There are a lot of products on the market for getting cats out of your yard-or at least for repelling them. I'm not sure how many of them work so I would suggest saving your money and using things around the house first.
Know your beast: Is this your cat or your neighbor's old crusty tom. You may not want to use certain items unless you know the cat. For instance, my ultra-high maintenance cat has asthma and is sensitive to the volatile oils in citrus or lemon products and peels that can trigger an attack. I'd be pretty pissed if I caught my neighbor chasing him around with vinegar spray and lemons. (He's indoors now in his dotage anyway, but he was quite the goldfinch killer in his heyday).
I haven't used scent deterrents much, but it's cheap and it might work it you keep updating the scent on a weekly basis. But in Portland, the rain might just wash it away constantly.
I have had success with mulches like cocoa mulch for certain areas. The kitty's fussy paws don't like the feel of the sharp edges underfoot (nor do slugs). Some say dogs are attracted to this and will eat it but I say that's one pretty dumb dog. Never hurts to try out a small area before you buy a truck load of it. And yes, it does smell like chocolate while it's fresh.
The main problem I encounter with cats is that they seem to know how to find your favorite perennial grass and chew the tender young shoots before the plant can really get going. A client with China Love Grass found that the grass was mysteriously chewed at every morning. I finally got smart after finding that spraying soap on the grass did nothing. I spent a few dollars at the store buying bamboo stakes and some plastic mesh (usually used for birds and squirrels) and I created a square shaped barrier so that the cat couldn't get at most of the plant. It worked and it was not an eyesore either. After the grass grew to a certain stage I removed the barrier and let the cat chew here and there-it was full enough that it wouldn't cause damage. Just a few telltale chew marks. My kitty loves chives. I will set out tender wheat grass for him and he will go straight for the chives unless I put them up out of his reach. But he has sophisticated tastes.
Water spray is good for strange cats that lurk amongst your roses. Just keep a hose or squirt gun handy when you're out in your yard-usually the cat will get the hint or at least come around less frequently.
But what about when cats use your shade garden for more discreet matters. This can be tricky. Try scent deterrents, mulch, and water spray. If you don't mind the cat using one small area, make a litter box area for it so that it uses only one spot of the garden. Dig a hole and sink a litter box and fill it with sand. Ah, the things we do for our little four-legged beasts.