Question: My garden has been invaded by what I believe to be three-cornered leeks. Any thoughts on how I can remove them? I have tried pulling out the plants just after flowering and before they set seed, but this doesn't seem to have any effect. Also, digging them out is really tricky as they have so many tiny bulbs and they get mixed up with the plants I would like to keep.
Answer: The three-cornered leek, Allium triquetrum, is rather a stubborn garden inhabitant once it takes up residence. The non-chemical method if the problem is just in a small area would literally be to dig up everything, including your garden plants, and start over. You would need to replace the soil to a spade's depth. This, of course, is not ideal, so you could try the chemical control approach. First, bruise the leaves with a gloves hand to make it easier for the chemical to be absorbed, and very carefully spray a glyphosate-based weedkiller such as Roundup or Tumbleweed just to the offending garlic. There is a also a Roundup 'spot' gel which may be easier to apply.
Send your questions for the RHS to: firstname.lastname@example.org.
Only a few questions per month can be answered. For further advice on handling problems in your garden, visit www.rhs.org.uk/advice.