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The Science Behind Southern Grasses, Including Turf
By Ginny Stibolt (Also, listen to the podcast the Science of Southern Grasses.)

Ginny's Freedom lawn during the summer.  Photo by Stibolt When my husband and I moved to northeast Florida in 2004, we inherited a St. Augustine lawn that had been weed & feed'ed, fertilized, irrigated, sprayed with pesticides and herbicides--all the typical actions people in Florida have been advised to take in order to maintain their lawns. We stopped all the chemicals and have irrigated only when needed. The irrigation system pumps water from a nutrient-rich lake, so each time we run the irrigation system, we essentially feed the lawn. We don't mow the lawn when it's dormant--from November to March. Under our no-chemicals care, there are still quite a few areas where the St. Augustine is doing well, but there are others where different grasses, clovers, and many other plants have moved in. It looks quite presentable as a lawn despite our not spending all that money on chemicals. This type of lawn, where we mow whatever is there, is called a "freedom lawn."

When researching my book, "Sustainable Gardening for Florida," I found lots of information on how to maintain turf so that it is green all the time. Most of the sources provided the same advice: fertilize several times per year, over-seed with annual rye in the winter, apply pre-emergence herbicide, and poison all the bugs. I was looking for information on more sustainable lawn care, but this advice varied widely. I finally decided to take the consensus approach and also relied on practices that worked for me in the Mid-Atlantic States. My advice on fertilizing was to do so only once a year, if needed, and to do so after the hurricane season is over in November. I was wrong. As one highly educated reader pointed out; I had not taken into account the science--that southern grasses use a more efficient photosynthesis process.

Photosynthesis

Photosynthesis is the process that green plants use to turn carbon dioxide and water into sugar and oxygen with the energy from sunlight.

Respiration is the reverse process whereby plants and animals use sugars from food and oxygen from the atmosphere to gain energy for living. The byproducts of respiration, carbon dioxide and water, are usually released into the atmosphere.

Heat will shut down photosynthesis in most plants when they close the pores (stomata) on their leaves to protect against dehydration. Carbon dioxide is not available from the air when the stomata are closed. When these plants respire, carbon dioxide and water are released into the atmosphere through the open stomata. But when the stomata are closed, respiration is also slowed. Because this method of photosynthesis creates sugars with multiples of three carbon atoms, this is called the "C3 pathway" for photosynthesis. This type of photosynthesis can take place in relatively weak light and cool weather, but not in the heat of a summer day.

Muhly grass is one of the more attractive C4 grasses, a Florida native.  Photo by StiboltIn contrast, many plants that thrive in hot weather have a different, more efficient process for photosynthesis than plants that use the standard C3 photosynthetic process. These hot-weather plants include southern grasses used for lawns such as Bahia grass, Bermuda grass, Centipede grass, Seashore Paspalum, St. Augustine grass, and Zoysia grass. Other plants in the grass family, such as corn, cord grass, muhly grass (See photo to the left.), sugar cane, goose grass, switch grass, and crab grass; plus some non-grasses including spurge and purslane also use this variant process.

These heat-tolerant plants use a more efficient method or the C4 pathway whereby the photosynthesis can continue during hot days even when their stomata are closed. They produce starch, which is a complex sugar with multiples of four carbon atoms, instead of a simple sugar. Also when C4 plants respire, the carbon dioxide and water are not released into the atmosphere, but are absorbed chemically and become available for photosynthesis. Since their respiration process retains water, C4 plants are also better equipped to handle drought.

The ideal soil temperatures that favor maximum root growth for C3 plants are 55 to 65 degrees, but the ideal temperature range is 75 to 95 degrees for the C4 plants. For the crown growth, C3 plants do best with air temperatures from 60 to 77 degrees, while C4 plants do best at 85 to 117 degrees!

On the other hand, C4 plants become dormant in cool weather and are not particularly shade tolerant. C3 plants do fine in cooler weather and many are quite tolerant of the shade because the lower temperature in the shade enables them to photosynthesize for more hours during hot days.

C3 vs. C4: What's the Significance?

The significance of this different chemistry is that the hot-weather-loving C4 plants should not be lumped into a one-size-fits-all type of lawn care. The typical advice for minimum lawn care is to fertilize with a slow-release fertilizer or compost in the fall. This is bad advice for C4 turf because it will be going into dormancy with cooler weather and shorter days. That fertilizer will sit there unused except by weeds, or the late fall storms will rinse it off the landscape into the waterways via the storm drainage systems. 

The better advice for minimum care of warm weather grasses is to fertilize (if you absolutely must) only once in the spring with a slow release fertilizer and/or compost after the grass begins to green up with the approach of warmer weather and longer days. So if you're managing a lawn with southern grasses, don't use the standard Yankee advice.

Resources:
· A column in the New York Times "Evolution by the Grassroots" by Olivia Judson.
· · The Lawn Reform Coalition has information on sustainable lawn care and ideas for non-lawn landscaping: www.lawnreform.org/
· Florida Extension Service provides numerous resources for turf selection and standard care. Here's one: Selecting a Florida-Friendly Turfgrass


Ginny Stibolt would like to hear from readers who have suggestions and questions. After all, there are more than a few transplanted gardeners here in northeast Florida trying to figure out what works and what doesn't in planting zone 8/9. She's written a book, "Sustainable Gardening for Florida," published by University Press of Florida.

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