Saturday, February 6, 2021

When do grasses break winter dormancy?

A, gerardii cultivar shoots growing with snow banks in the background

It's always a minor sort of miracle to me when Spring comes and the grasses that have overwintered by going into dormancy burst into new growth. This is especially true for the warm season grasses like Imperata cylindrica (Japanese Blood Grass, aka Cogongrass), Miscanthus sinensis, and Panicum virgatum, which "die back" completely during the cold season and re-emerge as new shoots. Last year, I noted down this process in late April, and I expected the same timing this year.

I had bought some native ornamental grasses during Fall, and I had been keeping them in my garage in order to lessen the possible stress of the pot bound plants due to extreme cold weather. Because I also store my tropical aroids in the same garage, I added a lamp that switched on every night for about 5 or 6 hours. The grasses were kept slightly away from the lamp, but they nevertheless must have gotten some illumination. Temperature in the garage was around 11-12 C.

A week or so ago I was surprised to discover that one of the Andropogon gerardii cultivars ('Blackhawk') had suddenly produced new shoots. This was unexpected since it was only late January, and in fact it snowed outside a few days later. Such early and premature breaking of dormancy normally would be a problem for the grass. Above ground shoots that had been prematurely produced could be killed by any sudden cold.  

I can only surmise that the warmer soil temperature in the garage might have caused the premature awakening of this particular cultivar. Triggered by the warmer temperatures, the grass had used its carbohydrate reserves to push out the culms. which caused me to wonder what would happen if global temperatures on average continue to rise. Would more and more grasses and other plants in colder northern climes break dormancy earlier during the year?

Some researchers did some experiments to answer this question. They took dormant Panicum virgatum (switchgrass) and Spartina spectinata, both of which are being studied as possible sources of biofuel, and grew them under ambient lighting, but with temperatures significantly higher than normal (Guo et al, 2017). 

They found that both species grew prematurely under the warmer soil temperatures, but the shortening photoperiod from early Fall to late Fall was confusing. They both ceased to grow, with P. virgatum maintaining the shoots above ground in a static condition, but S. spectina allowing the shoots to die back.

The result of the experiment confirmed the importance of photoperiod to the early growth of grasses after winter dormancy, and hinted that warming global temperatures might cause problems for grasses and other plants that break winter dormancy prematurely. The depletion of carbohydrate reserves in the rhizomes to power the new shoots, only to see these new shoots die due to sudden cold temperatures, might affect total biomass production and the overall fitness of the grass during the growing season.  

Literature Cited

Jia Guo, Arvid Boe, Do-Soon Kim, D.K. Lee. Growth and Development of Two Perennial Grasses in Ambient Light Conditions during their Natural Dormant Period. Crop Science, 2017; 57 (4): 2213 DOI: 10.2135/cropsci2016.09.0823

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