|Upon doing research on trying to get a wisteria to bloom (add triple super phosphate), I clearly added too much to my wiskey-barrel grown terrace garden wisteria... The leaves all fell off and I was sure I killed it.
Well, it is still alive but very much struggling. Because it is so late in the season (I'm in Brooklyn NY) the new leaves keep getting fried by the sun. I'm watering it twice a day. I've also given it some plant food.
My question is mostly this: is it a good idea to trim back the branches? I'm afraid of putting it through any more shock, but it seems that having less area to try to populate with leaves might be a good idea.
|Wisteria is truly "a survivor" among plants. I am not sure that an oversupply of triple superphosphate would cause those symptoms immediately because it is not an immediate release form of fertilizer but instead has a medium term effect. It will however continue to leach through the soil and will eventually leach away. In the meantime, avoid adding any more to the soil or by foliar feed.
These plants can take years and years to come into bloom. The average for seed grown plants is about eight years. In the meantime, there isn't much you can do except provide routine care and wait. These plants also tend to grow quite huge. Care for a container grown plant would include following a regular fertilization program with a complete fertilizer that include minor nutrients as well as the main three (N-P-K) and possibly top dressing with compost periodically. You would also need to replenish the soil in the container annually, either by repotting or by removing the top layer of soil and replacing it.
It is difficult to keep a potted plant healthy, even in a large container, for years and years. You need to try to keep the fertility high enough to support the plant and you need to keep the soil evenly moist, not soggy sopping wet and not alternating between wet and dry. Overly wet soil will cause problems as will dry soil.
Over time the potting soil compacts, the organic matter decomposes and it and loses its ability to hold water and air, it may become either waterlogged or too fast draining depending on its original composition and the nutrients become depleted. Salts will build up, too. Your plant may be suffering due to restricted roots or water stress or possibly something unrelated. Either of these could cause it to suddenly defoliate, as could an accidental event such as herbicide drift or a mismeasured foliar application of a water soluble fertilizer or even a reaction to an insect spray.
At this point, I would suggest sheltering it from wind, provide light shade or morning sun only, repotting into a good quality soilless mix and watering only enough to keep the soil evenly moist. Trim the plant back by about half to minimize the demands on the apparently damaged root system and use a compost tea and/or seaweed based water soluble fertilizer according to the label instructions to try to give the plant a gentle boost.