I got this from my mother. A few years ago I was having real problems with Potentilla Bushes and she suggested I soak the root area with soapy water, then wait about a week to see if there was any new growth. There was but she said to wait another week and do it again.
Now, every time there's no reason for a plant to be dying, I use this treatment whether it's a shrub, tree, or perennial. Only once has it failed and the reason was that there was a huge rock located under a planting which we had not detected while planting.
I live in Zone 7 of Central Virginia near Charlottesville. We've had four or more years of very dry weather and there are always Japanese Beetles around by now. This spring we've had lots of rain; sometimes raining hard every other day. So far July is still rainy and I haven't seen one beetle.
I'm comparing the lady's remedy and our rain; both could be drowning grubs. Some of you who have gardened longer than I may already know this. Well it's a blessing nonetheless! Rain and no beetles! What could be better. And additionally, this Mom's home remedy might be worth remembering.
