Q&A: Help! My garden is being invaded by Japanese knotweed

Horticultural advice from the experts at the RHS
Question: Will a systemic weedkiller get rid of Japanese knotweed? Is there anything that will see it off? My partner and I are prepared to work at this. We're scared because we've heard that you can't get a mortgage if you have it on your property.

Answer: If there’s one weed that’s guaranteed to strike fear into homeowners and gardeners it’s this one. For good reason too, since there have indeed been instances of mortgages being refused.

Persistence is the watch word here. The most practical method is to spray with a systemic weedkiller that contains glyphosate. The best time to do this is when the plants are a metre tall so that all parts are easy to reach. You will need to repeat the treatment in midsummer and then again in autumn as the plant starts to die down for autumn.

An alternative way to deal with Japanese knotweed is to cut the stems down during the growing season and apply Roundup Tree Stump & Rootkiller directly to the canes. Even then, the fight may not be over, so make sure you check for regrowth the following spring.

Regular digging up of small areas of knotweed will eventually weaken the root and burning of the material is the best way of disposing of the material as it is classed as ‘controlled waste’ and must not be sent to landfill sites.

Although a sap sucking insect was introduced in some trial areas of the country within the past few years, the march of this weed does not appear to have been significantly curtailed.

Send your questions for the RHS to: gardenproblems@standard.co.uk.

Only a few questions per month can be answered. For further advice on handling problems in your garden, visit www.rhs.org.uk/advice/index.asp.

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