Clematis

About Clematis

Clematis are mostly climbing vines (plus a few bushy kinds) with flowers from small bells to 7-inch stars in purple, blue, pink, red, white and yellow. Bees, butterflies and hummingbirds visit them. Sweet autumn clematis (Clematis terniflora) is invasive in at least 10 states, and old man's beard (C. vitalba) is a Washington noxious weed; choose the native virgin's bower (C. virginiana) instead.

Zones 3-9 depending on the kind. Sweet autumn clematis is invasive in at least 10 states (Clemson) and old man's beard is a Class C noxious weed in Washington.

When to plant

Timing is counted from your average last spring frost (and first fall frost). Sow & Tell works out the actual dates for your ZIP code.

Plant in early spring (potted plants any time in the growing season) with the crown about 1-2 inches below the soil, 1-2 ft from its support, and cut it back to about 12 inches (Iowa State, Clemson). New plants can take up to three years to settle in and bloom well.

Soil

Rich, moist, well-drained soil with plenty of organic matter; keep the roots cool with 2-3 inches of mulch or low plants over them.

Dig a hole about 18 inches deep and wide and mix in compost (about 20% by volume); test the soil first.

Feeding

• Spring, as new growth starts: All-purpose fertilizer with low numbers, such as 10-10-10 or 5-10-5. Scatter around the plant following the label and a soil test, then water in (Iowa State).
• During the growing season, every 4-6 weeks, but not while in full bloom: Low-nitrogen, slow-release fertilizer. NC State calls clematis a heavy feeder; Clemson says skip feeding during bloom and don't overdo nitrogen.

Too much nitrogen and feeding during bloom are common mistakes; give at least 1 inch of water a week in dry spells.

Harvest

Spring, summer or fall bloom depending on the pruning group. Group 1 (spring bloomers): prune right after flowering. Group 2 (repeat bloomers): trim lightly in late winter to the top plump buds. Group 3 (summer-fall bloomers): cut back to 6-24 inches in late winter.

Varieties (20)

Pests to watch for

Aphids, Twospotted spider mite, Slugs

Diseases and problems

Clematis wilt, Powdery mildew

In the kitchen

Safety: All clematis are poisonous, and NC State rates the severity high: the leaves and sap cause severe mouth pain and blisters if chewed, and the sap can redden and burn skin, so wear gloves (NC State).

Pets: Toxic to dogs, cats and horses (ASPCA): an irritant (protoanemonin) causes drooling, vomiting and diarrhea