I have no idea what the Houston garden centers are like so it is impossible to answer your question.

Green Acres Nursery has built some new stores in the area 2-3 years ago and has a really nice selection and helpful employees. Huge selection in a very clean and well organized stores. Their prices are also pretty good. If it wasn't such a long drive to them way up in Melissa off 75 (they also have a store in Irving) I'd shop there more often. I think they are based out of California. Seem to be very well funded. I guess maybe somebody made a ton of money off working in tech and decided to have a 2nd career selling plants.

Covington's (way out in Rowlett on George Bush tollway) is nice if you live out in that area. Very large selection of everything from annuals to trees, but there are 3 separate areas which are all pretty big, so if you don't know they exist you might not realize how big the store is. Prices maybe a little on the higher end, but they do have a membership program that gives discounts. Usually there are several associates wondering around the area who will help you if you need anything. Really nice store that I'd probably shop at more often if I lived closer, but it is a fairly long drive out there.

Cristina's garden center (in plano at george bush tollway/preston by Lowes) has a huge supply of flats of annual flowers (18 small containers of flowers) if you want to buy a bunch of annuals to fill a bed. Their prices for perennials and shrubs are pretty good also. They are primarily set up to sell to professional landscapers so plants are often not well labeled and there is very minimal help available from sales associates if you can even find one at all. They do have a large selection and I buy from their fairly often. I think they have 2 or 3 stores in DFW but I'm not 100% sure about that.

Bruce Miller Nursery on belt line in Richardson has a decent supply of annual flats (maybe 20% as much as Cristina's). They also sell a lot of shrubs, perennials, etc and generally their prices are pretty good, although I've found their tree prices to be pretty expensive. You can find helpful associates here more than at Cristina's , but it is very hit/miss. Kind of a messy nursery that isn't well kept, but it is a big nursery with a fairly large selection. They are fairly close to me so I buy perennials from them almost every year.

Moon Valley Nursery bought a nursery up in Frisco that I had been to a few times. I can't recall what the name was before, but I haven't been to it since Moon Valley bought them out. That nursery used to sell a lot of native plants, but I have no idea what the store looks like now. Moon Valley is a big company with locations all over the state and have been buying up local nurseries in the area over the last decade. I haven't been to one of their stores so can't comment

Calloway's has been around DFW for as long as I've been here and they have many locations. Generally their prices are a little higher than I want to pay, but I still buy from them on occasion. Unless they are very busy you'll usually be approached by a sales associate, or maybe they think that since I'm a man I have no idea what I'm doing and look like I need help.

I went to one of the Ruibal's locations down in Dallas once, but that is really out of my area. It seemed ok, but it has been years ago so I can't comment too much on them.

I certainly wouldn't dismiss Home Depot or Lowes either. Lowes especially often has a good plants, but for both of these chains it can depend a lot on the location. Sometimes Lowes does a terrible job of keeping their plants watered in summer and you'll go in there and find half of their plants wilting severely. I don't understand their business model which allows them to let half of their plants die in summer instead of paying one full time employee who's only job is to keep them watered. I generally only buy from Lowes in spring before it gets hot for that reason, but their prices are really good on what they have.