Thursday, January 26, 2023

It's Flowering Time for Imperata cylindrica!

It is January here and many of the Imperata cylindrica (cogongrass) stands are in flower.

Imperata cylindrica is known locally as Cogongrass here in USA, and it is a C4 member of the subfamily Panicoideae.

The species has a bad rep in many places. It is notorious for being listed as one of the top 10 weeds in the world, and its ability to compete and dominate its surroundings is well known, both in areas where it is an exotic, and even in its native range.

But a wind-blown field of flowering I. cylindrica is a beautiful sight.

The inflorescence at first are slim spikes of almost pure white, their pencil-thin shapes like missiles rising from the surrounding greenery. 

As the flowerhead matures it becomes more fluffy and rounded, the spikelets coming loose as the time for them to fly off to distant lands comes close. 

The callus hairs at the base of each spikelet are so fine and numerous that when the wind finally knocks one loose from the flowerhead, it usually drags others with it, so a somewhat roundish tangled ball of seeds is what one finds stuck to the ground once it alights. 

Seeds coming loose
Studies have shown that these seeds can be deposited by wind as much as 110 m away from the parent plant, with a possible total dispersion of up to 20 km away in open country!

Lift off!

The spikelets come in pairs, and each spikelet is 2 flowered, with the fertile and sterile flowers being enclosed in 2 similar glumes.

For a summary of spikelet components, you can read this short article.

I tried dissecting a spikelet using a pair of sewing pins, and I have to say it was quite difficult. Not only because it was so light and tiny (with a length of around 3 mm) that my breath alone caused it to be blown away, but because the callus hairs made it almost impossible to manipulate the tiny, fragile structure. It was a frustrating exercise.

Microscope image showing spikelet glumes surrounded by callus hairs

After repeated tries I finally managed to isolate some of the miniscule parts after cutting off much of the hairs. Under the pair of glumes, the fertile flower is protected by fragile looking and almost transparent lemma and palea, while the sterile flower has only the lemma.

Microscope image showing parts of spikelet (palea, stigma, and anthers/filament missing)

I separated out what looked to be the two lemma, as well as possibly the ovary with a long style. There were no anthers.

I have to say that after that arduous set of exercises, I have learned to really admire the fortitude and perseverance of grass taxonomists!

Rare inflorescence of Japanese Blood Grass (variety of I. cylindrica)

Finally,  I have to note that although the common varieties of I. cylindrica have an overall all-white inflorescence, an ornamental variety called Japanese Bloodgrass that I had in my garden in NJ had amazing looking purplish flowerheads. In this case, the numerous anthers were all fat and purple.

Rare inflorescence of Japanese Blood Grass (variety of I. cylindrica)

I. cylindrica is one of the few warm season grasses here in this semi tropical area that flowers during Spring. When it does flower it puts on quite a show, and I've gotten to look forward to its mass flowerings every year, much like I've eagerly anticipated the flowerings of the native Andropogon spp in Fall.

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