what's killing my hardy hibiscus?

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.

If you've got gophers that could be the problem--I've got a bad gopher problem here too and anytime I see plants wilting that I know have plenty of water it's usually because they've been chewing on the roots. Sometimes they chew enough to kill, other times just a little chewing but not enough to kill the plant. With plants like these that need a decent amount of water to begin with, a little root damage can cause significant problems when it's hot out.

On the one that rotted at the base...are you positive it was rot and not gophers chewing off a bunch of roots? If it was rot, that suggests you might be watering too much. If you don't water, do the plants perk up again in the evening when the sun goes away? Especially if the roots had been chewed a bit by gophers, there might not be enough roots to keep the plant from wilting during the heat of the day (roots can only take up water so fast, so if there aren't enough roots relative to the top growth then the plant can wilt even if there's plenty of water around). So if they perk up overnight, then that's a sign that they have plenty of water but the roots just aren't taking it up fast enough, and if that's going on and you water more, then you can end up overwatering. These hibiscus like a lot of water though so I think it would be hard to overwater them in hot weather in good draining soil, but wanted to throw it out there as a possibility.

Hello, and thanks for the reply. The plant that rotted seems to have done so at the crown. I was able to salvage a few healthy roots from it to see if I can get it started again. It was hard to tell just how much damage was actually done to the rootball, but I found an obvious gopher tunnel right on the edge of it.
I've got some good soil for about 12 to 18" down, with pretty fast drainage. Below that is either red clay or hardpan caliche. The big things like maple and crape myrtle don't have a problem with it, but I wondered about the more shallow rooted things like hibiscus and hydrangea. They both tend to wilt really badly in the intense heat and don't perk up again when it cools off. Of course, with an overnight low that's nearly 80 degrees, they don't get much of a cooling-off period. We've had temps hovering around the 100 degree mark for about 3 weeks, so it's hard on all the plants, but those seem to suffer the most from it.
I hate to have to stress them any more by taking up the mulch and digging around to see if I can find more evidence of gophers, but I may be forced to do that if the situation continues. One of the problems is that the gophers are not coming up and leaving any mound or sign of their presence, so it makes it hard to locate and eradicate them. But yes, I'm sure it's gophers and not moles. The tunnels I've been finding are big, among other things. I've never seen any sign of voles, and we don't seem to have ground squirrels either, thank heaven.
One thing I'm learning is that most folks in Oklahoma don't do any large or excessive flower gardening. Between the critters and the weather and the excessive vegetation, maybe it's an indication of why they don't. It's what you might call labor intensive. Still, I've worked 4 years to develop my big flower border and I'm not willing to give up on it.

If you're not seeing mounds, it could be voles. That's what my exteriminator has told me at least when I was seeing some new damage to my plants but not seeing new gopher mounds. If the gophers are digging tunnels there will always be a mound. Both gophers & voles will do similar things to your plants though so in some senses it doesn't really matter which one it is.

I guess you're right. It could be voles. I thought they were smaller, but I really don't have much experience with them at all. Can they be baited, if necessary, just like gophers?

Another question I had is regarding mulch. I'm using the DeWitt Weed Barrier Pro with shavings over the top, and I wondered if maybe I didn't allow enough space around the crown as the plants expanded and put out new stalks. Maybe they don't have enough room to breathe? Could that have caused the one to rot off at the crown? Plus the rodent problem, of course.

I just did some reading on voles and it doesn't look like it. I have never found evidence of any burrow, entrance, or mound of any kind. That's what has me so puzzled. The only thing I DO find are tunnels that are a foot or so underground. They all seem to be nearly 3" across, which seems to me to be pretty big. They don't ever seem to be close to the surface.
I'm more than familiar with what a gopher normally does, and the characteristic mounds. I've never seen them behave like this. However, I found a dead one a few days ago in the separate veggie garden, that had apparently been caught by a cat. Still no mound or other evidence of where it came from. I didn't notice his top teeth, but he had the characteristic big undershot lower ones.
We live out in the country, so it isn't a case of them coming in from a neighbor's yard or anything like that. One side of the garden area has about a 10' border between it and the street, and the other side is a giant concrete parking lot.
Does anyone have any idea of how far a gopher can travel underground before he comes up and makes a mound? I've heard that they can decimate several acres.

With voles there will be holes (without mounds) but they can be hard to see sometimes if you've got a lot of plants.

Do you have plants that you know for sure have been eaten, or are you just finding tunnels? At my old house, I never once found a sign of an existing gopher, but whenever I would dig down in the ground I would dig into old tunnels.

I think gophers can travel a ways...my yard is built on a hill with multiple terraces. I have a big open field behind me and I know that's where they come from, but I've had them come in all the way to my top terrace which is a good 6-8 ft higher than the field behind me and about 15 ft into my yard. So they've basically dug upwards through 6 ft or so of dirt. I don't know if they could tunnel all the way under a parking lot though.

You can bait for voles as well, although I think my exterminator uses something different for them vs the gophers (for the gophers, he doesn't use bait--he uses something that when it gets wet releases a gas that fills up their tunnels and I don't think that stuff works for voles)

If your plants died from rotting at the crown though vs the roots being chewed then you can't blame the gophers or voles or any other critters. The mulch could have been a problem--you always want to leave a little space between the crown of the plant and where the mulch starts so that you don't end up with water being held right around the crown. Some plants are more sensitive to that than others--I'm not sure about your hibiscus.

Hi, and thanks again.
I've never been able to examine the roots of any of my plants to see if they've been chewed, but last year I had two roses nearly topple over from something tunneling under them. I caught it in time and was able to correct it but one of plants has never really recovered. I suppose I'll have to take it up and see what's going on or move it to another location. I also lost a big spiderwort when it looked like something ate its roots.
No, the plants are not close enough together that I wouldn't be able to see evidence of any critters. I put the mulch down last year, so that makes it harder to locate a problem, but it began before the mulch was in place. The attached photo shows the distance between the plants in a section of the garden. The whole bed is about 22' deep and 80' wide. It's big.