(This is in the Dallas area)

My backyard is 80% undesirable grass, and weeds. I am ready to put down sod, either Bermuda or St. Augustine, I figure. I'll have it put in professionally. The yard is 30 X 50, with a 6ft. fence and a very large cedar tree in the middle that keeps the middle of the yard in heavy shade.

Regarding sod selection, I know St. Augustine does better in the shade. My only concern about it is, neither of my neighbors does any weed control and I'm a little concerned about grassy weeds migrating under the wooden fence, back into my yard. Is that a reasonable concern, and is it possible to control grassy weeds in St. Augustine with chemicals? My recollection from years ago was that that was a problem, whereas it's pretty easy with Bermuda.

Also, I'm concerned about the existing weeds and their seeds remaining in the soil and sprouting in the new sod. Would it be advisable to kill the entire yard with Roundup, maybe even two treatments a couple of weeks apart, or would that be dangerous or a waste of time?

I had sod installed a few years back and it was done professionally. They did spray it first with a strong Round Up that was available to the profession. After it died then they tilled it and added some new top soil.
The sod was laided and rolled. This was a zoysia which is similar to Bermuda. The thickness of the sod keeps a lot of weeds down but you have to pull some , especially on the seams.
I did get an irrigation system to water it as it needs watering frequently at first.

I had a terrible fescue lawn before and I just love my zeon zoysia. I get a service to apply fertilizer and weed control about seven times a season.

I'm in the Atlanta area , not too unlike Dallas.

Don’t listen to the northerners with their Kentucky blue grass as St Augustine is a different animal.

Yes, you can do weed control in St. Augustine.

I use barricade a pre-emergence weed preventative 3x per year. You’ll have to check Neil Sperry’s publications for Dallas dates.

When it’s less than 75, I use Bonide’s Weed Beater Ultra. I have 1 slacker neighbor and I spray the hell out of the fenceline plus spot treat anything Barricade didn’t prevent.

When it’s 75 to 85, I use Bonide Weed Beater for Southern Lawns. Use same way as cooler temp product.

Above 85F, I only use Barricade as killers can kill everything.

Yes you can weed control with st Augustine but it is a different chemical. Palmetto is a more cold tolerant variety if that’s a concern.

Bermuda also an option of course as is zoysia. Zoysia, once established, is resistant to weed infestation but it comes with other characteristics such as periodic dethatching.

Do you have Irrigation ?

Something to consider now if you don’t.

When I was in Houston early to mid 2000’s, had St Augustine and loved it. I used the Scotts product specifically designed for weed and feed on St Augustine grass. It had a white and purple label at the time and something about dollar weeds. HD would have pallets of this in all the area stores about this time of the year. It’s what everyone used. Had a weed or two, just pulled by hand. Otherwise lawn was best I’ve had in many years.

I’m NW Georgia now and have Bermuda. Horrible grass. Can’t take the sun or heat. Won’t grow between house due to lack of sun, too much shade. Doesn’t do well under the trees. Goes dormaint in winter and if it gets a lot of foot traffic, we’ll that just destroys the area. Then in spring/summer your fighting to grow the bare spots along with everything else going on. And heavy water area suck also.

It’s throughout my neighborhood and all the neighbors hate it. Several hundred homes. Many want to reseed or resod. I have my weed control company, local guys, coming out to to make suggestions on other types of grass to grow here. And this stuff is in every neighborhood and commercial green spaces around here. They all have the same issues. It’s not suited for this areas climate. My backyard is gone at this point even with the lawn care. And now my front yard is getting patchy. Just like all the neighbors. Bermuda sucks.

I started off here doing the recommended Scott’s for the bermuda just like I did in Houston. It never worked here. After a few years I started using the local lawn care company, like most of the neighbors do, to get the weeds under control and have a somewhat decent yard. I hate the monthly bill but so far it has worked for few years far better than doing it myself.

I utilize a local DIY lawn and pest control company, Barricade as mentioned above is good, we have St Augustine and my lot backs up to a golf course with Bermuda, its a big fight to keep the Bermuda out of the St Augustine.

The key is to keep up the fertization and pre emergence weed control applications on a timely manner. If you have a local DiY they can setup a plan and email you when it's time to apply.

Also there are variations of St Augustine, some is very sun friendly and some are highly shade tolerant. If you tree cover is pretty heavy and well shaded you'll want a different varietal called Seville in the shade section.