'Dear Garden Answers, we have a 200ha site in East London which we recently discovered to be infested with Japanese Knotweed at various locations totalling 4 hectares. We have to start building on this ground next summer. What should we do? Would mulching help?' Yours sincerely, David and Tessa.
Dear David and Tessa Thank you for your interesting enquiry. The usual question we ask of property owners of this kind is why on earth did you buy it if there was such a bad problem?
Surely you must have noticed the difficulty, since Japanese knotweed grows 2m high. But as you are stuck with the plot now we must find a solution.
It is important to get rid of all the underground parts of the weed since even small pieces left in the soil will grow and you will be back to square one.
It is no good trying to remove it with a hand trowel - you need to do some serious digging to get the roots out because, if established, they will be woody and diffi ult to extract. Over such a large area you may even need earth-moving equipment!
And to make sure you get the lower roots out, you may need to excavate deep enough for an Olympic-sized swimming pool.
Perhaps you have considered that.
Mulching is really not much use because the new shoots will grow up, undeterred by even a generous layer of bark or compost. The only real way to prevent growth is with a thick mulch of something permanent - perhaps some roads, houses or a velodrome may be effective.
If you do not want to excavate the whole site there are some chemicals that may be effective.
Glyphosate is a possibility because several applications will control and finally kill the growth. Herbicides containing ammonium sulphamate are also worth considering. Although they poison the soil in the short term, giving total weed control, the chemical is converted to the fertiliser ammonium sulphate after several months.
Of course, if you are concerned about maintaining the land organically and do not like the thought of applying chemicals to what may be pristine agricultural land, you may not want to use any of these.
The only other alternative is to dig it out by hand. This would be back-breaking work but, if you know of a source of cheap labour, perhaps from eastern Europe, it may be worth considering.
However, it may take a long time and could delay any plans you have for your plot.
It depends how important the completion date is for your project and if there would be any problem in it not being ready in time.
We hope that helps The team at Garden Answers Garden Answers is a monthly gardening magazine available in all good newsagents.