often put them back in the wrong pots.