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The best worms for vermicomposting are red wigglers and red earth worms (Eisenia fetida, Eisenia hortensis, or Perionyx excavatus).
That's because they eat food scraps and they hang out where the food is, even on the surface, whereas worms like night crawlers burrow down deep which wouldn't be the most effective for composting.
While you can find these worms in your yard, when starting a compost bin it's best to just purchase them. It can be hard to tell worms apart so start off on the right foot with buying them.
Don't get too caught up on having the right amount of worms. You don't want to overdo it though!
A good rule of thumb recommended by professionals is 1 lb. worms/ square foot of worm bin. Or, 1 lb. worms/1 lb. food scraps. This can be a lot and not necessary!
We added a handful of worms (less than 100) to a large cooler-sized bin and they've been doing great for years.
Worms multiply!! If they're happy then you'll see lots of babies soon. With that said, I'd recommend starting out with a small amount, say 100 worms (unless you're doing a huge outdoor bin). When starting out with a worm bin you want to start out lightly with the amount of food scraps you're adding in anyways. If you have a big family or a lot of food scraps, then go bigger with a 250 count. We've never bought a large amount and it really isn't necessary.
As you grow your worm bins and add more, you can just take a portion of worms from a thriving bin!
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The worms should be able to eat a "serving" in a couple of weeks. It's best to feed them about every 2-3 days to not over feed.
Do a rotation of food placement: make it easy and do clockwise. Start in a corner, dig a hole, place food, cover. When you return in a few days/week move to the next quadrant or corner of the bin. This allows the worms time to finish the first portion of food before adding more. If you're keeping your bin indoors, leave it outdoors every couple of weeks to allow in new organisms.
Where to Buy Worms for Vermicomposting
Prices online have gone up considerably recently. If you're wanting something local, check a feed/farm supply store near you, Walmart, or tackle shop. Just remember you'll want Red Wigglers or red earth worms.
Buying online is a great option, you just want to go through a reputable professional breeder! Here are some great options.
To learn more about vermicomposting see my full post A Guide to Vermicomposting.