Wisteria, Soil pH, Soil Testing

My more than 20 year old Wisteria longissima ‘Alba’ hasn’t grown in over 10 years. An arborist and numerous others have given advice but nothing works. I’ve replaced the soil, enlarged the area around the trunk, and fed/supplemented in many ways without results. In sum: it flowers and grows leaves, but produces no new shoots. If by chance I get a leafy tendril, it’s short and, as the season progresses, it fades to pale yellow, the leaf tips turn brown, and it dies.  I even planted a second wisteria this spring in a more favorable location thinking it would take over where the first one hadn’t, but now even that one is getting brown tips too. Any thoughts?
— Lynne BeLusko, Palos Verdes
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Wisteria longissima  ‘Alba’
Let me first say that I love your question because it does not suggest an easy answer.
Wisteria longissima (or floribunda) originates in Japan where one of its specimens is 1,300 years old.  This wisteria species, which blooms over a period of months, has powerfully fragrant racemes (multiple-stalk flower clusters) that can grow up to eighteen inches long.  By comparison, Chinese wisteria (Wisteria chinensis) blooms all at once with shorter, but more abundant, flower clusters.  The best local example of Chinese wisteria is the world famous specimen in Sierra Madre that was planted from a one-gallon container in 1894, today covers one acre, and produces one million flowers annually in early spring.  Tendrils assist wisteria in climbing and clambering over trellises, pergolas, and arbors.
Knowing that wisteria can live a long time is an indication that, as far as care is concerned, wisteria plants are probably best left alone, a strategy sometimes called “benign neglect,” based on the approach that, when it comes to certain plants, “less is more.”
However, the fact that you have not seen your plant grow in more than ten years does arouse concern.
The photo you sent shows a plant with mostly vigorous green foliage topped with pale yellow new growth.  When new growth on any plant shows that much yellow, mineral deficiency is the best explanation.  Since wisteria is a member of the legume family, the likelihood that it lacks nitrogen is small since legumes live in symbiosis with bacteria (dwelling in root nodules) whose function is to convert atmospheric nitrogen (taken from soil air pockets) into nitrate, the form of nitrogen that is absorbed by plant roots.

availability of minerals to plants at different soil pH levels

The fact that your plant grew well for ten years and then stopped growing may be useful in diagnosing what is going on, together with the fact that a newly planted wisteria shows similarly yellow growth with brown tips.  Keep in mind that there are 15 mineral nutrients required by plants and that if any one of them is missing, or present to an excessive degree, you could have problems.  It could be that, after ten years of growth, your roots reached a patch of soil with mineral imbalance that is also present in the soil where the new wisteria was planted.  Alternatively, this patch of soil could have an alkaline pH, whereas wisteria prefers a slightly acid soil pH.  Micronutrients such as iron, manganese, boron, copper, and zinc are less available to plant roots when soil pH is higher than 7, which is the neutral point on the 14 point acid to alkaline pH scale.

Foliar fertilization may be used to correct mineral deficiencies where soil condition is problematic.  A hose-end feeder is a hand held device, which may be attached to any garden hose, into which granular or liquid fertilizer is diluted with water and then sprayed onto foliage.  Special fertilizers designed specifically for hose-end feeders are available at most nurseries and garden centers.
There is another possible explanation for your problem, which could also be ameliorated through foliar fertilization.  This problem is known as graft failure and may first express itself many years after planting.  Because the graft union is imperfect, minerals are eventually prevented from passing through the graft and reaching the upper parts of the tree or vine.
Last but not least, keep in mind that a 20 year old wisteria is extremely drought tolerant and should not need any water other than what it receives from winter rain.
Tip of the Week:  Where your soil condition could be a problem, consider doing a soil test.  Locally, this can be done by sending two or three cups of soil in a zip lock bag to Wallace Labs in El Segundo.  The standard test is $80 and results include “a narrative report of the major soil properties and recommendations.” For instructions on how to take the sample and where to send it, go to us.wlabs.com.

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