Dispersal by Bulbils – A Bulbous Bluegrass Story

The main way that a plant gets from place to place is in the form of a seed. As seeds, plants have the ability to travel miles from home, especially with the assistance of outside forces like wind, water, and animals. They could also simply drop to the ground at the base of their parent plant and stay there. The possibilities are endless, really.

But what about plants that don’t even bother making seeds? How do they get around? In the case of bulbous bluegrass, miniature bulbs produced in place of flowers function exactly like seeds. They are formed in the same location as seeds, reach maturity and drop from the plant just like seed-bearing fruits, and are then dispersed in the same ways that seeds are. They even experience a period of dormancy similar to seeds, in that they lie in wait for months or years until the right environmental conditions “tell” them to sprout. And so, bulbils are basically seeds, but different.

bulbous bluegrass (Poa bulbosa)

Bulbous bluegrass (Poa bulbosa) is a Eurasian native but is widely distributed outside of its native range having been repeatedly spread around by humans both intentionally and accidentally. It’s a short-lived, perennial grass that can reach up to 2 feet tall but is often considerably shorter. Its leaves are similar to other bluegrasses – narrow, flat or slightly rolled, with boat-shaped tips and membranous ligules – yet the plants are easy to distinguish thanks to their bulbous bases and the bulbils that form in their flower heads. Their bulbous bases are actually true bulbs, and bulbous bluegrass is said to be the only grass species that has this trait. Just like other bulb-producing plants, the production of these basal bulbs is one way that bulbous bluegrass propagates itself.

basal bulbs of bulbous bluegrass

Bulbous bluegrass is also propagated by seeds and bulbils. Seeds form, like any other plant species, in the ovary of a pollinated flower. But sometimes bulbous bluegrass doesn’t make flowers, and instead modifies its flower parts to form bulbils in their place. Bulbils are essentially tiny, immature plants that, once separated from their parent plant, can form roots and grow into a full size plant. The drawback is that, unlike with most seeds, no sexual recombination has occurred, and so bulbils are essentially clones of a single parent.

The bulbils of bulbous bluegrass sit atop the glumes (bracts) of a spikelet, which would otherwise consist of multiple florets. They have dark purple bases and long, slender, grass-like tips. Bulbils are a type of pseudovivipary, in that they are little plantlets attached to a parent plant. True vivipary occurs when a seed germinates inside of a fruit while still attached to its parent.

Like seeds, bulbils are small packets of starch and fat, and so they are sought ought by small mammals and birds as a source of food. Ants and small rodents are said to collect and cache the bulbils, which is one way they get dispersed. Otherwise, the bulbils rely mostly on wind to get around. They then lie dormant for as long as 2 or 3 years, awaiting the ideal time to take root.

bulbils of bulbous bluegrass

Bulbous bluegrass was accidentally brought to North America as a contaminant in alfalfa and clover seed. It was also intentionally planted as early as 1907 and has been evaluated repeatedly by the USDA and other organizations for use as a forage crop or turfgrass. It has been used in restoration to stabilize soils and reduce erosion. Despite numerous trials, it has consistently underperformed mainly due to its short growth cycle and long dormancy period. It is one of the first grasses to green up in the spring, but by the start of summer it has often gone completely dormant, limiting its value as forage and making for a pretty pathetic turfgrass. Otherwise, it’s pretty good at propagating itself and persisting in locations where it hasn’t been invited and is now mostly considered a weed – a noxious one at that according to some states. Due to its preference for dry climates, it is found most commonly in western North America.

In its native range, bulbous bluegrass frequently reproduces sexually. In North America, however, sexual reproduction is rare, and bulbils are the most common method of reproduction. Prolific asexual reproduction suggests that bulbous bluegress populations in North America should have low genetic diversity. Researchers set out to examine this by comparing populations found in Washington, Oregon, and Idaho. Their results, published in Northwest Science (1997), showed a surprising amount of genetic variation within and among populations. They concluded that multiple introductions, some sexual reproduction, and the autopolyploidy nature of the species help explain this high level of diversity.

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Interested in learning more about how plants get around? Check out the first issue of our new zine Dispersal Stories.

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