Slugs and snails can ravage a garden. Here are a couple of examples in my garden, where companion planting almost eliminated snail damage in swiss chard and beets. If you do a search using "intercropping' you're more likely to get fact based results.
We had a very wet spring and early summer and the slugs and snails loved it. Everything was green and luscious. The snails prospered and multiplied. Snails can reproduce very quickly because they are hermaphrodite and both parents can become pregnant and lay eggs. I you have 2 snails they can breed together and reproduce. Many of my plants started looking like lace handkerchiefs in no time at all.
I started patrolling and picking off the offending snails and this helped. I also noticed that some birds were carrying off adult snails. The snails and slugs are very fond of plants in the cabbage family. Collard greens, broccoli and brussel sprouts, kale and turnip, are all favourites. They will also happily munch on beet leaves and swiss chard. A few hungry snails will devastate your greens in just a few nights.
One day as I was frantically hunting the slugs and snails, I noticed something very strange. The turnip were being shredded but right beside them, and touching, the swiss chard was practically un-munched. In spite of the large number of snails around there was almost no damage to the chard nor to the beets one row away.
A few inches beside them the turnips were disappearing as I watched. When they first came up the turnip was blindingly fast and quickly got to the business of making lots of leaves and making the turnip. This happened before the snails had a chance to build up their populations but they slowed right down when the snails showed up. The chard quickly caught up.
