There are a few reliable ways to kill rhubarb. Oddly, applying RoundUp is not among them. Others, as some suggested here (motor oil, salt) will seriously damage your soil for some time to come and injure any other plants whose roots enter that soil.
The most reliable method with the least collateral damage to soil and surrounding plants, is to 1) dig up the central root. (Just cut down up to 12" straight down around the edges of where you see the stalks emerging. This cuts the lateral roots, it does not have a tap root, then cut it into quarters or more by simply cutting straight down into it with a shovel.Tthe roots have a consistency similar to potatoes and cut easily.) Drop a log or thick piece of wood on the ground for a fulcrum and use a pry bar to lift out the root chunks. Each chunk will provide many roots, easily 20+ for a 3 year old plant...and they sell for $15 each retail if you have a good plant. 2) drown the remaining roots. Keep the area soaked with water for a few weeks until the roots rot.
Attached is a photo of a 3 year old root ball from www.HighAltitudeRhubarb.com
I just stumbled across this and I am shocked that someone would advocate pouring motor oil into a hole in a tree. This means that the motor oil will end up in your soil. Not only is that illegal in most states, it can cause a health hazard and contaminate your yard for years to come.
I could understand someone trying this with vegetable oil or thick salt water, but motor oil is toxic and hazardous for humans and animals as well.
Rhubarb is not truly a woody plant, so I don't think you need a tree stump removal firm. But neither is it a relative of convolvulus, so you don't need to get rid of every single root particle; it needs the crown to continue, so if you just get rid of the crowns and most of the roots you should probably be O.K. See Oregon State's posting on the web. http://oregonstate.edu/Dept/NWREC/rhubarb.html
At your garage sale sell rhubarb plants...it sounds like each year you could dig a piece out and each year it will regrow (because of deep roots). You will have a plant that will make you money. Selling the rhubarb stalks to people who live in apartments, that do not have access to rhubarb, will also make you money.
I do not know if Round-up will kill rhubarb, but if you really want to get rid of the plant...try it.
Why not try selling it? In our local store it sells for $3 a pound.
Cutting off all above ground growth and then drilling a hold in the stump and filling hole with motor oil may work. This works on tree stumps and doesn't pollute the ground.
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