Perennial Weeds

Perennial weeds form a storage system in their roots or stems and can persist for many years. They are typically difficult to eradicate as they have a number of methods to ensure their survival:

They may produce seed that may be dispersed to reach new ground as well as the surrounding area.
Many can regenerate if the top-growth is removed, by the means of underground fleshy roots, rhizomes or bulbs.

Perennials can survive through the winter, either by storing food in their roots or rhizomes (field bindweed, perennial stinging nettle and ground elder) or by storing food in stems and branches (common elder and bramble).

This means that to eliminate perennial weeds, it is essential to remove and destroy the underground fleshy roots, rhizomes, and other storage organs to prevent re-growth. Digging, hoeing and mechanical cultivation often sever the roots of these weeds, allowing them to survive and re-establish themselves. This is particularly true of those weeds with very deep roots, such as Japanese knotweed. This plant produces rhizomes that can extend to over 15 m (50 ft) in length. If chopped up, it can regenerate from as little as a 2 cm (1 in) piece of rhizome.

With most perennial weeds, the most practical and effective approach form of control is to use systemic weedkillers. The plant absorbs the chemical and transports it through its entire system via the sap, eventually killing the whole weed.