While most annual weeds are easy to keep under control by hoeing and mulching, perennials tend to be strong and fast-growing, so a little more effort is required to eradicate them. Perennial weeds include wildflowers such as spear thistle, red valerian and celandines, and more ornamental plants like golden rod, curled tansy and some species of grass. You can’t always predict what’s going to be a problem, as some weeds thrive in a particular soil type or situation.
Perennial weeds can be identified and treated accordingly by their root type. Being aware of what you’re dealing with, whether it’s deep tap roots or short, spreading weeds, will help you take effective action.
Plants with tap roots include dandelion, dock and thistle. The thick, main root grows deeply to retrieve moisture from deep in the soil. Use a daisy grubber or an old knife to remove the root from lawns and paving. Alternatively, spot-treat foliage with weedkiller.
Grasses, nettles and yarrow spread under the soil surface and form dense clumps. Lever them out with a spade or fork. Ensure you remove all of the roots, as even the smallest piece can regrow. You can also water the area with a weedkiller solution.
‘Woody scrub’ describes brambles, sycamore and ivy, which self-seed and quickly form robust roots. Dig out young plants and cut down stems and new growth of established shrubs to ‘starve’ the roots. Alternatively, spot-treat new growth with brushwood killer.
Deep-rooted plants include Japanese knotweed and horsetail. Their roots can extend more than a metre down in the soil. Clear all ground and cover soil with black polythene for two years, to deprive the weeds of light and kill them. Weedkillers don’t work.
Lesser celandine, oxalis, ransomes and Spanish bluebells are propagated by small bulbils and spread quickly. Regularly hoe off the leaves before the plants flower, or cover soil from March to June with black polythene to suppress growth.
Plants with creeping roots include chickweed, clover, couch grass, creeping buttercup and speedwell, and are common in lawns. Rake the lawn to lift creeping stems, then mow it twice, in opposite directions. Raise mower blades so longer grass shades out weeds.
The roots of bindweed, ground elder, creeping thistle and willowherb break easily, producing shoots from the smallest pieces. Dig up plants and sieve soil to remove roots. Keep removing foliage to weaken roots, or spot-treat new shoots with weedkiller.