I use round up on a regular basis next to my dahlias, and have yet to have an issue or stunted growth, but I am very cautious and use a cardboard blocker about 20x24 to protect the dahlias (and blueberries, by the way) from overspray. It also assists in creating a straight line down the grass line, spraying into the cardboard AWAY from the dahlias. I don't use edging or ditches to separate lawn from dahlia rows, like I do for my perennial gardens.( I have resorted to extra strong poision ivy round up to kill perennial thistle next to them, too, with no problems because of cardboard) i should also say that i don't spray on windy days.

Would the overspray damage your dahlias from the farmers herbicide? It depends on wind conditions and his equipment. I most certaintly wouldn't put any of my most coveted varieties there for a season at least, until I'm sure the damage would be negligible in the worst case scenario. But some extras that would be gifted or composted otherwise? Why not?

Hello BlueBird

You may need to clarify your questions in order to get responses. I can't tell if you are asking a specific question about 3 specific applications or 3 questions wrapped in one.

You mention fertilizer - certain fertilizers (dosed at the proper time) are great for dahlias. The wrong one (heavy on Nitrogen) at the wrong time can stunt the growth of dahlias.

Then you mention pesticides. Again, pesticides can be beneficial for dahlias but you would be spraying for specific pests. I have almost 100 dahlias and have NEVER sprayed mine - but I live in the concrete jungle and nowhere near any cornfields.

Lastly, you ask about weed killers. Assuming you are identifying 3 very different subjects purposely (and not lumping fertilizers and pesticides in with weed killers) I would suggest using weed killers is VERY harmful for dahlias. Most vendors will tell you to hand weed around dahlias. I personally do not use RoundUp anywhere NEAR my dahlias because the slightest whiff of it could severely damage the growth of the dahlia, plus it would likely travel to the tuber and kill it eventually.

So in general, it would (IMHO) be a waste in regards to the weed killers but not necessarily for the pesticides and fertilizers, if you take precautions with them.

Corn growers use herbicides that kill broad leaf plants(corn is related to grasses). The drift from the spray and even the "fumes" from the spray will drastically affect dahlias that are super sensitive to these herbicides. The "over spray" will probably not kill the dahlias but will cause them to have terrible twisted leaves and stems and not to grow properly and the symptoms can last the entire season.

I have used Round up around my dahlias but have been extremely careful and only do so rarely. Dahlias are not as sensitive to roundup as they are to the broadleaf herbicides used on corn. Still , why use it when a garden hoe does the job?

If weed grasses are the problem, there is a chemical herbicide that just kills grasses and does not affect dahlias at all. I have by mistake sprayed the dahlias and there was no effect on the dahlias. It works reasonably well on the grasses but takes over a week to see the results and may have to be applied a second time. Again, hand weeding and a hoe is probably a better solution.

Hi, Dave. Thanks for the advice. I was intentionally vague because I'm not sure what the farmer uses exactly so I was hoping for comments on all three.
Hi, Ted. That was very helpful. I'm afraid it would be a waste. I might have to make space for them elsewhere.
I planted one in a giant container. So I guess I will start there and see how it does. At least it won't be sprayed.
Thanks, again!