Plant Doctor Archive

Bank covered in weeds

CAN you help us with a weed that is overtaking everything planted on our bank? It has pink flowers like a sweet pea, climbs to a considerable height and when you cut it down it just grows away again from the root system. We've tried spraying it with all kinds of weedkillers but nothing seems to work.

I CHECKED with sweet pea expert Keith Hammett and it sounds like your problem is caused by the perennial sweet pea Lathyrus latifolius. It comes from Chile and really shouldn't be called a sweet pea at all as its flowers, which can be pink, rose or white, have no scent. The popular scented sweet peas are mostly bred from forms of Lathyrus odoratus which originate in Italy.

Keith suggests you should be able to control this weedy variety by cutting them back to the ground, allowing new growth to reach 30cm or so high, then spraying it with a glyphosate-based herbicide such as Roundup or Glyphosate. It will take several weeks for the foliage to die down and for the next year or two you'll almost certainly have to keep spot-spraying new growth until the fleshy root system has either absorbed enough of the spray to kill it or dies of exhaustion.

There will also be seeds in the soil which will continue to germinate over the next few years and you'll need to either pull these seedlings out while they're still small or spot spray them.

When spraying, make sure you thoroughly cover all foliage but avoid getting it on desirable plants.

Other products you could try include Activated Amitrole and Escort. Adding a sticking agent such as Pulse or Sprayfix to the mix helps improve the effectiveness of these sprays, so check the label and follow the recommendations.

Also perhaps worth a try is Vigilant which comes as a gel that is applied directly to the cut stems at the base of the plant immediately after you chop off the top growth.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.