How to remove wisteria
How can I get rid of a healthy old wisteria that was planted too close to the house? I cut off major branches. What remains is a woody trunk that will be cut down to the ground in the fall. How do I prevent suckers from cropping up? Do I have to resort to chemicals? I don't believe it's possible to dig the roots, as they're mostly covered by driveway and cement pavement. Also I planted some native shrubs nearby, so trying not to harm those in the process.
Multnomah County Oregon
To control re-sprouting of aggressive trees and shrubs (hawthorn, holly, etc.) as well as larger-diameter aggressive vines (blackberry, wisteria) there is a non-chemical strategy that works, a chemical strategy that works, and a very common chemical strategy that doesn't work.
The non-chemical strategy that works is to cut the trunk, and then keep an eye on the stump over the next several years. Every time it sends up a shoot, prune it off. This strategy basically involves starving the plant to death: it costs the plant energy to send up each shoot, and if you cut off the shoot before the new leaves can start feeding new energy into the plant, the plant loses. But if you forget it, and allow new leaves to recharge the plant's resources, the plant wins, and you lose, and the clock starts over.
The chemical strategy that doesn't work is to cut the plant, then go back later and treat the stump with an herbicide, even a strong or concentrated herbicide. Woody plants, it turns out, are very good at "compartmentalizing" a problem, even a very serious one. Within seconds, literally, of being cut, a woody plant or vine seals up the cut areas with residues that block the intake of the herbicide toxins into the remaining parts of the tree or vine.
The chemical strategy that works is to apply an appropriate concentrated herbicide IMMEDIATELY upon making the cut, before the plant has the (minimal) time it needs to block uptake via the severed circulatory system. (One published study says the window of opportunity for blackberry vines is 15 seconds; another author says that window is only four seconds!)
Recommended herbicides include a 20% concentration of glyphosate (start with the concentrated bottle that says "41%" and create a working solution by dluting 1:1 with water). Or use an 8% solution of triclopyr. In either case, you are starting with a bottle marked "concentrate", not with a pre-mixed spray bottle. In the bad old days the solution was applied with a small paint brush; today the weapon of choice is a cheap household hand spray bottle. And of course you should be wearing rubber gloves and eye protection.
When using any herbicide--or any garden chemical, even those marked "organic"-- ALWAYS READ AND FOLLOW THE LABEL INSTRUCTIONS AND WARNINGS. In addition, note that herbicides should NOT be used when air temperatures exceed 85 degrees, or will exceed that temperature in the next 8 or 12 hours. In hot weather, herbicides can evaporate and form a small toxic cloud that can drift for hundreds of feet.
The following link leads to an excellent article from the Alabama Extension Service (Alabama A & M) on cut-stump herbicide treatments. That part of the world has more nasty vines than most of the country, and the advice they give is also valid for our area. http://www.aces.edu/pubs/docs/A/ANR-1465/ANR-1465-low.pdf