I have a stubborn collection of weeds that have colonized my rock garden. It is about 8' X 8' in size. I have a spiderwort issue in that they grow so densely that other plants can not compete.
If I use Round-up soon to kill the "undesirables", how long do I need to wait in order to plant other things like zinnias, basil, phlox and others? I am trying to attract pollinators only. I will not be eating the basil. I have basil growing in containers that will be used in the kitchen.

For Roundup to be effective you need to wait for temperatures to be at or above 60F for at least four hours a day. At lower temperatures its efficiency is much reduced.
Generally speaking you can plant edible plants 3 days after using Roundup, so I see no problem waiting the same for ornamentals as well.

Thank you so much for the quick and positive reply.

Here in SE Michigan, I won't be planting much of anything in this area until May 1st at least. So when it warms up sufficiently, I should have no issues in the bed.
Prior to the exploding Spiderwort problem I had planted Ice Plants, Dianthus, Hen & Chicks and others in order to create a little garden, now I am going to reclaim control!
Die you little buggers! 💀💥⚡️

If your chief problem is Spiderwort you may want to consider using a triclopyr-based herbicide (such as Corteva Remedy): it's what is used to selectively control Spiderwort and other invasive brush like Black locust, Twisted acacia and Poison Oak in pastures.
Of course the flip side is many other noxious weeds such as cinquefoil are unaffected by Triclopyr, while Glyphosate (Roundup) kills pretty much everything indiscriminately.

Thanks very much for the suggestion but I think that I may "want to lay waste" to everything in that bed. It also has seedling trees in there like red maple, mulberry, some Queen Anne's Lace and some grass plants.
With a complete die off, hopefully I can get ahead and stay ahead.

In my 2 raised beds out front, maybe 5-6' wide X 18' Long, it got in there too. They're are a pain in the butt. A deep rooted thistle is in there as well. The Spiderwort went from 20 stems out there late 2019 to 75 stems last year. Those darn things are impossible to pull and get the entire root. Fortunately they are confined out in those raised beds but if I failed to address it this Spring, I am fearful that I would lose control in one of those beds!!

I have not found Roundup to be very effective on spiderwort, but it is your only option if you want to replant. Starting to treat the spiderwort while young will be best.

Some of those tough weeds aren't completely killed by Roundup. In an agricultural setting they don't need to be because the crop just needs enough of an advantage to shade out the weeds and out-compete them. Two applications might be needed in your case and the weeds need to be actively growing for it to be effective. The bad thing about that is you might get a later start on your other plants. The good thing is the Roundup isn't residual so you can plant very quickly after you apply it.

Here's what I'd do: Put clear plastic over that bad so it heats up and makes a beautiful greenhouse environment for the weeds. You want them to grow like weeds! When they're growing happily, hit them with the Roundup. If your improvised coldframe speeds up the weed growth enough, you won't even have to delay your planting season at all.

Sigh.....
The good thing is the Roundup isn't residual so you can plant very quickly after you apply it.

There is no residual that hurts plants, but there is residual! The chemical is still there, just like any other chemical (that doesn't evaporate). It decomposes with time, just like any other chemical. Not saying you should or should not use Round Up, just saying you should know the truth.

Thanks leftwood.
I figured that there will be a residual effect. I was hoping to get these weeds/flowers under control. If I reduce it all by 90%, it will be a huge success! At the end of June, this 64 square foot bed was 85% Spiderwort.
If I beat it back to where pulling by hand completes the control, I'll be happy.
It's either a spray herbicide or I hire 2 Workman to trample them over the course of 3 hours after I tell them to be careful!

BigBill said:I have a stubborn collection of weeds that have colonized my rock garden. It is about 8' X 8' in size. I have a spiderwort issue in that they grow so densely that other plants can not compete.
If I use Round-up soon to kill the "undesirables", how long do I need to wait in order to plant other things like zinnias, basil, phlox and others? I am trying to attract pollinators only. I will not be eating the basil. I have basil growing in containers that will be used in the kitchen.

Hi BigBill,
Have you tried pouring boiled water over the weeds? I've heard that it kills them without having to use poison. Maybe try it before using Roundup? Hope either way you can get rid of them. :-)
VernelleV

I'm sure with enough boiling water you could certainly kill any plant. How many gallons would it take to treat 64sq'?

When using Roundup one needs to use it when temps are over 70 degrees to make sure your kill the
item that you want. It is a top killer that will travel downwards to kill the roots. On spiderwort I leave
only an inch above the soil before spraying or if it's a tight area I'll use a paintbrush and paint the area
in order not to kill other areas. Also be advised that Roundup has been linked to cancer and must be
used with per the directions, do cover up and remove clothes that might have been in contact with the
spray. As an environmentalist whom tries not to use chemicals, one learns what one should use if there
is an issue. If we have a large area such as gravel walkways or gravel driveways we will use vinegar to
save money and the use of Roundup.

Boiling water is great for some plants also. We also use our steam cleaner for insect control.

BigBill said:If I use Round-up soon to kill the "undesirables", how long do I need to wait in order to plant other things like zinnias, basil, phlox and others? I am trying to attract pollinators only. I will not be eating the basil. I have basil growing in containers that will be used in the kitchen.

As soon as the Roundup dries its job is done. The poison is in the roots, doing its thing but cooler weather does slow down the process. Spray them, wait a day or so and pull up the bodies - even if you don't get all the roots, they will continue to die. Or, as you have to pull up the bodies anyway (to make room for your plant additions) why don't you hire those 2 workmen to pull up the Spiderwort instead of just trampling it.

Plants with waxy surfaces aren't affected by Roundup - it just rolls right off.

First of all, let me thank all of you for your thoughtful assistance!

Trying to pull Spiderwort is next to
Impossible. Experience tells me that they sprout from bulbs or corms and just keep coming back.
64 Square feet is a lot of boiling water! I am thinking 64 gallons minimum. The time involved would be ridiculous. Plus I do not believe, or think that it would be a thorough kill.
I am 72 years old everyone, 72! Not 27! I am not going to be spraying acres and acres, year after year with Round up!! It is just one massive spraying around April 1st. Maybe a touch up a week later. I will take the risk of exposure and dying of cancer! ( records seem to indicate that repeated exposure is bad)

I'll state again for the record that I have refrained from using any poison for almost 40 years, at least. I am willing to tempt fate just this once. The worse thing I use is Isopropyl alcohol or Neem, and both of those rarely.
I may have to resort to this a few more times before I depart this Earth but I'll chance it.
I figure the time to strike is when they are less then 3" high and thin like grass plants. And if you guys are correct, destruction should be massive. They kind of remind me of crocus when they are young. I feel certain enough that the poison will seep into the leaf axils.

There are no vegetables involved. Just new plants that will be used to attract butterflies and bees.
Again ALL, Thank you so very much.

I just read two articles published by universities. Both say Roundup is semi affective for getting rid of spiderwort but that's not the problem. Its the millions of seeds left behind.

PS: Thick fibrous roots like daylilies.

Adding a generous amount of surfactant will help it stay on the slippery leaves.

Alternatively, how about a flame thrower?

porkpal said:Alternatively, how about a flame thrower?

Ooh! I like that idea! You probably wouldn't even have to worry about burning your yard down.

And more effective than boiling water, I would wager.

I thought about a tactical nuclear weapon but the last one was sold on e-bay last month! Rats 🐀

I always wanted to mount a machine gun turret on the deck to shoot squirrels. I couldn't find one of those either.