How to solve the eternal problem of 

perennial weeds

Perennial weeds are those that die down in winter and re-emerge in spring.  They can be difficult to control, particular when growing among cultivated plants in beds and borders.  Weeds such as bindweed, ground elder, couch grass, nettles and Japanese knotweed are among the toughest problems gardeners have to deal with. The best time to combat these troublesome invaders is over spring and summer when they are actively growing.  Weed control is not easy, but there are methods that can make success more rapid.  Here are various methods to try, both cultural and chemical (weedkillers).  Combining several eradication techniques may be the best way to ensure good wee control and minimal regrowth.


Digging out roots

What it does: removing just the leaves of perennial weeds only gives temporary control, as most have extensive root systems that will re-shoot. Digging out root systems with a hand or border fork may be an insurmountable task (and only feasible in badly weedy areas if cultivated plants are lifted and grown elsewhere until the area has been cleared).
When: spring/summer, when weeds are growing strongly.
Effective against:  weeds that spread by rhizomes, such as bindweed, ground elder and stinging nettles, tubers (lesser celandine), and taprooted species such as dandelion.
Typical situations: most effective in areas not too closely planted, such as new borders before planting, or where neglected allotments or vegetable beds need overhauling.
(and, according to Bob Flowerdew, if for any reason you can't dig out the roots, just keep taking off the tops as it does eventually weaken the plants, see article below. This appeals to lazy side of me!)


Loose mulches
What they do: depending on the material, mulches can suppress annual weeds, improve soil texture, help retain soil moisture, encourage beneficial  soil organisms - and look decorative.  To avoid perennial weeds re-emerging clear the area and cover with a barrier first (see ...**)
Typical materials: garden compost, leaf mould, well-rotted manure, wood chips or gravel.
When to apply: spring or autumn, should be a minimum 10 - 15cm deep.
Typical situations: new plantings or existing beds.  Keep the mulch clear of woody plant stems.
(and with mulches look out for slugs as these menaces love to hide in the dark damp areas)


Weedkillers
Synthetic weedkillers can be divided into three types, based on their mode of action.

1. Unselective formulations kill any green plant material.  Some are persistent, remaining active in soils for months. (Good for paving or driveways, either ready-to-use or concentrates to dilute in a sprayer)
2. Selective weedkillers target certain weeds, such as broad leaved weeds in lawns with little effect on grasses. (for specified weed species, such as weeds in lawns - products often contain more than one chemical)
3. Systemic formulae's active ingredients travel through treated plants, killing both the top growth and the roots (well-known glyphosate is both unselective and systemic).  Systemic weedkillers do not persist in the soil, so the area treated can be replanted immediately. (for tough weeds such as ground elder and horsetail, several treatments may be needed)

Applying all types of weedkiller when temperatures are warm and weeds are actively growing gives the best results.

Barriers
What they do: vertical or horizontally laid barriers that physically block the spread of perennial weeds into cultivated areas.
Vertical barriers: many materials can be used: edging boards or strips, paving slabs or bespoke root-barrier fabrics, made of spun or woven material, sunk vertically around the areas to be protected.  Plastic sheeting is a short-term solution as it tends to deteriorate relatively quickly 
Horizontal barriers: clear weeds from the area to be planted, then lay a weed barrier in the form of a permeable weed-suppressant fabric over the soil surface.  Plant through x-shaped holes cut in the fabric.  Cover with a layer of soil or a thick mulch to disguise the membrane and help protect it (how long it will last depends on the material).
When to apply: ideally, later winter or early spring, to be in place before weeds start growing.
Effective against:  unwanted grass growth into borders and suppressing regrowth of old weeds.  Installed vertically, barriers are particularly useful for stopping the spread of persistent weeds into your plot from neighbouring gardens. 
Suitable for: all weeds, but especially ground elder, horsetail, bindweed and couch grass, which will be weakened over tie. However, ornamental plants may need to be dug up and moved to other weed-free areas for up to a year.

Repeated cutting
What it does: repeatedly cutting foliage to ground level with a string trimmer or sickle may - eventually - weaken and kill weeds.
Effectiveness: only for large weedy areas, and definitely not a short-term solution.
Typical weeds to treat: horsetails, thistles, nettles and brambles.


Other methods
Heat: hot water left from boiling vegetables, or the kettle, is useful to knock back weeds on the patio.  A flame gun (powered by gas canisters or paraffin) is another effective method, but can be quite slow. (when I was a small child I can remember an elderly (or so she appeared a the time!) neighbour who used to always pour her vegetable water onto the weeds at the front of her cottage)
Scraping or brushing off: this clears weeds from cracks in paving but you may get tired of repeating the job before the weeds do.  Residual weedkillers are longer-lasting alternatives (for gravel and other permeable surfaces only).


By the way, although they're weeds,  nettles are brilliant for adding to the compost heap - or making into a lovely stew to fertilise the plants.  

In spring or summer, soak chopped-up nettles in a bucket of water for two weeks, preferably in warm sunshine.  Then dilute, at a ratio 1:10 and water leafy salad crops (not carrots as then you'll just get lots of leafy bits and tiny carrots).  It'll give them nitrogen, iron, magnesium and sulphur (but not phosphate).  

Even better, add some comfrey (and even some horsetail).  Later, pour any dregs onto the compost heap.

So for that reason - and because helpful ladybirds love them - it's a good idea to always leave a few nettles in a secluded part of the garden so you can call upon them whenever you like.


From an article by the RHS.