Answer: The most commonly asked question about wisterias is why some individuals do not bloom satisfactorily. There is more than one possible cause. In very cold climates the flower buds, which form in fall, may be winter-killed. Plants raised from seeds do not flower for many years, often twenty or more. Because of this, seedlings should not ordinarily be planted. They are not sold by reputable nurseries. To bloom well, wisterias must not be shaded for more than a minor part of each day. They grow, but do not flower in shade. A common cause of nonflowering is the production of excessively exuberant vegetative growth induced by faulty pruning or fertilizing. It is too much to expect the vines to make a rambunctious growth of stems and foliage, aswell as an abundant bloom. To flower freely, vegetative growth must not be more than moderate. Unfortunately, the usual means tried to curb exuberance, heavy pruning in winter or spring, is self-defeating. Such treatment merely encourages vigorousshoot growth, as does fertilizing and watering.
If you have a young vine that has not yet filled its allotted space, by all means fertilize and water generously to encourage growth, but do not expect bloom. Train selected shoots along wires or other supports in directions you wish them to assume as part of a framework of permanent branches, so spaced that they adequately furnish the area you wish to cover, but do not excessively crowd it. From 11/2 to 2 feet between major branches is about right. Prune in summer and in winter. Summer pruning consists of cutting off the ends of all side shoots just beyond the sixth or seventh leaf as soon as that leaf develops and of cutting the ends off shoots that develop from the cut-back side shoots, immediately after they have developed their first or second leaves. Allow one strong leader shoot to grow from the end, or from near the end, of each main framework branch without cutting it. Stretch it tautly along the wire or other support in the position you wish the branch to be. In late winter cut the leader shoots back to two/thirds or one-half of their lengths (the more drastic treatment is for weak shoots) and cut the side shoots that were pruned the previous summer back to within an inch or two of their bases. In this way the permanent branches will be extended each year by one-half or more the length of the annual growth of the leader shoots, and side shoots will be converted into short flowering spurs instead of developing into hopeless tangles.
Once the vine has occupied its allotted space, follow the same pruning practices, except in winter prune the leader shoots in the same way as the side shoots, by cutting them back to within an inch or two of their bases. Also, after the vine has covered the area vou wish, do not use fertilizer unless shoot and foliage growth is definitely scanty, and water only if there is danger of the foliage wilting, which will only happen under drought conditions.
Old, neglected wisterias that fail to bloom present a different problem. Often they're tangled masses of intertwined stems, branches, and dense foliage. If faced with taming one of these, take drastic action. In winter prune away all except a framework of the most advantageouslyplaced stems and branches. Not all will be exactly where you would like them to be, but choose those that best fill, but do not crowd, the area you have in mind. Shorten the other branches drastically, being quite ruthless that are truly ill-placed or obviously crowded. When you have finished you vine should look like a skeleton of its former self. It should consist of a gaunt framework of fairly well-spaced branches. Do not fertilize, and do not water unless at a later date the foliage begins to wilt. If the soil is acid, an application of lime, at the rate of 1/2 to 1 pound per 10 feet, lightly forked into the surface, will be of benefit. The drastic pruning will stimulate vigorous growth. Treat this growth in the manner described above for younger vines that have not been neglected. If there are too large gaps between the old framework branches, train shoots selected as leaders, treating them in the manner described above for young, newly planted vines. Prune side shoots in summerand in winter. Under this treatment the vine is likely to flower within two to three years.
Root-pruning is sometimes advocated as a means of bringiing nonflowering wisterias into bloom, but unless combined with summer pruning of the shoots, as described above, it is ineffective. If done in addition to summer pruning it can be helpful, especially if the roots have access to large bodies of fertile soil. To root-prune, dig a ditch 2 to 3 feet deep around the trunk of the vine at a distance outwardof 4 to 8 feet. The distance should relate to the size of the vine. Severing the roots checks top growth and favors flower production. Mix superphosphate with the excavated soil at the rate of approximately 1 pound per 10 feet of ditch and back fill it into the trench.
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