Paddy Lucerne (Sida rhombifolia) is a weed that is beginning to ring alarm bells for me. It is locally common in some coastal areas of the Opotiki district. It is generally a weed of rough pasture, and is particularly invasive in horse paddocks. However I am finding more and more infestations of this weed in relatively good pasture land throughout the coastal area and pastoral farmers particularly should be on the lookout for it as it is unpalatable to stock. It is a problem weed in tropical and sub-tropical areas worldwide and here it could be the next gorse.
The plant can grow to 1.5 metres high, but on poor ground is usually much smaller. Plants on good land at Omaio are about waist height, but those on the stony land near the beach at Torere barely reach knee height. The plant is bushy with tough, twiggy stems. It can form very dense stands, eventually completely dominating pasture. The leaves are dark green, small (2 cm) and serrated. The flowers are very small, pale orange and arise from the leaf axils. In late summer the plant is covered in small, hard, round seed cases. It tends to lose much of its upper foliage in the winter but greens up again in the spring.
Control of the plant is difficult. Having hard, woody stems puts it in the brush weed category. It could be controlled by mowing, but this usually only checks the plant, and it will sprout away again from the base. Attempting to pull it out will only result in a sore back!
My experience so far suggests that paddy lucerne is quite resistant to most herbicides and only metsulfuron (eg. Escort® or Meturon®) gives good control. As with any pasture weed, keeping a healthy pasture without overgrazing is the best defence.