Without doubt one of the most important jobs for any gardener is the suppression of weeds. They will, given half a chance, destroy our desired plants; out-competing them for light, space and nutrients. However they are also an inextricable part of our local ecology and we may achieve better overall results by aiming at their rigorous control rather than their total elimination. (To be fair though there are some such as bindweed and Equisetum which can never be tolerated at all or they will quickly take over entirely.)
The reasons why weeds must be controlled are numerous and within any garden it is absolutely essential that good and effective control is established initially, and confidently, before any attempt is made to allow some weeds back in again. But if this proviso is met then some weeds can be utilised as an extremely valuable resource. Often simply because of their sheer fecundity and robust ability to survive in adverse conditions. This is after all what made them weeds in the first place!
Weeds make excellent ground cover as green manures on soil that would otherwise be bare. True, a selected green manure could be used but why bother when nature offers a better alternative. Providing the natural weed cover can be incorporated or stripped off for composting before any aggressive perennial weeds become too well established then the mix of native weeds is likely to provide a more varied and better material than conventional green manures. And there are few green manures as hardy and vigorous through winter as our native weeds. Because our weeds are self selected those that infest a piece of ground are the most well suited plants available for that piece of ground, to repeat myself-that is why they have become weeds in the first place!
The weeds advantage becomes more so when the soil is lacking in any particular element, the weeds that predominate on such a soil will be those plants that either least need that element or are most successful at obtaining it. Weeds are often good accumulators of such scarce elements and it is this ability that makes many of them so detrimental to our crop plants who can never obtain any quantity in competition with such weeds. However if a 'burden' of weeds is grown before the crop and incorporated in situ then the materials accumulated become available to our crop in greater quantities than could be extracted by that same crop from the soil unaided.
For example one rare but pernicious weed, thornapple (Datura/ Brugmansia stramonium), is extremely effective at extracting phosphorus from the soil solution. As thornapple removes the phosphates a fractional amount dissolves from the surface of soil mineral particles which only slowly relinquish it. In addition the soil solution's phosphate levels are added to by the soil micro-life which liberates phosphates (again mostly derived from insoluble mineral particles) to also be removed by the thornapple. If the thornapple were not to continually remove such free phosphates as exist from the solution these would rapidly become locked up as an insoluble mineralised form again. The thornapple's tremendous attraction removes the phosphates and converts them into green material, this once composted and returned to the soil now carries high levels of phosphate but in a more stable and assimilable form. Thus allowing a weedy cover of thornapple plants can increase phosphate levels in a phosphate deficient soil.
As with any green manuring a flush of such weeds must be utilised before the nutrients are converted into another inaccessible form; seeds. Once weeds have set seed the goodness is locked up inside and it takes a very hot composting, water logging or the passage of time before the seed coat breaks down and the soil micro-life can gain access to this wealth. The corollary of this is that a vast amount of goodness is stored up in dormant weed seeds in the soil. Until they die or germinate the nutrients inside may as well not exist. If we allow the seeds to germinate then they become available and can be restored to the soil. Thus allowing a flush of weeds to appear and then promptly dealing with them is probably doing more good for soil fertility than merely suppressing their germination under a mulch or with deep burial.
A stand of succulent weeds can also act as windbreaks and nurse crops to small seedlings that would find the conditions rather bleak unaided. By removing strips of weeds and sowing in their place a crop can be established earlier as now protected by the micro-hedges created on either side. For example a row of early peas only needs a clear space a handwidth or so across, the weeds on either side can be left until the peas are an inch or two high and have toughened up. Then the weeds can be killed and incorporated under a grass clipping mulch dumped on top of them, their fertility soon becoming available to the peas.
An even more complicated argument concerns weeds and pests. We are told we must remove weeds as they harbour pests, this is true but these pests automatically provide an ongoing supply of their own predators and parasites. Furthermore the weeds themselves may also be necessary for providing those predators and parasites with the nectar or pollen they need to be able to breed so that their offspring can go on to control the same, or other, pests on our crop plants.
Because weeds will often thrive where other plants fail then they can be allowed to prosper in places that would be hard to plant up otherwise. Shady and/or dry places are notoriously difficult to grow anything desirable yet left neglected they are soon colonised by weeds. These may provide for the wildlife and then later become compost material to no detriment to the garden as a whole. Indeed some weeds such as dandelions, lesser celandines and dead nettles are flowering earlier in the year than most of our garden plants and can provide an attractive display while feeding up the bees. Even the much damned stinging nettle is well known for being a host to some of our prettiest native butterflies.
Most useful of all deliberate weed applications are grass bunds. I used to keep the sward under my rows of hybrid berries, around fruit trees and in front of hedges immaculately neat and well cut. I found that allowing a foot or so wide strip to become 'weedy' dramaticallyincreased the number of ground beetles (and grasshoppers), without damaging the yield of the established crop. (Ground beetles are important predators of pest eggs such as slugs, snails, cabbage root fly et al.) I have 'tarted up' the bunds by planting them with spring flowering bulbs so now I cannot cut them too early or too neatly. And I am well pleased by the wild flowers (weeds) that have also thrived in the miniature meadow these bunds create.
Of course there is one other aspect of weeds we tend to overlook and that is their nutritional value. As many of them are rich mineral accumulators so some are also valuable additions to our diet. Indeed too many of our weeds are edible to list them and our ancestors did not think it strange to eat stinging nettle soup, dock pudding or to add red and white dead nettle flowers to salads containing blanched dandelion leaves. And that bane of modern gardens, hairy bittercress, is as nice to eat and as nutritious as watercress!