Snakes, I think, are kind of territorrial. You might get 1 to stay around that size space if you are fortunate. What we did was to make a border completely around the garden with wildflowers etc. so the animals will be able to travel all around & have 'cover', which is what they need if you want to keep them there. Also put piles of rocks & some wood or boards but not rodent apartments just things for lizards, toads & snakes. It has worked for us, I enjoy sighting them on occassion. I brought the lizards in & have relocated a few little toads recently.

Sounds good! Remember that the fence will prevent them from leaving and will also keep rodents out. Should I just put in one or two garter snakes and a few toads caught nearby?

They need to have access to water. You can put out ground-level bird baths, which you need to keep filled and clean, or you might go for a full-blown pond or water garden.

Provide the proper habitat for them and they will move in with no need for more assistance from you. Unless you provide the proper habitat even if you import them they will move on quickly.

Thanks for the good ideas! Remember that the fence will keep critters from moving in (or out); I'll have to place them there. Any other thoughts on what the proper habitat would be, or what to import?

Snakes can climb, though garter snakes are mostly ground dwellers. It seems to me they would be more likely to climb out once you put them in than to climb in on their own, so you might think about "restocking" the snakes from time to time.

And if there is enough for them to hunt, they'll likely just climb in on their own, anyway.

They'll climb over a 3-foot hardware cloth fence? That's pretty impressive.

They'll climb over a 3-foot hardware cloth fence? That's pretty impressive.

If they really felt the need they would. If you've ever tried keeping snakes in a cage you know how deft they are at climbing and squeezing through tight places. Also baby garter snakes could probably crawl through a 1/4" mesh with little trouble.

The first thing to do is learn about the pest, then you can learn how to manage it. There's a link below that might be helpful. Voles climb and tunnel. If you put up a fence you need to bury the bottom 6-10" below the surface so they won't tunnel under. That won't keep them from climbing though.

I use traps in my greenhouse, cold frames and garden. We have a lot of them this year. The cat will line them up on the porch (compost ingredients) but she's not able to keep up with them.

Here is a link that might be useful: Vole Management Guidelines

I don't deal much with the voles but I've heard you can trap them with a conventional mouse trap, baited with peanut butter.
I do have several toads in the garden and once in a while see a snake but not often. You need to give toads shelter from the sun and a water source. You can buy fancy little "toad houses" but I just use terra cotta flower pots turned on their side and burried half way. For water I have a few terra cotta saucers buried to ground level and as a bonus they attract butterflies. I always transport any toads I find in the yard to the garden and some seem to stay. I've caught a few snakes and put them in the garden area but I don't think they take up residence there.
Good luck...

Makes sense. I've been catching the voles with traps, but after catching 30 in just a week decided that I need a fence, which is nearly completed. I often see snakes moving about the garden, and thought I'd ask here about the wisdom of importing them as permanent residents if they can no longer move in and out because of the fence. Saucers of water, half-buried pots on their sides, and propped-up flat rocks or boards ought to make good shelter for a few of these friendly reptiles.