Pampasgrass and Jubatagrass Threaten California Coastal Habitats

What makes them invasive?

Jubatagrass. Germination generally occurs in spring and requires sandy soils, ample moisture and light. The temperature range is typical of coastal environments. After germination has occurred, initial growth is slow, but increases rapidly once seedlings become established. Seedling growth and establishment is most rapid on bare sandy soil and exposed road cuts, but requires cool foggy climate and moist soil.

Jubatagrass seedling

Seedling survival is low in shaded areas or in competition with grasses or sedges. Jubatagrass does not tolerate winter frost, warmer summer temperatures, more intense sunlight, and moderate drought. This may account for its inability to become established in the inland areas of California. Individual plants are capable of surviving about 15 years.

Jubatagrass plants have only female flowers, but are able to set viable seed without the need for pollen. This characteristic, called apomixis, is an important factor in its weediness. Thus, no pollination is necessary and all seeds produced are genetically identical to the parent plant. The development of seed occurs before flowering. The apomictic nature of jubatagrass is probably the most important characteristic responsible for its weediness as well as its limited range. An individual inflorescence or plume can produce 100,000 seeds and large clumps can make a million or more seeds. Flowering generally occurs from late July to September, often in the first year of growth. Some plants can flower twice during the same season. The tiny seeds are spread up to 20 miles from the mother plant by wind.

Pampasgrass. Seedlings usually become established in spring and require sandy soils, ample moisture and light. Like jubatagrass, seedling survival is low in shaded areas or in competition with grasses or sedges. Unlike jubatagrass, pampasgrass can tolerate winter frost, warmer summer temperatures, more intense sunlight, and moderate drought. This accounts for its success as an ornamental in the hotter and drier inland areas of California.

In the nursery industry, pampasgrass is primarily propagated through division of mature plants. In nature, however, reproduction occurs only by sexual means with flowers produced 2-3 years after germination. Flowering usually occurs from late August though September, but occasionally in winter.

The species is considered functionally dioecious, that is, flowers of only one sex occur on an individual plant. Over the years, selection for ornamental plants in California has been for the showier plumes of the female plants.

Within most populations in California the two sexes do not co-exist. Consequently, very few seed are produced. However, in areas where flowers of both sexes are present, seed production is high and escaped weedy populations are common. In New Zealand, selection for female plants has not been as rigorous. As a result, more seed is produced and pampasgrass has become a significant weed problem.

Jubatagrass (left) and
female pampasgrass (right)

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