How about Dandelion and Chicory- both edible and will grow anywhere and have long, strong roots. Comfrey is also a valuable herb that grows fast and has strong, deep roots. It can be chopped down several times each season and used for very fertile mulch.

From my reading online about green manures, the general concensus is that cereal crops, especially annual rye, grow the fastest and most extensive root systems.

Alfalfa is also said to be good for deep rooting but many of the sources I've read suggest growing it for two years instead of just one season.

Buckwheat is often suggested for rapid growth and production of large quantities of organic materials, but it requires warm day and night temperatures.

You may find a mix of a cereal and a legume to be worth exploring, it's a common practice in agriculture.

All the best,
-Patrick

Here is a link that might be useful: Overview of Cover Crops and Green Manures (ATTRA)

Heres the conundrum, strong and deep roots typically take a long time to build, so plants with those types of roots generally not considered "fast" growing.

While they are building roots the top growth is usually minimal.

You may want to check out the Native Plant Society of Texas. They have a cool interactive plant selection thingie.

Here is a link that might be useful: Native Plants of Texas

Thank you all so much for the clear, concise answers! It has helped me to better organize all the info swimming around in my head! I will be checking out the links and researching the specific plants.

After my original post, I accidentally found a post on another forum listed parsnips, white clover, chicory, and field beans are good for breaking up soil. I don't know if "field beans" is a general category or a specific bean--my guess is it's general.

I would suggest sunflowers. They have an extensive root system that is known for breaking up soil. This quote was taken from the following website:

'Grow deep rooted plants in your rotation to break up the soil and bring nutrients up from deep below: annual alfalfa, beet, sunflower, okra, flax, turnip.'

Added bonus is that you (or the birds) can eat the seeds.

Here is a link that might be useful: How to build up nutrients

To my knowledge, field beans area a specific type also known as fava beans. Anyway, I gardened in Tx a few years ago and if your soil is that heavy clay gumbo that I had I don't think you can get away with no-till. I'm sure there are people who will disagree with me, and cover crops are always a good idea, but that clay gumbo is too heavy to grow much of anything without heavily amending with compost and turning over either with a fork or a rototiller (which you can rent). Maybe once you get a good base going after tilling for a few years you could get away with no till.

I would agree with Kris. That heavy concrete-like clay is very workable with just the right amount of moisture. Wet it, get lots of organic matter on it, till it once, then grow some favas or some of the other plants suggested here and you will get much better, faster results. You'll probably never need to till it again.

IMO, organic matter is the only way to deal with Western clay soils.

Happens I went to a cover crops day last Thursday. Listed "subsoil looseners" are alfalfa and oilseed radish (Raphanus sativus). The fastest growing in my experience are yellow mustard and buckwheat, but they don't have deep roots. A good compromise might be cereal rye that has "massy" rather than deep roots. The problem with alfalfa is that you would want to keep it for 2 to 3 years to get the most benefit, and then it would really have deep roots. Rutabaga have really deep roots -- like up to 12 feet -- but they aren't very fast growing. Regards, Peter.