Advantages and disadvantages
Subtropical grasses have been introduced from warmer continents (notably Africa and South America) and are now well-established in northern regions of New Zealand. These grasses need relatively high temperatures for photosynthesis compared with the more-common temperate grasses in New Zealand pastures.
Subtropical grasses grow coarser, less palatable foliage than the temperate grasses, and are vigorous during the warm seasons. However, they are frost-tender and grow poorly in the cold seasons. As a result, subtropical grasses have a mixed reputation and, as they often suppress higher-quality pasture species, many farmers consider them to be weeds. If farmers can maintain good clover content in pastures dominated by subtropical grasses, then the pasture feed value is adequate. Otherwise these species offer poor nutrition for grazing animals.
Annual subtropical species (commonly called summer grasses) in northern pastures often colonise dead patches caused by summer drought.
Some species can exist for many years as dormant seeds in the soil, whereas ryegrass seed tends to be short-lived, so cultivation or livestock treading can result in reinvasion by the less valuable subtropical grasses.
Kikuyu grass (Pennisetum clandestinum) is a very vigorous, coarse, low-quality grass, which can take over pastures in frost-free regions. It is present throughout Northland.
A perennial grass, it is low-growing and deep-rooted, with stolons (stems that grow along the soil surface) and rhizomes (roots that grow horizontally in the soil). It forms a dense turf, which is very resistant to heavy grazing and attempts to remove it.
Why everyone hates kikuyu
Almost everyone in Northland hates kikuyu. Farmers hate it because it has poor feed value and crowds out better pasture species; orchardists hate it because it can climb up and partially smother fruit trees, and gardeners hate it because it can invade gardens and lawns. Even worse it is perennial and drought resistant, so once established is really difficult to get rid of.
Other subtropical grasses have steadily advanced south into new regions, possibly because of adaptation, farm management changes or climate change.
The most widespread of these is Paspalum dilatatum, which in recent decades has advanced in pastures (and lawns) well into the South Island. The content of paspalum in northern pastures has also increased.