When purchasing your rye seed, make sure that you purchase cereal rye, not annual ryegrass. Annual ryegrass is a whole different plant, one that is most commonly used in lawns. While it can also make a good cover crop for your garden, it doesn't grow as tall or have as massive a root system as cereal rye, therefore, it builds your soil up less.
Adding organic matter to your soil is a good way to improve it, and rye can produce more biomass than almost any plant that can be grown in the fall and winter. In fact, rye can produce over 5,000 pounds of organic matter/acre! That means that a rye cover crop can add about 25 lbs. of organic matter to your 200 square foot garden bed.
Rye can tolerate extreme cold and is commonly grown as far North as Canada. It also does well in the South: I've grown tremendous rye crops here in North Carolina and have read about agricultural scientists in Alabama who are encouraging farmers down there to plant rye as a cover crop. Rye does best in cool weather, so it is usually best to plant it in fall, let it grow all winter and then mow it or till it into your garden in the spring.
Once springtime begins to arrive, your rye will turn lush and green. It's likely to be the best looking grass you've got at that time of year! Unless you have a very heavy duty lawnmower or small tractor, make sure to mow or till your rye into your garden beds before it gets too tall. It can get over 5' tall, but don't let it unless you have the equipment to deal with it!
1. It will increase your soil's organic matter content, which will help your garden grow!
2. It's roots will ease soil compaction and will make channels in the soil, allowing air and water to get to your plant's roots.
3. It will compete with garden weeds and can smother them out, thus reducing your garden's weed problems.
4. It will provide habitat for beneficial insects: I have noticed that ladybugs love rye!
5. It will prevent your precious garden soil from eroding -- either washing away in the rain or blowing away in the wind.
For all these reasons and more, you should consider giving rye a try.