I have not added worms to my garden, I feed them, and they come.
This is what I have learned from reading, having a compost bin, observing my garden, watching about a thousand YouTube videos...and reading.
They will eat almost any decaying plant matter. A worm will eat about half of its body weight every day. So if you get a pound of worms, you will need to provide them with 3.5 lbs of food every week.
(I love this video. Worms eating sawdust!)
They do not like onions or oranges. (I'd extrapolate that they probably wouldn't like anything citrus.) Don't try to feed them anything spicy hot. Oh! And nothing with salt.
They are drawn to sweeter things. They especially like bananas, banana peels, watermelon rinds, and pumpkin. (I remember watching a video about a large-scale worm farm that fed them cornmeal, molasses, and... something else, I forget.) Worm farmers will tell you that a diet of sweets is not good for their overall health. But if you want to get them in your garden, there's your hook. (Pun intended.)
They do not have stomachs, they have gizzards, like birds. They need a bit of grit in their diet to help them
... pass their castings. In my compost bin, I give them coffee grounds, or finely crushed eggshells. (I'm not sure - maybe the sand in your soil would be enough?)
They will need some kind of bedding. Dampened shredded paper, or wet cardboard will work. In my own compost pile, I have seen them lay eggs in the corrugations of cardboard, they use it kind of like a nursery. (As a bonus, if their feeding schedule is disrupted, they will eventually eat the paper, too!) I watched a video once where the guy put corncobs in his bin, apparently they not only love sweetcorn, but will lay their eggs along the cob.
If you are going to buy worms, I'd suggest you only put about half in the garden. Because if you want to easily harvest fishing worms, you should start a worm bin.
You can make a DIY worm bin - a lot of the videos show using two plastic totes. There's a video of one guy (I think he's Rob from Australia) - he uses an old bathtub!! There are all kinds of fancy systems with stackable trays,
as the worms fill the tray with castings, you add a tray, and they will move up into the new tray, once the food is depleted in the first one. For a DIY tray system, I saw a video where they used an old dresser, and just drilled access holes in the bottom of the drawers. (About a half inch diameter.)
Before I started my compost bin, I made "worm feeding stations" in my garden. I'd use my trowel to dig a small hole between my plants, or in the rows. I'd put in a couple handfuls of shredded bills
, then pour in my kitchen scraps, and push the soil loosely back over the hole. (I bought a $3 blender at a yard sale, and pureed my kitchen scraps, partly as a way to dampen the paper, partly because worms' mouths are very small, and I wanted them to eat the stuff quick and leave me castings in the garden.
People also make worm towers, where they drill holes in either a 5 gallon bucket or a wide section of PVC pipe, then dig a hole, and sink it into the garden. They just drop their kitchen scraps in there. Would be a good way to compost if you didn't want a bin or pile, (or didn't have space for one,) and seems like it would keep the worms in your garden. Could put one every 5 feet or so, and just fill them in succession, to keep the worms moving from one end to the other. ("Hey guys! Over here!!")
If you do go with the worm bin idea, then even if they do migrate out of your garden, you'll have more to add in a short period of time. That, or you'll get to do a lot of fishing!!
You would probably want to keep them somewhere that is fairly climate controlled. They don't survive temperatures outside of 40-85 degrees F. (I think that range is right. I may be off a few degrees, I'm working off memory.)
I've never noticed a disappearance of worms in my garden, but once it's planted and HOT out, I don't do a lot of digging, either. That upper range of 85 degrees makes me think - I wonder if they just go deeper once your soil gets too warm for them. Do they seem to disappear mostly while it's hot?
Have you noticed any kind of pattern? When they are there, as opposed to not?
I am not affiliated with Uncle Jim in any way, and cannot endorse his product. I quoted that only to show that you will want to do something
with your excess worms!