Answer: Perennial weeds can be difficult to get control of, once they have become established. They are tough and durable, and can withstand conditions which would kill most other plants. Like most weeds (and other plants), perennial weeds produce an abundance of flowers... and thousands of subsequent seeds. Unfortunately they also reproduce and spread by fleshy underground roots and rhizomes. Attempts at pulling up perennial weeds may seem successful at first, but the cold reality is that the plant will regrow from any (and every) portion of the root which remains in the ground.
The most effective way to rid yourself of a perennial weed is by carefully digging up the entire root system. Because of their ability to regenerate from even a very small piece of root, be sure you get them all out. Once you've rid the area of weeds, place a 3-4" layer of organic mulch over the bare soil. This will help keep weed seeds from germinating, which will keep your perennial bed free of weeds.
Best wishes with your garden!
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