What to do when it's taken over a lawn

Crabgrass control before repairing a St. Augustine lawn.

Can you tell us if this is a good idea? We live in South Florida and have a substantial amount of crabgrass in our St. Augustine lawn - roughly 50%. Life got in the way and the crabgrass got out of control. We want to get rid of the crabgrass and repair those areas with new sod.

I read an article on crabgrass control that suggested pulling all the weeds out first, you know, digging them out piece by piece, but I’m not sure if that’s the most practical option. We have such a large area to contend with, we were contemplating tilling to loosen everything up and raking up the debris and then replacing the sod. We did this once before in a smaller area with good results.

We laid down baking soda first to kill the crabgrass and after a week when the crabgrass was dead we tilled the area. But is that a good idea on such a large area? If so, should we treat the soil prior to sodding? Or add some topsoil or what????? By the way, that area is not part of the bad 50%.

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Garden Counselor ANSWER:

I can’t comment on your chances for adequate crabgrass control at this point, because I’m not sure about the layout of your problem. So bear with me while I try to clarify and offer a few different perspectives about typical lawn care techniques and “no-no’s”.

Is the entire lawn area half crabgrass inter-mixed with the St Augustine? In other words, is it like a casserole all jumbled together? Or is half of the area predominantly crabgrass while the other half is basically fine with the good grass thriving there? Let’s look at both of those options.

1. If the crab grass section is mostly isolated
from your nicer lawn area, and is quite heavily infested, it would make sense to follow your approach to kill it first by spraying an appropriate herbicide. Most commonly, soil gets tilled to loosen it in preparation for planting, but if you found it to be a handy way to dislodge the crabgrass and get it out, that’s fine. I’m assuming you are referring to the use of a power roto-tiller.

Personally, I hate the way that weeds or grass clumps get buried and mixed in with the dirt when the rototiller works through a ‘populated’ area. It seems very troublesome to get all the debris out so the soil can be leveled, but that’s your choice. This could provide some pretty effective crabgrass control for a large area, but I still have some concerns about your description, so we’ll come back to this approach in a bit.

2. If the crabgrass is mixed throughout the entire lawn area, intermingled with the good, it is a judgment call which way to go. The big concern is to avoid damaging or extracting the desired lawn grass, which is a difficult thing to isolate.

Since you had success with the baking soda treatment (I've never tried that), go ahead and repeat that, then clean out the debris as much as possible without getting too meticulous, (and without using a tiller in there).
Then cut out plugs or stolons from good patches of the St Augustine, replant the bare spots and let those young’uns spread and fill in.

I mention not to re-till the soil in this case, if in fact your intention is to use a deep-tine power tiller to turn over the entire area, good grass and weeds churned up together.
It seems a shame to sacrifice any of the good grass. Since St. Augustine is so effective at spreading, I would not disturb any healthy sections.
Just clean out the bad and encourage the good to get better, even if it is just a patch here and there. It’s always better to avoid disturbing the roots of any plant, if possible.

NEXT CONCERN: Regardless of which scenario actually describes your situation, they have one thing in common. A key factor regarding your long term success with crabgrass control will be the timing.

If you can get the crabgrass out before it forms seeds, you have a good chance of success. If this crop of weed has started to reseed already, you are going to have continued problems this season and probably next year as well. That might affect how deeply involved you want to get with a renovation project now.

Let’s go back to situation #1.
If  the problem is a separate half-section that needs to be renovated, I also am not sure what you had in mind by "treating the soil".
If you mean treating (a) with a weed killer or (b) with a pre emergent (a weed-preventer), then consider these points separately:

(a) No further weed killer will be needed, if you are able to clear out the crab-grass. That is, unless you hold off on replanting and deliberately allow new weeds to sprout so they can be eliminated. (Like a revenge thing!) 

This is an effective approach to crabgrass control, but it does delay significantly your progress on getting the St. Augustine re-established.

(b) If you want to use a pre-emergent to prevent new weeds, it could affect the growth of your new sod or replanted stolons, depending on the chemical found in different lawn care products. 

So check the label carefully before purchasing any herbicides.
Be very cautious taking this approach so you don’t interfere with your new grass getting established.

Another point to clarify, or your crabgrass control could be anything but that. If by 'tilling' you really meant just loosening up the soil in the bad spots with a hand tool, like a cultivator, that is fine. That could help to get out the crabgrass more easily and prep the soil for new grass — as long as you're not dealing with existing seed heads on the weeds. You want to avoid leaving any seeds behind in the soil as much as possible.

3. There is no need to add topsoil, except to fill in low spots. Some people do add a layer of compost or topsoil, sometimes thinking or hoping that it will bury any existing weed seeds and eliminate the next generation of crab grass. That is a questionable benefit.

In a practical sense, it would be awkward to implement if you are only adding soil to a partial area and not the entire yard.

It also is not recommended to just add a layer of good quality material on top of older soil, without mixing it in thoroughly.
Layers cause problems, as the roots always prefer to hang around in the new, lush living area, rather than “work harder” to penetrate down into the bowels of the earth like they should.

4. Finalize your sod plans. Some people use the term ‘sod’ to refer to planting grass from seed. Is that your plan, or are you actually planning to purchase pre-grown sod to fill in the bad/cleared areas?
That might be the most effective approach, giving you the greatest degree of crabgrass control immediately.

It is the most expensive solution, but it eliminates the jeopardy that comes with the long waiting period for grass seed to fill in and mature.

You may need to remove a slight amount of loosened soil (about 1” of depth) to lay the sod at the correct level. This would remove some weed seeds, and whatever is left will typically not germinate through the thickness of sod.

Your only concern with new crabgrass would be from new seeds distributed over the top from adjacent areas, not from underneath.

Thanks for submitting your crabgrass control question to the Garden Counselor Lawn Care site.

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