We are often advised that as perennial weeds will survive home composting using cold composting techniques they should not be composted at home but put in the “Green” bin to be collected by the Council as part of their kerbside collections. This will means that the weeds are hot composted by the council and then purchased back by the home gardening as part of commercially available composts.

However in most cases there are options available to the home composter faced with perennial weeds involving pre-treated so as to make them safe to be cold composted. These options include drowning, desiccation and starvation/light deprivation for at least two years.  

Before considering composting plants that may be toxic or invasive please check the advise on the  Toxic Invasive Plant page.

While normal cold composting, using a one year cycle, will not kill the roots of perennial weeds composting in a covered heap, from which the light is excluded, for a period of two years has been reported as being effective for many perennial weeds.  When I took on my first allotment plot it was covered in perennial weeds and I took the lazy approach of making a pallet bin using four pallets, filling it with weeds and leaving it until the pallets rotted. Although there were a few roots struggling to survive on the surface where some light had seeped in, the compost was excellent. When using this and any other method of treatment on this page it is recommended to check the compost at the end of the treatment to ensure the absence of living roots before use.

ASK Organic  (http://askorganic.co.uk/composting/perennial%20weeds.pdf) recommend using a separate plastic compost bin so as to ensure that all light is excluded which may not be possible if using a wooden bin. The material is kept covered in the bin for two complete years to rot down. 

An effective variation involving light exclusion is recommend by Alys Fowler

(https://www.theguardian.com/lifeandstyle/2014/mar/01/alys-fowler-perennial-weeds)  writing in the Guardian

The weeds are bagged using biodegradable bin bags and stored for two to three years.  The bags are covered with black plastic to exclude light and air and allowed to degrade. By the time the bags have degraded rich compost should be produced. One of the advantages of using biodegradable bags is that as the bags degraded worms gain access to the material assisting the composting process. Again the finished compost should be checked to ensure rots have not survived.

 Garden Organic suggests a quicker method using plastic bags but it is only suitable for use during the summer when the lawns are mowed regularly. This involves mixing the weeds with fresh grass mowings in a plastic bag e.g.  an old compost bag. The bag containing the weed/grass mixture is tied up and left in a sunny spot until the weeds are no longer recognisable. Some suggest the weeds will be sufficient decomposed to add to the compost bin but I would recommend leaving it a month or two. Take care when emptying the bags as the contents may best be described as slimy. 

Drowning is one of the quickest ways of dealing with perennial weeds and it has the advantage of producing a compost activator that can be added to the compost bin or used as a liquid plant food.

The perennial weeds are put into a lidded bucket, old Bokashi bin or water butt depending on the quantity involved and weighed down with a brick or stone. The weeds are covered with water, the lid put in place to exclude light and the soup left for four weeks. The liquid is then strained off to be used as diluted and used as a plant food or the complete contents of the bucket added to the compost bin.

This method requires a hot sunny summer to work most effectively.  Spread the perennial weeds, or their roots, on a dry flat surface such as a concrete paving slab in full sun.  To be on the safe side I like to smash and flatten them with a club hammer at this stage to assist the drying process.  Once smashed they can be left on the concrete slab or moved to dry on a wooden cover on the top of the compost bin if a new Zealand or pallet bin is being used.  After two or three weeks the roots will be sufficiently  baked to be  safely added to the compost bin.