I’ll be planting some trees (Canadian Hemlock) and ground cover (phlox and others) in about 1 week. If use grass killer on the existing grass now will it be safe to plant the new stuff 1 week later?
a product like that will have a detailed label on how to use and the cautions/hazards involved.
some herbicides might allow planting in a week. it depends on what you used and the dosage and your conditions.
Ok, I’ll have get out to the store and look at the labels. Thanks.
Every weed killer I can think of (Roundup, 2,4-D, etc) is a defoliant. You should be able to plant anytime. Certainly this would apply to seeds. As long as the leaves of the tree are shielded from contact with affected areas, this should pose no problem.
Some “weed killers” include chemicals designed to inhibit seeds from sprouting in the first place.
My Roundup says 3-4 months, but it’s the formulation that promises you only have to spray once a summer.
Even then, I wouldn’t count on it. I had a patch of dirt next to my driveway that always grew weeds. One year, I decided to go “nuclear” on it with a mix of Roundup and Casoran and in quantities about triple the recommendation. This STILL failed to kill/suppress all of the weeds for very long. Almost two years after nuking that strip (and eight months since applying anything), I decided to plant some groundcover there… which promptly died.
So the moral of the story is that the effectiveness of weed killer is proportional to how much you want to keep a plant alive. In your case, this moral dictates that you will have healthy grass surrounding a dead hemlock. 
Roundup is *never *a defoliant. One of the major marketing points for Roundup was that is wasn’t a defoliant. If a plant treated with Glyphoste has dropped its leaves, it’s because the conductive tissue has broken down, ie the shoot is dead.
Just because a herbicide is a defoliant, that doesn’t tell you anything about when you can sow. Tebuthiuron and Hexazinone are notorious defoliants, and both will persist in the soil and prevent re-sowing for at least 2 years.
I had a feeling somebody would call me to task on that one. A buddy of mine who has a bachelor’s in ornamental horticulture told me that about 20 years ago. I’ll have to Google all of these compounds and straighten out any misconceptions before I make a costly “garden misapplication.” So the two compounds you mention—tebuthiuron and hexazinone—can they actually be used to sterilize a piece of ground? Every time I use one of those herbicides that say “will prevent regrowth for up to one year,” I swear there are weeds there a month later reproaching me with faint taunts of “neener neener.” Is there anything I can buy in consumer quantities that will actually work for a year or more?
Okay, I decided no to do it. The different types of stuff at the Lowe’s said it wouldn’t harm all sorts of things, but we changed things up so it won’t matter. We didn’t have enough ground cover coming to fill in the whole area in front of the trees. It would take a long time for it all to grow out and cover the space, and I didn’t want to have to be weeding and mowing around it until then. So we’ll use the ground cover somewhere else and transplant ivy to put in front of the trees. We have plenty of ivy, and it will grow quickly and choke out any remaining grass. I may just put some landscape cloth under the trees to stop weeds from growing there.
Thanks all for the advice and info. Continue arguing if you like.
We’re not arguing. Blake just done whacked me upside the head with the stupid stick is all. I have to edumacate myself before I make some sort of serious blunder that kills an expensive ornamental. What I’ve already learned about glycophosphates just from the wiki page on Roundup is pretty interesting. As far as I can tell, you can use Roundup with complete impunity if you’re sowing seed, and it shouldn’t have any effect on mature ornamentals as long as you don’t let any of the leaves touch the ground. That’s rarely a problem with ornamentals in small (like 18”) planter boxes or 15 gallon liners.