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I have 5 acres of backyard that used to be farmland. I planted grass, but I think I used the wrong kind, as it doesn't seem to be spreading. Now ~5 years later, the yard looks decent, but the grass is fairly thin and the weeds (dandelions mostly) are starting to get heavy. So I sprayed Weed B Gon on it, and now want to over-plant grass seed.

I'm not concerned about getting it perfect. If 10% of the seed I put down comes up and starts to spread over the years, I'd consider that a win. Every amount above that certainly welcome, but...

I just sprayed today, and it's supposed to rain in 2 days (and a few days in a row). I was hoping I could toss down the grass seed tomorrow before rain, but I wasn't sure if that's too soon, or if it matters with this particular chemical.

Can I over-plant grass seed a day after spraying Ortho Weed B Gon? If not, how long do I need to wait (and why)?

  • I found one comment on one site that says "Plant bare spots with grass seed one week after application." Any thoughts on it's accuracy and reasoning? – Dave Apr 19 '17 at 4:48
  • Found another that says 3 weeks. Neither is clear as to why, but it looks like I will probably not be able to seed tomorrow. Still interested to know for sure and what the chemical does to stop seeds from growing. I thought it only killed the plants it was sprayed on. – Dave Apr 19 '17 at 5:22
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The product you used contains triclopyr as the active ingredient - this is a synthetic auxin, and its effect on seeds is either to cause them to germinate with abnormal growth, or more likely, prevent growth even if the seed does germinate. That's why the recommended gap between using it and sowing seeds is three weeks.

I'm surprised the recommended gap between treatment and seeding is only three weeks - with most chemicals used on lawns, it's six weeks.

  • Great, thanks! If it rains 3-4 times in a week, does that help speed up the required wait time by washing away much of the chemical? – Dave Apr 19 '17 at 14:58
  • If they say three weeks, they're allowing for that - probably if its dry all that time, it'd be even longer, but up to you if you want to chance it – Bamboo Apr 19 '17 at 17:16
  • How can the chemical still be an issue after multiple rains? Does it really stick on through a few good spring thunderstorms? – Dave Apr 21 '17 at 16:48
  • I daresay if Noah was building his Ark and that sort of rain arrived, your weedkiller would be removed earlier, but that sort of rain would preclude sowing seeds anyway. The fact is, you need to wait three weeks - if you want to believe something else, that's your right, but the fact I have given cannot be altered or it would no longer be a fact. Take a chance if you want, as I said already... you might get away with it. – Bamboo Apr 21 '17 at 16:58
  • I understand that you're telling me I need to wait 3 weeks, through rain or shine. My questions is not insinuating that you're wrong, I'm merely wondering HOW the chemical can still affect new grass seed after (for example), the 2+ days of thunderstorms we've been having. Saying "because I said so, and therefore it's a fact" doesn't answer the question. While I'm inclined to believe you, I'm also very curious, as this seems counter-intuitive. I would think the large amounts of rain would wash away the chemical and then therefore not be a problem to new seeds. – Dave Apr 21 '17 at 17:04
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3 Weeks should be enough. I reseeded after 2 weeks and the new seeds sprouted and are doing okay. Make sure to spread starter fertilizer to boost the new growth.

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