St John’s wort

NZ Lifestyle Block - - Contents -

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.

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