How do I prevent my cairn terrier digging up the plants?

Bailie is a male dog and 2 years of age.

Some dogs love to dig and it would be a shame if he wasn't allowed to dig at all. You need to teach him that the plants are a no go area.
The only way to do this without confusing him is to supervise him at all times in the garden. Tell him off if he heads for the beds and praise him when he comes away from them. Channel his energies into games with toys instead. Over time, you will break the pattern of behaviour and he will become more reliable.
However, if he twigs that he gets told off when caught digging - but not when you are not there - you will have difficulty stopping him in the future - so its important that you supervise him closely and stop him from approaching the beds - rather than rush out to stop him when he is already indulging in a good dig.
You'll get there in the end!

A loyal happy little dog. My Cairn aged 12 loves digging - it's part of their make-up, why take that away. She used to wipe all four paws (as if doing a dance with tiny jumps) on my low growing plants, they went. Now she's digging under a bush at our rented property. It gives them something to do. Forget your plants and let him enjoy himself.

all the ideas above are good as a deterrent, but you also need to look at the cause. cairn terriers are very very active dogs, if your dog is digging, or otherwise misbehaving, hes bored. almost all terriers need a good long walk everyday, and interesting toys to play with. try to give your dog two half hour walks everyday, and buy him a selection of semi interactive toys, like a kong. a kong is especially good, because you fill it with treats, and your dig gets to play with it til they fall out. another idea is one of those hollow rib bones, take it and fill it with peanut butter. take all your dogs toys, and keep them in a box or basket where he cant get to them, then give him one or two from it each day, and take the old ones from yesterday. that way theres a "new" toy everyday and he wont get as bored.

Have you tried sprinkling pepper down in the area(s) he digs. We had this problem with our doggy and a bit of pepper sorted him out right away. We always knew when he had tried to dig as well as he came in sneezing like a gooden..

Sadly the word terrier comes from the Latin word for terrafirma (earth) they were named terriers because of there tendency to dig holes, you could try some of the products from you nearest pet supply store like pepper dust etc, but this is one habit you will find hard to break.

im not answering the ques, um asking you a ques. theres a pet store where i live and they had a cairn terrier that they were selling and i named him,"todo." i really like him but i already have two other dogs, but is this breed a good dog breed? thank you

you can lay chicken wire just under the soil surface it helped for my dog because they dont like the feeling of it when they scrape it

Cayenne Pepper works a treat! Only every time it rains you will have to replace. A much better way would be to occupy your dog. Provide toys/ distraction or set aside a nice cosy spot in the garden of freshly tilled earth or sand where he can dig to his heart's content.

sprinkle pepper on the plant. [this is a ture fact my aunt did it and it really works.!!]

I'm going to assume their outside plants, because this would be kinda nasty to do inside. Well,,I guess if you want to..It works the same as when they dig holes, put his poo in the area of digging. He wont dig in his own poo. BUT... you dont want to leave this under your plants when it rains or the chemicals will soak into the plants. This is only a fertilizer with grain eaters, not meat eaters. But he should stop.

Give him a cat to chase!
Seriously - are there not some pepper sprays on the market that will make him not want to go near them?