Judith Tromp, of Hampden, asks :-
Can you tell me what this weed is please and what we can do to eradicate it from my garden?
Roy Edwards, a horticultural botanist at Lincoln University, responded.
The weed in the attached photos is Galinsoga parviflora (commonly called galinsoga or yellow weed, although I have also heard it called gallant soldier). Galinsoga is one of many weed species that originate from the daisy family (Asteraceae). Think of dandelion, lawn daisies, thistles and yarrow. A feature of this family that assists the spread of plants, are the bristles or scales attached to the seeds, making them often easily blown to other places.
Galinsoga originates from South America and was first recorded as an adventive in New Zealand in 1896. It is now quite widespread, particularly in cultivated areas, but may also be a weed of pastures and waste sites. In a home garden situation galinsoga is a relatively easy weed to deal with. Galinsoga is weak stemmed and fibrous rooted and may be best forked out and simply composted with other weeds. It does not have any rhizomes or stolons (underground or over ground stems) that make weeds with those often more of a challenge to remove.
In a home garden I would just keep removing galinsoga, it is not a particularly difficult weed to pull out, especially when it is small. Persistent weed removal is the answer, over time, after all seeds have finally germinated and the seed bank depleted, there will be no further problems.
Pulsar(r) is listed in the 2009 'New Zealand novachem agrichemical manual' as a herbicide to control galinsoga in pastures, other herbicides are listed in this same publication to control galinsoga in other situations, many of these are unavailable to home gardeners. For the home gardener the commonly sold glyphosate based herbicide is likely to work if the weeds are sprayed while still young. As always, care is needed in applying herbicides and the directions on the product should be strictly followed.