Worrying weeds

A fair description of a weed is that it is a plant that is growing where it is not wanted. Weeds come into the country by various methods. Some are brought in unintentionally, often mixed with the seeds of pasture grasses.  Many are allowed in as ornamentals, and it often becomes obvious that a foolish decision has been made. Others might be intended for food for humans or farm stock and yet others for the aquarium trade. Here are five plants, or groups of plants, to look out for and discourage their proliferation … 

Climbing asparagus (Asparagus scandens)
Climbing asparagus is probably the worst of our many weeds as it is so hard to eradicate. This is due to the tuberous roots, the thin, twining stems, and the very many small palatable orange berries that are produced.  Once established, it infests the roadside vegetation, the bush or wherever it germinates. Valiant efforts have been made in various places to deal with it – Sandspit comes to mind – but it is a never-ending job, where there will be a vast seed source in the ground, and, thanks to the birds, re-infestation is always on the cards. Digging out all tubers and disposing of them carefully is probably beyond most people’s ability. For plants taller than 60cm, cut down to 30 to 60cm and spray this lower vegetation carefully with glyphosate. Return to spot spray within 30 to 60 days.

Loquat (Eriobotrya japonica)
Presumably loquats were introduced as a food source, though the rather insipid taste of the round yellow fruit seems to appeal only to hungry children and kereru. Originally from Japan, loquat has also found the climate of northern New Zealand to be favourable for growth. Several planted trees on the roadside of McKinney Road, within a kereru flight of Parry Kauri Park, are the source of a weed problem within the park. For anyone not completely sure, the seedlings can be confused with seedlings of taraire, but loquat leaves have small teeth around the edges, whereas taraire do not. The only good thing about loquat seedlings is that they are easy to pull from the ground, unlike monkey apple or privet seedlings. Eradication of trees is the same as for Taiwan cherry.

Ladder fern (Nephrolepis cordifolia)
It is understandable why people plant ladder fern (sometimes known as tuber sword fern), as, together with agapanthus, it completely covers a bare bank or a long driveway, leaving no room for other weeds to penetrate.  An introduced fern from the tropics, it is the only fern in the country to produce tubers. The long rhizomes (mass of roots) and these plentiful tubers enable it to spread rapidly. The many tubers also make it a very difficult plant to keep under control or eradicate. There are two reasons why it becomes a nuisance, the first being that it knows no boundaries, so readily slips under the fence and infests places where it is not wanted. The second is that when a gardener wants to get rid of it, they are inclined to dump it over a bank somewhere, often into the bush. This has occurred in the bush at the eastern end of the Omaha causeway, and on the side of the road leading to the Cement Works. To get rid of ladder fern, remove it by hand. Alternatively, spray with metsulfuron-methyl 1g/1L and then leave for four months to allow the herbicide to translocate to the tubers.

Taiwan cherry (Prunus campanulata)
Taiwan cherry is a very attractive deciduous tree which displays deep pink bell-shaped flowers in the spring.  The flowers attract tui in large numbers, and the fruit is spread by kereru and other fruit-eating birds. It is the spreading of the fruit that is the problem, as conditions in northern New Zealand are perfect for germination, and Taiwan cherry is fast becoming a very bad weed. If one travels north of Warkworth in August, a very obvious ring of pink surrounds Whangarei, Kawakawa, Kerikeri and Kaeo, and at Pukenui a gully on a side road is full of the “pink plague”. It is beginning to appear locally on Sandspit Road, on the main highway south of Warkworth, in Morrisons Reserve on Kaipara Hills Road and in Parry Kauri Park. The deep colour of the bell-shaped flowers is the distinguishing feature – other flowering cherries with flatter, softer pink flowers are no problem. To get rid of Taiwan cherry, cut down the trees and apply herbicide to the stumps.

Oxygen weed (Egeria, Elodea and Lagarosiphon species)
These three species of oxygen weed have been introduced to New Zealand by the aquarium industry and have infested many of our waterways, although none of them has been known to fruit here. They move to new sites by various methods, including by fragments clinging to boats and trailers, to the feet and feathers of water fowl, to scuba divers, and by overflows from ornamental ponds. Various manual methods of control have been tried, with limited success. Diquat herbicide can eliminate them from lakes, as can grass carp. Grass carp eat everything in the lake, but seeds from native water plants can survive in the mud at the bottom of the lake.  Grass carp do not breed in New Zealand, so they can either be left to die out (about 30 years) or they can be fished out. From the Sesquicentennial Walkway, by the Bridgehouse in Warkworth, it can be seen that Egeria grows in abundance in the slow flowing fresh-water reaches of the Mahurangi River, and in summer the white flowers on the surface of the water are quite noticeable.

For a comprehensive guide on weed identification and control, visit the Department of Conservation website at: doc.govt.nz/nature/pests-and-threats/weeds/identifying-weeds