I have some back beds with mature crepe myrtles that have become overgrown with weeds. I've cut all the weeds down but I want to kill them before putting down landscape fabric and mulch. Any recommendations for effective weed killer that won't harm the crepes? My beds are pretty big. Thanks.

If you can take your time and be patient, go at them with roundup on a sponge. If you have a sprayer that can make a direct spray nozzle, you can take a round tube of card board (paper towel tube works good for small weeds) to surround target weed and hit with roundup sprayer. Or take several pieces to box around the trunks and then spray weeds.

Roundup breaks down upon contact with dirt and won't have any residual harm upon other plants

Roundup. Just avoid contact with desireable plants. The active ingredient is tightly bound to the clay portion of the soil therefore, has no residual activity.

If you keep the roundup to just touching the bark on the crepe myrtles you should be fine. It needs green tissue to work and even at that doesn't work well on woody plants.

And adding to what AgResearch said, it last in the soil. Microbes eat the stuff up and will be broken down in a matter of days so you will be good to go.

Thanks all!

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Thanks all!

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As an addendum: let the weeds grow back for a couple days before applying. As mentioned above roundup needs active green growth to work. Spraying them if they are cut back down won't be as effective as if they have a little new growth

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Roundup. Just avoid contact with desireable plants. The active ingredient is tightly bound to the clay portion of the soil therefore, has no residual activity.

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so I can spray an area with Roundup to kill existing grass and put sod down in 3-5 days?

I use round up all the time in my beds.

Just spray it on what you don't want and you will be fine. I've lost a couple annuals and perinnials in the past but never an established shrub or tree.

** TIP - Don't do it on a windy day

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Roundup. Just avoid contact with desireable plants. The active ingredient is tightly bound to the clay portion of the soil therefore, has no residual activity.

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so I can spray an area with Roundup to kill existing grass and put sod down in 3-5 days?

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Technically yes you could but the grass may not be dead in 3-5 days. Roundup takes 7-14 days for complete kill.