St John’s wort

WHEN I WAS first asked to contribute weed articles to NZ Lifestyle Block magazine, I had no idea that five years later I would still be finding weeds and writing them up.

But like all good things, everything has an end. I have recently been given a promotion and the time is right for me to let someone new come through.

I have loved hearing from you, which makes it fitting that I finish up on a question from a reader. Beth recently wrote me an email seeking help identifying a weed.

It turned out to be St John’s wort ( Hypericum perforatum), a common perennial weed found all over New Zealand especially in high country or very dry country like Central Otago (where Beth found it). It’s originally native to Europe and Asia but is now widely distributed around the world.

St John’s wort is a member of the Hypericum family of weeds which are commonly known as the St John’s wort family. Confused? You should be. All 500 or so individual species of this family are known as St John’s wort which doesn’t make identification easy. Thankfully, most of the time in NZ, you will be finding common St John’s wort.

St John’s wort is a particularly toxic weed to livestock, especially horses, with a number of confirmed deaths reported. Thankfully, animals tend to avoid eating it.

It has a long history of being used as a herbal medicine, although modern research has found a number of unforeseen interactions with other medicines so professional advice and care are needed around its use.

One last fun fact: the name St John’s wort comes from its tendency to flower in the northern hemisphere around St John’s day ( June 24).

It germinates in spring and forms stemmed plants up to around a metre in height. These are covered in 1-3cm long, oval, stalk-less leaves growing in opposite pairs up the stem. From December to March, plants produce a large number of small, yellow, star-like flowers with (usually) five petals, at the very tops of the stem. These produce a large amount of seed. During autumn, the stems die back leaving small shoots that run along the ground and these can form dense mats of vegetation. If this wasn’t bad enough, plants also produce rhizomes (underground stems) that can aid in colonisation of the area.

How to control it

This is tricky as it behaves more like a brushweed than a broadleaf weed. The rhizomes prevent hand-pulling from working effectively, but this can be a useful tool if you get to plants early.

You can spot spray it or weed wipe it with glyphosate but you need to do this before it flowers. This solution will kill all other plants around it though.

You can also use Tordon 2G granules. These can be sprinkled around the plants and give excellent control but they will kill any clover in the patch.