Although blackberry (Rubus fruiticosus complex) is found in most districts of New Zealand it is mainly a problem weed in Northland, Central and Eastern districts of the North Island, Nelson and the West Coast. There are several varieties or strains of blackberry found in New Zealand but they all have long trailing stems or canes, covered with thorns. These canes grow in an arc from the crown of the plant and where they touch the ground their tips may take root. The flowers are in clusters at the end of short branches and have five pale pink, mauve or white petals. The fruit is the well known succulent blackberry.
Blackberry seeds are spread by birds and often infestations arise around stumps, rocks etc. Seedlings develop very slowly and are taprooted for a year or so before becoming rhizomatous i.e. producing underground spreading rooting systems.
The preferred method of controlling blackberry is spraying. Goats will graze blackberry and keep it in check, but they need to be well fenced and confined to ensure they do eat it. Because goats will rarely eliminate blackberry, spraying of grazed bushes will still be required.
The best time to spray blackberry is from flowering until early leaf fall when ample soil moisture is present and the blackberry is growing actively. This is usually in the period of January to May and coincides with sap flowing back down to the roots as the plant builds up a reserve for the following year. Variable control will be achieved if blackberry is sprayed in the spring. Where goats have grazed the blackberry one years uninhibited growth should be allowed before spraying.
When spraying blackberry, coverage of of both leaves and stems must be complete. This allows maximum uptake of herbicide in to the crown and root system.
Management after spraying
Maximum control of blackberry is obtained using a programmed approach. Not only should the spraying be taken into account, but also the follow-up work which includes burning, topdressing, oversowing and fencing.
When dense bushes are sprayed it is best to remove the old canes by either crushing or burning. This helps to prevent new plants using the old canes as shelter when becoming established. It also helps pasture to establish more quickly. It is recommended that grass seed be sown where the old plant was growing.
Grazon and Tordon Brushkiller are ideal herbicides for blackberry. They provide quick knockdown, letting you to see where you have been within a few days. They are also grass friendly, meaning they are great for small blackberry as they do not damage high quality pasture grasses like ryegrass.
Varieties of blackberry growing on the East Coast (Hastings through to Gisborne), or in Northland, are hard to kill. These require the use of metsulfuron to achieve a good kill.
Penetrants are not required when spraying blackberry.