Knotgrass

How to recognise it

Growth habit

Annual with sprawling to rising, much-branched stems 10–100 cm long, often forming flat mats.

Leaf

Small, elliptical to lance-shaped, 1–4 cm, bluish green and short-stalked. The leaves on the main stem are larger than those on the side branches.

Flower

Tiny flowers of 2–3 mm, greenish with a white or pink edge, 1–6 together in the leaf axils, from June into October.

Fruit/Seed

Small, three-sided, dark brown nutlets of 2–4 mm, enclosed by the persistent perianth.

Stem

Tough and striped, with swollen joints and silvery, papery, ragged sheaths around each joint. At the base a tough taproot.

Confusion with

Equal-leaved knotgrass has leaves that are all the same, smaller size and forms even flatter mats. Black-bindweed twines and has arrow-shaped leaves. Redshank stands upright, with larger leaves and pink flowers in spikes. Common chickweed has opposite leaves and white starry flowers. Knotgrass: tough, sprawling stems, small alternate leaves and silvery sheaths at the joints.

Flowering: June–October

Why grow it?

• In autumn and winter its seeds are an important food for seed-eating birds such as sparrows, finches, greenfinches, larks and partridges.
• The leaves are eaten by the caterpillars of a few moths and by a small, metallic leaf beetle (Gastrophysa polygoni).
• The flowers are tiny and give little nectar; now and then small flies and small bees visit them. Its value for pollinators is therefore modest.
• A true survivor that tolerates trampling, drought and compacted soil: it greens places where nothing else grows.
• A handy indicator: where it dominates your lawn or vegetable plot, the soil is probably compacted.

Knotgrass: where does it grow best?

• On trampled, open ground: paths, farmyards, car parks, paving joints, fields, vegetable plots, building sites and sandy banks.
• Full sun; it grows poorly in shade.
• Any soil, from sand to clay, including compacted, rich and dry soil. It also tolerates soil that is wet for a while.
• In a wildlife garden it belongs on a bare, trampled spot, a wild strip along the vegetable plot or between the paving of a little-used path.

Goes well with

• Greater plantain: Another plant of trampled paths, with broad rosettes and seed for birds.
• Annual meadow-grass: A low grass that keeps the same trampled spots green.
• Pineappleweed: Fragrant, greenish-yellow knobs on the same hard paths.
• Shepherd’s-purse: An early-flowering little plant with heart-shaped seed pods, also good for seed-eaters.

Knotgrass: when and how to plant it?

1. Usually you need do nothing: knotgrass arrives by itself, from seed that can lie in the soil for years.
2. If you want to give it a place for the birds, leave an open, trampled strip or an edge of the vegetable plot unweeded.
3. If you want to sow, scatter seed from your own garden in autumn or February–March on bare soil: it germinates early in spring, often only after a cold period.
4. No watering or feeding needed: it grows even in the hardest ground.
5. Choose a spot where it can self-seed without taking over your vegetable plot or border.
6. Do not take plants or seed from nature areas: only collect seed in your own garden or with permission.

Care

Water and feeding

Not needed. On rich soil it only grows bigger and produces more seed.

Weeding

In the vegetable plot, hoe or weed the seedlings from March to May while they are small. Older plants have tough stems and a deep taproot: pull them out when the soil is moist.

Spreading

One plant produces hundreds of seeds that stay viable for years. Do not let it set seed in the vegetable plot, and keep the soil covered with mulch or a green manure.

Soil

Where it dominates, the soil is often compacted. Loosen the soil, work in compost and walk on it less: grass and other plants will then gain ground.

Between paving

Scrape it out with a weeding knife or brush the joints regularly. Do not use chemicals.

Self-seeding

In a wild corner, let a few plants set seed: sparrows and finches will come to pick at it well into winter.

Tip: Knotgrass in your lawn or path is a sign of compacted soil: loosen the ground, route the path elsewhere and it will largely disappear on its own. Meanwhile, keep one wild corner for the sparrows.

Pruning and mowing

When: No pruning needed. In the vegetable plot, pull the plants out before they set seed, ideally before July.

How:

• Hoe or pull out young plants in spring: that is when it is least work.
• In a wild corner, leave a few plants until the seed is ripe: birds feed on it into winter.
• Leave dead plants lying in the wild corner. Do not put plants with ripe seed on the compost heap, or you will spread the seed with the compost.

Pruning calendar and basic rules

Propagation

Seed

Knotgrass reproduces only by seed. It germinates mainly early in spring, and the seed can survive in the soil for many years.

Spread

Seed sticks to shoes, tyres and tools and travels with soil and compost. That is how it turns up everywhere, especially on disturbed ground.

Knotgrass: how does it get through the winter?

Hardy to −15 °C: protect young plants.

• Knotgrass is an annual: the plants die in autumn, at the latest with the first hard frost.
• The seed overwinters in the soil; much of it only germinates after a cold period, early in spring.
• Seed left on the dead plants feeds birds in autumn and early winter.

Diseases and pests

• Weed in the vegetable plot: Forms mats between the vegetables. Hoe early in spring and mulch the beds.
• Between paving: Grows in the joints of paths and patios. Scrape out the joints or pour boiling water over it, then fill them with fine sand or mortar.
• Tough stems: The stems wrap around hoes and mowers. Weed when the soil is moist and pull out the whole plant, root and all.
• Compacted soil: A sign that the soil is compressed. Loosen it, add compost and lay a fixed path.
• Powdery mildew: In late summer a white powder sometimes appears on the leaves. Harmless: leave it.

Through the year

• March: Seedlings appear. Hoe in the vegetable plot.
• April: Weed while the plants are small. Mulch the beds.
• May: Pull out when the soil is moist.
• June: Flowering begins. In the vegetable plot, pull out before seed set.
• July: Flowering. Scrape out paving joints.
• August: Flowering and seed set. Leave standing in a wild corner.
• September: Seed ripens: birds come to feed.
• October: Last flowers. Sow seed from your garden on bare soil.
• November: The plants die. Leave them in the wild corner for the birds.

Did you know?

• In Shakespeare’s A Midsummer Night’s Dream a small character is insulted as being made of ‘hindering knot-grass’: people once believed the plant stunted the growth of children and animals.
• The name knotgrass refers to the knotty stem joints; the species name aviculare means ‘of the little birds’, after the birds that eat its seed.
• The seed can survive in the soil for years. That is why knotgrass often turns up unexpectedly where the ground has just been dug.