I don't recommend putting down black plastic. It won't let any moisture through and will eventually decay, leaving you with strips of plastic working making a big mess. If you want to try to kill any weeds or stop the seeds from sprouting, covering with a layer of cardboard or a thick layer of newspaper, wetting it down, and covering with a 3-5" layer of mulch should work.

I have two websites that I like that are plant databases. http://www.wildflower.org/plants/ is run by the Lady Bird Johnson Wildflower Center, while http://www.txsmartscape.com/index.asp is run by the North Central Texas Council of Governments. Both allow you to pick plant type, habit, water use, sun exposure, etc and will give you a list of plants.

Good luck with your "new" gardens!

Meryl had emailed me yesterday with this question. In case there are others facing similar situation here is my reply to her.

I have one bed that is much the same shape as yours. The others much less so. I'll be working on that bed intensely this spring.

First I wouldn't put plastic down. It will cause more problems than solve. The soil needs to breathe and it needs water. Plastic will prevent that from happening and it will breed all kinds of bugs, snakes and mosquitoes! I'm sure you don't want that on top of everything else you have to deal with.

Up north some put black plastic over garden beds in the early spring to quickly warm them up. But its temporary. Down here its not necessary to do that. Our ground doesn't get that cold and warms quickly enough with out the plastic treatment.

I have read that some have used black plastic to kill off grass and weeds.. but its only a temporary fix... It all comes back with that method and it looks terrible in the mean time plus the issue of bugs and snakes and mosquitos.

First step is get rid of the weeds and grass. This is going to take a while. But you don't have to do it all at once. You can do it section by section as you are able. Or hire the help to get it done all at once.

Either pull them or poison them or both. (grass you'll have to poison a few times... ) Then mulch. But before you mulch I would suggest you lay a layer of heavy cardboard down. Over lap edges so nothing can get any sun to it. Cut around existing plants. Wet it down really good then put your mulch down. It won't look bad, you won't see the cardboard after the mulch is in place. The cardboard will feed the worms and break down. The worm activity will soften your soil. Most of the weed seeds already there won't be able to grow from no light getting to them. Some seeds blown in afterwards will sprout but will be very easy to pull out.

If you have the money for it, you can put down a nice layer of compost before the cardboard. Or you can layer it like a lasagna bed. But first you have to kill everything or it will all be a waste of time. :( Grass and weeds down here will come right up through the cardboard and the mulch. You HAVE to kill it all before you do anything else.
If you don't have access to enough cardboard, you can use thick (2-3inches) of wet (really really wet) newspapers. I soaked them in a trash can of water before laying them in place.

It might be May or June before you're sure everything is dead and gone. Especially the grass. By then its going to be too hot to plant anything but the most stout of plants so just let everything sit the summer. Keep watering what is established.

You can start getting your plants lined up and plant out in the early fall.. This gives you the summer to think and plan and research what plants you might like.
Mid Sept is usually cool enough and gives the plants plenty of time to set down good roots to make it through the winter.

I have to run right now, but I will answer more later.

In the mean time I would urge you to post your question to the Texas Garden Web and let others chime in. I'm sure there are many other facing the same battle you are! Your post could help a lot of other people!

Think about attending our Denton Plant Swap end of April. We can get you started on some good plants that do well here.

Oh here's 2 sites you might get some ideas from:

http://texassuperstar.com/plants/index.html

http://allthingsplants.com/thread/view/3050/Gardening-with-Texas-Native-Plants-and-Wildflowers-The-Front-Yard/?offset=0

I've faced your problem a couple of times in my gardening life.

Once I used heavy clear plastic for about two months (July and August) over areas of heavy Bermuda grass and it did do a good enough job that I could work work the soil underneath and have a more manageable weed situation afterwards. This was on blackland clay in the Austin metro area. I wouldn't really recommend this unless that is your only alternative. Not because of mosquitoes or snakes or bugs, it rained little if any during that period, but because the heat kills all life in the upper inches of the soil and I just don't like the the whole plasticulture movement in gardening. I think we have enough of that in our lives, environment.

Another time while living down in Harlingen, I wanted to start a vegetable garden. The site was very compacted, clay loam full of Bermuda grass and Silver-leaf Nightshade. I had a tree trimming company deliver enough rough bark, clippings, wood chips to cover the site to a depth of 12 inches or so. By June, I was able to rake the rapidly decomposing rough mulch and find a friable, moist soil full of earthworms and planted a crop of Okra and Southern Peas to get my garden started. The weed situation was completely controllable. It was a typically dry spring in the Valley and I did water the plot regularly. A year or so later, I found a research report that recommended this technique for the semi-arid portions of west Africa.

Hope this helps.