Cut and Stump Treat
The “cut and stump treat” method has been developed particularly for the control of Giant Rhubarb (Gunnera). After successful trials in New Zealand and Ireland, it is now considered the most effective and environmentally sensitive of treatment options. Giant Rhubarb also responds to spray treatment but, because of the rough and leathery consistency of the plant's leaves, a significant amount of herbicide has to be applied to both the top and underside of the plant's leaves, with the inherent risk of run-off and ground contamination. The very large size of the plant, and the height of its leaves and stems, may also cause excessive herbicide drift if being sprayed. It is also important to note that Giant Rhubarb’s stronghold is across the western counties of Ireland and is therefore often located within National Parks, Special Areas of Conservation (SAC’s), Special Protection Areas (SPA's) and Natural Heritage Areas (NHA’s). In many of these situations, a licence is required from The National Parks and Wildlife Service for treatment to be carried out, and typically foliar spraying will not be permitted.
Giant Rhubarbs’s rhizome, or root system, grows above ground but is very tough and difficult to penetrate. Therefore the “cut and stump treat” approach entails cutting the plant's stems as close as possible to their base where they join the rhizome, and then applying the prescribed dose and concentration of herbicide to the exposed stump. The herbicide is applied using a swab or sponge, and must be carried out as soon as possible after cutting the stem, to ensure maximum absorption into the rhizome.
An alternative or complimentary strategy is to introduce herbicide into the rhizome by drilling directly into the root and pouring the herbicide mix into the trough created. For very mature plants, a number of drill holes will be required in each rhizome. It should be noted that because of the resistance and durability of the Giant Rhubarb's rhizome, the plant will often retain some regenerative capacity after the initial treatment. In these circumstances, some additional treatment may be required in the following seasons.