The Burden of Weeds

by Three Hagges Wood Meadow in Flora (Plants), Management, Weed control 1

Whenever anyone asks that question about what exactly is a weed, the commonplace and altogether unhelpful answer is usually that it is simply a plant/wildflower in the wrong place. This is at best a half truth.

Weeds, and especially those pernicious and obnoxious weeds that we expect to have to deal with on the site of Three Hagges Jubilee Wood, are a group of particularly robust –even aggressive ‑ plants that have developed a number of strategies to ensure their own successful survival.  They are seldom fussy as to soil type, and often are expert and hardy colonizers of bare ground.  They are prolific in the production of fertile seed, and profligate in its distribution, often blown by the wind over considerable distances – as with most thistles, and the ragwort, Senecio jacobaea, which flowers from June to November, producing a potential 150,000 seeds from a single plant.

Ragwort, Senecio jacobaea seedheads. Photo: John Potter

Anyone familiar with the burdock, Arctium lappa, will also know how efficient they are at spreading seed on fur or clothing, attaching themselves obdurately by tiny entangling hooks.

Burdock, Arctium lappa, seedheads backlit and showing tiny hooked seeds. Photo: Gwen and James Anderson, licensed the Creative Commons Attribution-ShareAlike 2.0 license

 

The broad leaved dock, Rumex obtusifolius, can bear up to 60,000 ripe seeds per year, shedding them between late summer and winter.  Weeds seeds germinate freely, often at low, late autumn temperatures, and can grow on even in the cooler months, thus getting a head start on less forceful species in spring.

Dock, Rumex obtusifolius,in seed and unmanaged. Photo: Evelyn Simak, licensed under the Creative Commons Attribution-ShareAlike 2.0 license

 

The most pernicious weeds are perennial, and can be very long lived. Like the dock, Rumex obtusifolius, common nettle, Urtica dioica, and couch grass, they are fiendishly clever in the rapid formation of deep, fleshy and extensive root systems.  The creeping thistle, Cirsium arvense, not only produces seed in profusion, it persists from year to year, spreading by creeping underground roots that may be 5m or more in length, so that it can easily form a colony 6‑12m across in a single year.  It can also regenerate new plants from the tiniest root fraction, either having been chopped up during cultivations, or by root pieces collected and distributed by small mammals.

It’s not only flowering plants that can be aggressive colonizers.  A number of grasses, such as false oat grass, Arrhenatherum elatius, and rye grass, Lolium perenne, are noted for their vigour, and indeed that is why some are grown as crop plants, for silage.  Cocksfoot, Dactylis glomerata, is a stout, clump-forming grass that may reach 60-150cm in height. It starts into growth early in the spring, with a deep and extensive root system, and tolerates shade and flooding.

Yorkshire fog, Holcus lanatus. Photo: Robin Stott, licensed under the Creative Commons Attribution-ShareAlike 2.0 license.

The very familiar Yorkshire fog, Holcus lanatus, has similar attributes, and in addition flowers from June to September, producing 177,000 to 240,000 seeds per plant.  Seeds are viable within 10‑20 days of flowering, and germinate immediately over a wide range of soil temperatures.  It tillers profusely, spreads by producing new shoots and roots at the nodes, forming a dense mat of runners on the soil surface that physically excludes other plants.  And as if that weren’t enough, it may also produce chemicals that suppress the growth of other plants in its vicinity.

So that just about covers what makes a weed a weed.  Their exploitation of all their various survival strategies is a wonder of evolutionary biology.  They are successful and highly efficient competitors.  Their aim in life is to sustain their own rapid root and shoot growth, and they will deprive neighbours of light, nutrients and water as they do so, whether these are other perennials, grasses, or small, newly planted trees. They crowd out less robust wildflower seeds, and make life impossible for any that are newly germinated.

Weeds have no sense of social responsibility, they are brutes, and the lives of those around them often short.  We have to discover ways and means of subverting their survival, and evening up the competition if our more choice wildflowers are to survive and thrive. And our strategies will need to be multiple, cunning, and consistent, to be applied from the bare soil stage onwards. The first will be preparation of the stale seed bed to help reduce the weed burden. The second is sowing a seed mix whose components are carefully balanced with regard to their competitive potential, and their suitability for the site.  The third, fourth and fifth will be management, management and management.