Beginner Gardening: what's killing my hardy hibiscus?, 1 by patsdogs

patsdogs wrote:
Two years ago we had 4 hibiscus moscheutos in the garden. Now we only have two of them left. They came up just fine and began to bloom, and then the canes began to turn brown, dry up and die back, one at a time, but over a very short time period before the whole plant was gone. The plants are in full sun, are mulched and get plenty of water, and they are in a fairly light soil with good drainage. It tends to stay damp under them but not soggy.
When I dug one of them up, I found that the culprit had probably been a gopher tunneling under it. The other one simply rotted at the base. It still had a couple of live roots that look pretty healthy, so I clipped them off, dusted them in RootTone and planted them in a pot to see if I can start them again.
Today I noticed that one of the remaining plants also has a few dead canes. I don't want to lose them!
The other problem I've noticed is that they've been going limp in the recent heatwave, in spite of daily watering. I use a probe to check for soil moisture and there seems to be enough, but the plants are not happy. I worry about over-watering, but if I didn't give them a daily drink, they'd be lying flat on the ground, injured beyond recovery. Does anyone have any ideas or suggestions?
I'm not a novice gardener. I've been at it for over 35 years. Only the past 5 have been in Oklahoma, and it's a major change from conditions in northern California. It's been a real learning curve.