Crossville Chronicle, Crossville, TN

October 22, 2012

Plateau Gardening: Pruning and fertilizing impact flower production

By C. Rae Hozer
Chronicle contributor

CROSSVILLE — When and how you prune and/or fertilize flowering perennials impacts blooms. The concepts apply to most flowering landscape plants, but since a clematis vine question triggered this article, that plant family will be used in examples.

Experts say don’t prune young, recently transplanted clematis. Wait about three years. Suits me. I say do no pruning without a good reason. Knowing the cultivar’s botanical name helps determine to which of the three clematis pruning groups a particular vine belongs. Without a positive ID, time-of-bloom helps in making an educated guess as to which pruning and fertilization practices promote better flowering and which may hinder blossom production.

Before blooming there must be flower buds. Know when a plant forms buds. Time fertilization and pruning, accordingly. Do not apply a fertilizer heavy in nitrogen while a plant is generating flower buds. Nitrogen disrupts bud set directing plant energy instead to green leaves and shoots. Weather affects flower production, too. Natural events like shoot dieback in severe winters or from spring frosts and freezes impact early flowers. Drought in summer or autumn can also make blossoms sparse on trees, shrubs and vines forming flower buds at that time.

Know the terms "blooms on old wood" and "blooms on new wood." Woody plants that blossom once in springtime or early summer develop flower buds on shoots from this season which don’t normally bloom until early the following year. This is blooming on "old wood." Prune such specimens soon after their flowers fade. If you wait too long, cutting them back between Aug. 1 and their spring bloom time, flower buds get tossed along with the pruned branches, and there will be few or no flowers.

Trees, shrubs and vines that flower once per year in summertime or have repeat periods of bloom from summer through autumn develop buds on new growth produced earlier that same season. They bloom on "new wood." Prune them in late winter to stimulate more spring shoot growth (and more flowers). Do not repeatedly cut back plants that bloom on new wood during the season. That eliminates flower buds. You’ll have few or no blossoms. Some woody stemmed perennials bloom on both old and new wood.

Clematis vines in Pruning Group 1 flower on the prior year’s growth (old wood). The group includes Clematis alpina, Clematis macropetala and Clemantis Montana cultivars. Montana Rubens is one that grows vigorously to 30 feet and bears two to three inch, dark pink flowers in late spring or early summer. Be sure to have adequate support if you grow this large vine! 

Clematis vines in Pruning Group 2 produce large flowers on both old and new growth. The first flush of blooms is usually bigger and showier than those borne later in the season. Prune mainly to shape and maintain a smaller size. Shaping should be done in February or March. Do a hard pruning (cutting stems back to three sets of buds nearest the soil line) in early spring to rejuvenate older, overgrown plants in this category. Trim immediately after petals on the first blooms drop if necessary, so as not to interfere with bud formation for the second wave of flowers in summertime.

Two popular and readily available vines in this category are Henryi which has seven- to nine-inch, creamy white blooms and Nelly Moser with bi-colored flowers about that same size in pastel pink with darker pink stripes. Shorter cultivars (six to 10 feet) can be grown in containers without pruning. Try Belle de Woking (silver-blue double blooms on 10-foot vines) or H F Young (six- to nine-inch blue flowers with white stamens on eight-foot vines).

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Plateau Gardening is written by Master Gardeners for gardeners in Tennessee’s Upper Cumberland Region.  UT Extension Cumberland County at P.O. Box 483, Crossville, TN 38557 (484-6743) has answers to horticulture questions, free publications and details on how to become a Master Gardener. Send email comments or yard and garden inquiries to Master Gardener Rae, mgardenerrae@frontiernet.net.