Does that plant go dormant and thin out in winter and can it handle overnight frost and 100+ degree mid summer afternoon temps? It would be in the shadow of the house part of the day, but it would still get mid afternoon full summer sun.

Most winter days and nights are well above freezing, but every year there are at least a few days where temps get into the 20s overnight. Record low temps could even go lower for a night or two.

What part of USDA 8 geographically?

Unfortunately, Carrissa is not frost tolerant. Whenever there is a colder snap in my area, they are one of the first shrubs to show the damage and defoliate. Also, be aware that some dwarf/groundcover cultivars do not have thorns, or almost no thorns.

Some gardeners have reported effective deterrent properties from using liquidambar seed pods as a spiky mulch. Additional information would be useful. Your Sunset zone would be better than USDA for this purpose.

Dyckia? Sure will keep the cats off but might not be your idea of aesthetically pleasing. There are hundreds of them, many of which have interesting foliage and as an added bonus, attractive flowers once a year.

This is in the Sacramento area.

The "Sacramento area" could be any of three Sunset zones.

Where is the sunset zone map? I didn’t think Sacramento country was that large.

Can we see a photo of the area you need cover? How large is it?

Although I have never tested it out for this purpose, I would consider using Rubus calycinoides (aka, Rubus hayata-koidzumii) or creeping raspberry. Moderately drought and shade tolerant once established, evergreen and with a low, layering habit. Has pretty bristly stems that I would think would be rather uncomfortable for cats to walk through or on. You will need to leave other deterrents in place however until the GC is established and fills in.

btw, most of the Sacramento area is Sunset zone 14. Higher elevations could put you into zone 9. The rubus should work in either.

I see nothing on that page other than generic information. No actual map.

http://sunsetwesterngardencollection.com/climate-zones

you can Google 'Sunset zone maps' if you don't like this version. There were several Sunset pages.

It's within the area where that map says is Zone 9, not far from the border of Zone 7.

Try this: scardey cat plant - coleus canina

LOL!! You can certainly try that but it doesn't really work.....one of many repellent plant gardening myths, like gopher plant.

I need a physical deterrent that will work year round. Sharp thorns that cannot be ignored by small animals will work.

Odors from plant blossoms and cayenne pepper seem like wives tales and wishful thinking.

Other than a dwarf barberry, I don't have much to offer. Most groundcovers are thornless or at least foot traffic tolerant so will not be much of a physical barrier (although cats in my neighborhood just focus on bare mulch or soil and avoid messing about with any plants to do their business).

And some scents really do deter cats - citrus peels for one. But you need to keep replacing with fresh peels. Also pepper spray (capsicum), which is a common ingredient in commercial dog and cat repellents.

You could pour and spread cobbles there and be done with it.

Cobbles won’t deter the cats here. They will still use it as a litter box

Maybe thorns are not necessary if the ground cover is thick and about 12 inches tall. Not tall enough to hide in but still tall enough to be a problem to walk on top of.

I need something that would be very awkward for a cat to walk through even without spikes.

Coyotes will do the trick.

What's wrong with the Dyckia idea? No cat will mess with Dyckia!

I haven’t seen any Dyckia that look good.

They may be too ugly.

Will they survive and spread out to cover the area so the cats can’t just walk around them?

There still has to be spots for humans to step so they can trim the hedges.

Some people use plastic forks sticking out of the ground.

Well...I think that they are beautiful but de gustibus non disputandum.

I'm starting to think that you need an inorganic solution. is it really true that cats will use an area as a litter box if they cannot bury their scat?

Most cats will try to bury, but "most" is not all and many try very half-halfheartedly so you still see and smell it.

I have seen cat poop on concrete.

What about Genista ' Lydia'?

When I wanted to keep my cats from climbing up a screened enclosure, I covered the planting beds at the base of the wall with Red Lava Rock. The kind from Arizona I believe. They refused to walk on it.

>Cobbles won’t deter the cats here. They will still use it as a litter box<

Maybe you don't actually understand what cobbles are, cobbles are rocks (term is also used for stone pavers). As in partly flattened rocks out of a river bottom. Or you are thinking in terms of a shallow layer with spaces between the stones. No cat is going to use soil under a complete heavy covering of rocks as a litter box.

https://www.bing.com/images/search?q=cobbles&qpvt=cobbles&FORM=IGRE

"No cat" is an overstatement. As I said before, I have seen cat poop on concrete. So, that proves that just because they can't try to bury their feces there, doesn't mean they 100% won't still go there. Cats go outside their litter boxes inside people’s homes and damage carpets all the time. So, it’s a myth that cats always bury their waste.

And I know it wasn't from a dog because it was a location that requires hopping over a 6 foot fence to access.

However, I will still consider it since I only saw that happen once in 10 years and maybe heavy river rock could be the best compromise between effectiveness and attractiveness vs something 100% effective, but an eyesore.

Cat pee stinks worse and I don't know that rocks would do anything to prevent cats from peeing there.

The other issue with rocks, is that there would need to be some method to keep the rocks from slowly sinking into the dirt and eventually becoming buried.

You might consider a meidiland groundcover rose, such as scarlet or fire meidiland.

My dogs would the trick but I’m in no.

haha

seriously, I understand your frustratio.

good luck