Yellow powder appeared suddenly and covers !eaves and stems 100%. What to do?
Unfortunately, your photos did not come through. I am unable to see the issue at hand. Perhaps there is a tree nearby that has dropped pollen!
In the meantime, this collection of articles will help you with the care of the shrub:
https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush
This is not under the walnut tree. It is probably a good 30 feet or so from the tree itself.
I can't find a source that lists butterfly bush as juglone tolerant. It is recommended, though, that plants be 50 to 80 feet from the black walnut if there is any sensitivity.
I know butterfly bushes are on the prohibited list. But is the above species/variety included in that? I grew it once and it did not spread at all. Thanks, kate
Some butterfly bushes do reseed and can become invasive. The new sterile varieties such as Buddleia Lo & Behold, Buddleia Flutterby Grande and the Miss Series do not spread.
I don't find Silver Spring on a non-invasive list, but if you grew it before without any trouble, go ahead. The problem areas seem to be in the Northeast and the Northwest, which would include your area. You may want to contact your city government to be sure there aren't any restrictions.
https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/controlling-butterfly-bushes.htm
It is in a pot at the moment, but desperately needs to be planted in the garden, was given to me by a neighbour
You have the correct ID, it looks like. I see the telltale signs of downy mildew, as well. The yellow spots and graying of some of the leaf edges are the signs, here. Unfortunately, there isn't a real treatment, and the fungus will have to be killed of through environmental means. It requires moisture to carry out the lifecycle, so first is trying to keep the area a little more dry. Next is keeping dead debris picked up and discarded, removing heavily infected living growth, and watering ONLY from below the canopy. If the area remains wet, then pruning out some of the middle can help to improve airflow.
Here is an article that will give you some more detail:
https://www.gardeningknowhow.com/plant-problems/disease/downy-mildew.htm
my bush was trimmed in october ,, now in may it looks dead, what can i do
Prune it down to about 6 inches from the ground and see if new growth starts. It may not have survived the winter. Butterfly bushes vary in their hardiness. I am also in zone 6 and I often lose my butterfly bushes.
https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-varieties.htm
Last year I planted a Buddleia in a large pot in a catch-all generic compost. It seemed to do well the first year and did flower. I also gave it a couple of feeds of potash. Seeing as this was the first year I didn't prune it un March. Unfortunately this year it isn't doing as well. It is not troubled by pests or anything of that nature however the leaves are turning brown and falling off. There is still some growth there but I was wondering if there is any way of saving it?
I'm wondering if it's the container. It may not be draining well or is being overwatered. Can you remove it from the container and plant it in the ground? Since you didn't cut it back in spring before new growth started, cut it back now by at least half. They benefit from being cut to about 8 inches in spring because they bloom on new growth. If you don't cut it back, all the blooms will be at the top. Spider mites can be a problem with butterfly bush. Spray the leaves with Neem oil weekly for a few weeks and see if that helps.
https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-container-care.htm
https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/brown-leaf-spots-on-buddleia.htm
https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/how-to-revive-butterfly-bush.htm
My plant is in its second year. It did not start growing until very late in the spring - I thought it was dead. Do I cut the old wood or leave it alone?
Yes, cut off the stems from last year. Butterfly bush blooms on new growth.
Typically, in early spring, cut back your butterfly bush to about 6 inches to 1 foot from the ground. New growth will form.