An introduction to the arable weed seedbank

Soil contains many weed seeds – this is called the ‘seedbank’. For optimum management, it is important to know the weed species present, as well as their abundance, status and distribution in the soil profile.

Weed seeds are distributed throughout the soil profile. Like a conventional bank, deposits add to, and withdrawals subtract from, the seedbank.

To take a snapshot of a field’s seedbank, allow weeds to grow in representative untreated areas and count the species present. Typically, this best reflects the weed profile in the top 5 cm of the soil.

Adding to the seedbank

• Local weeds (most significant source), sometimes scattered by the combine and cultivations
• Imported seeds from contaminated machinery, crop seed, manure, slurry, sewage sludge, compost and anaerobic digestate

To reduce weed-contamination risks associated with manure, compost it, dry it and store it for at least eight weeks. Any compost that conforms to the BSI PAS 100:2011 standard should be free of weed seeds; however, the equivalent standard for products of anaerobic digestion, BSI PAS 110:2010 (digestate, separated liquor and separated fibre), does not contain a requirement to test for weed seeds.

The most difficult-to-control weeds produce high levels of seed and can establish large viable seedbanks in just one season. 

Subtracting from the seedbank

As seeds age, they become less viable. The rate of natural decline varies dramatically between weed species.

Under 1 year: Soft brome, rye brome, barren brome, volunteer cereals, oats, sunflower and linseed.

1–5 years: Perennial rye-grass, black-grass, winter wild-oat, chickweed, crane’s-bill, creeping thistle and mayweed.

Over 5 years: Wild-oat, loose silky bent, Italian rye-grass, orache, black bind-weed, charlock, common poppy (can persist for over 50 years), speedwells and volunteer rape

Seeds are also consumed (e.g. by birds) and germinate, mainly from the top 5 cm. Seed that are buried deeper, apart from a few larger seeded species, seldom emerge.