What is it?
Don't be fooled by its name - climbing asparagus (Asparagus scandens) has very little in common with our delicious garden asparagus. It is, in fact, a climbing vine that can't be eaten, and which has the potential to smother native forests.
It is easily recognised by the bright green leaves and stems, which scramble or climb across the ground and into surrounding vegetation. The thin wiry stems can be up to four metres long, and are covered with many spiky leaves.
From October to February, climbing asparagus will also bear lots of small round berries that ripen from green through to orange and red. The flowers are generally inconspicuous, being small and white, appearing from September to December.
A further way of identifying the plant is to look at its roots, which are small white tubers.
Why is it a problem?
Smothering native vegetation
Like so many other introduced climbers, climbing asparagus is capable of smothering areas of native vegetation through its scrambling habits, and is difficult to control once established.
Climbing asparagus particularly affects the forest floor and the understorey up to a height of about four metres, especially in lowland and coastal forests. Once established, it can prevent the growth and regeneration of native species. It can even strangle and kill softbarked shrubs and trees as it wraps itself around their trunks.
Climbing asparagus is hard to control once established, because birds tend to eat and disperse the seeds in their droppings. In addition, any tubers remaining in the ground will often resprout even if the rest of the plant is removed.
Methods of control
Small areas of climbing asparagus can be pulled out by hand. However, to be effective, the tubers must be dug up and removed as well, otherwise they will simply resprout. Even after digging them up, it is likely that some fragments will remain in the ground, requiring follow-up treatment at a later date.
Careful disposal is required. Tubers should be buried deep into the soil, burnt, or placed in a sealed black plastic bag and left to rot in the sun. Stems do not require this treatment.
Under the Biosecurity Act 1993, it is illegal to sell, propagate or distribute climbing asparagus and smilax.
Glyphosate-based herbicides, such as Roundup, Zero and Touchdown, are particularly useful for controlling climbing asparagus. Penetrants, such as Pulse and Boost, should not be added.
Take care to spray only in still conditions to avoid wind drift to non-target plants, and don't spray when rain is expected. During spraying, non-target plants can be shielded with cardboard, plastic sheets or a large plastic container. The use of a marker dye helps to avoid double spraying and wastage, and a foaming agent can be added to the spray to help prevent spray drift. As with all spraying, you should read the instructions on the manufacturer's label closely and always wear protective clothing.
The herbicide should be applied at the rate of 20ml per litre, and is best done during spring and early summer. The entire plant does not need to be covered in spray, so only a light coverage is required. If it is necessary to spray in autumn or winter, the herbicide should be applied at the rate of 30ml per litre and should only be undertaken in frost-free areas. In order to reduce the amount of herbicide being used, cut larger vines at a height of about 30-60cm above ground and spray this lower part of the plant only. The remaining cut material will die naturally if left in place and will not need spraying. It will be necessary to follow up any control work at six-monthly intervals, as it is likely that some plants will resprout from the tubers remaining in the ground. After two to three treatments, it may be useful to replant the treated area with a suitable ground cover to suppress any further regrowth.
A related problem plant is smilax, or Asparagus asparagoides. Like climbing asparagus, smilax is a scrambling perennial that can smother low-growing plants and seedlings in lowland and coastal areas. It can be distinguished from climbing asparagus by its leaves. Unlike the short spiky stems of climbing asparagus, smilax has flattened leaves up to 35mm by 15mm, with a pointed tip. Control methods for smilax are the same as for climbing asparagus.
Contact any Department of Conservation office for further information on the identification and control of invasive weed species.
A useful reference book is "Native Forest Restoration: A Practical Guide for Landowners" by Tim Porteous (Queen Elizabeth II National Trust, 1993).
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