Rain, Green Roofs in Florida and Climate Change Issues

Drought everywhere!  Jacksonville received only a small amount, way less than a tenth of an inch of rainfall last night.

MetroVerde Florida Green Roof, Breaking Ground

Here in Jacksonville we've had little if any rainfall for weeks now.  The saying spring showers brings May flowers does not hold true for the geographic climate here.  As weather fronts push down and across the US they loose momentum and dissipate just before reaching Jacksonville.  All the dry weather is rough on green roof plants.

However, being situated on the Atlantic Ocean we are constantly exposed to high winds.  When the weather fronts come through they drop all their moisture west of Jacksonville typically but retain enough wind turbulence to stress unprotected green roof plants.

Over the past year on the Breaking Ground Green Roof I have watched in amazement at the amount of water the dry winds were stealing from the plants as breezes whipped across the flat roof at 4 or 5 meters per second.  Are we seeing the results of climate change or is this an expected cycle?

Of course the CAM plants were fine and this is why we suggest planting a belt of CAM plants around unprotected green roof perimeters.  CAM plants generally keep their stomata closed during the day.

C4 plants like the Poaceae hold their own, as long as they are established.  The corn, lemon grass and native grasses don't seem to mind the desiccating winds though I am sure after time they too would suffer without some rainfall.  I love the wave action native grasses produce in the wind.

The Asteraceae too do well, again as long as they are established and again, most are considered C4 photosynthesis possessing plants.

But the C3 plants can dry out so quickly.

Because we are doing an install I water the plants to help them settle into their new home.

Typically, wind is so drying and strong I can hold the hose to where the stream of water was flowing vertically up and evaporating or being stolen by the wind before the droplets could ever make it back down to the plants.

Watching the C3 plant leaves move in the wind I could see the same occurrence happening, water being quickly vaporized out of the leaves just as the droplets from the hose where also 'gone with the wind'.

Sitting there with the hose I witnessed some of the more tender vegetables quickly wilt even with the hose  water directed at their roots.  The plants vascular system could not keep up with the wind induced evaporation of leaf moisture.

Without water in the leaves plants can not conduct photosynthesis.  Without photosynthesis plants die,

Wind impacts are so significant across green roofs.

In MetroVerde's opinion wind exposure is, with light, availability the most important consideration for green roof design.

We've posted numerous articles here on how to use CAM plants as wind breaks.  Understanding wind impacts on green roofs is critical for long term green roof design success.

Wind, wind & wind across Green Roofs in Arid Areas

The central states along the Mississippi have been receiving more than their fair share of rain recently as evidenced by the US Army Corps of Engineers blasting of levees to alleviate flooding.

MetroVerde Florida Green Roof, Breaking Ground

Here in Jacksonville we've had little if any rainfall for weeks now.  The saying April showers brings May flowers does not hold true for the geographic climate here.  As weather fronts push down and across the US they loose momentum and dissipate just before reaching Jacksonville.  All the dry weather is rough on green roof plants.

However, being situated on the Atlantic Ocean we are constantly exposed to high winds.  When the weather fronts come through they drop all their moisture west of Jacksonville typically but retain enough wind turbulence to stress unprotected green roof plants.

Up on the Breaking Ground Green Roof for most of yesterday I watched in amazement at the amount of water the dry winds were stealing from the plants as breezes whipped across the flat roof at 4 or 5 meters per second.

Of course the CAM plants were fine and this is why we suggest planting a belt of CAM plants around unprotected green roof perimeters.  CAM plants generally keep their stomata closed during the day.

C4 plants like the Poaceae hold their own, as long as they are established.  The corn, lemon grass and native grasses don't seem to mind the desiccating winds though I am sure after time they too would suffer without some rainfall.  I love the wave action native grasses produce in the wind.

The Asteraceae too do well, again as long as they are established and again, most are considered C4 photosynthesis possessing plants.

But the C3 plants can dry out so quickly.

Because we are doing an install I water the plants to help them settle into their new home.

Yesterday the wind was so drying and strong I could hold the hose to where the stream of water was flowing vertically up and evaporating or being stolen by the wind before the droplets could ever make it back down to the plants.

Watching the C3 plant leaves move in the wind I could see the same occurrence happening, water being quickly vaporized out of the leaves just as the droplets from the hose where also 'gone with the wind'.

Sitting there with the hose I witnessed some of the more tender vegetables quickly wilt even with the hose  water directed at their roots.  The plants vascular system could not keep up with the wind induced evaporation of leaf moisture.

Without water in the leaves plants can not conduct photosynthesis.  Without photosynthesis plants die,

Wind impacts are so significant across green roofs.

In my opinion wind exposure is, with light, availability the most important consideration for green roof design.

We've posted numerous articles here on how to use CAM plants as wind breaks.  Understanding wind impacts on green roofs is critical for long term green roof design success.

Green Roof Vegetables - A List of Recommended C4 Plants

Let's take another look at growing rooftop food gardens.  Rooftop permaculture is a rapidly growing movement across the world.  Highly productive vegetable gardens can be found on green roofs in nearly every city. 

Choosing vegetables that possess C4 photosynthesis capabilities can help ensure rooftop garden success. 

Examples of C4 plants include;

Vegetables & Flowers -

• Cabbage
• Broccoli
• Cauliflower
• Turnip
• Mustard Greens
• Wasabi
• Cress & Watercress
• Rapeseed
• Horseradish
• Most lettuces
• Chicory
• Artichoke
• Jerusalem artichoke
• Yacon
• Echinacea
• Tagetes marigold
• Marigolds
• Zinnias
• Chrysanthemums
• Heleniums
• Artemesia
• Dandelion
• Goldenrod, and more

The beauty of C4 plants is their ability to withstand drought and the harsher environment of many rooftops.

Generally, C4 plants loose only one third the amount of water to evapotranspiration as C3 plants, giving them a great advantage on the roof.

Remember, we've discussed C$, C3 and CAM photosynthesis processes in prior posts.  

Review:

Photosynthesis occurs in most C3 plants by:

• 'Breathing' in CO2 and combining the CO2 with a 5-carbon sugar
• The reaction is facilitated by an enzyme, Ribulose Bisphosphate Carboxylase Oxygenase or RUBISCO abbreviated
• Now there are 6-carbons
• The 6-carbon chain breaks into 2 molecules of a glyceric acid called PGA - 3-phosphoglyceric acid
• The completed reaction is called the Calvin cycle and the PGA is the base for glucose synthesis, along with other carbohydrates

However, some plants have an supplemental or alternative method of implementing the above food production cycle. 

C4 plants generally conduct photosynthesis by:

• Absorbing CO2 into the mesophyll - 
• Once the CO2 is in the mesophyll it combines with a 3-carbon compound called PEP, phosphoenolpyruvic acid to form a 4-carbon substance called oxaloacetic acid - hence the C4
• The C4 is moved into the deep bundle sheath layer of cells away from the leaf surface
• Inside the bundle sheath the C4 breaks down into the C3 base and eventually PEP for the Calvin Cycle food production

CAM plants are different from C4 plants in several ways such as CAM plants taking in CO2 only at night. 

Interestingly, many C4 plants have developed the capabilities to switch back and forth between C3's 'full-steam ahead' photosynthesis on the leaf's surface (where the process can be interrupted by environmental conditions) to the C4 process where the photosynthesis required compounds are protected by layers of plant cells.

The process of switching back and forth between C3 and C4 gives a plant the opportunity to grow quickly, a survival advantage, during good weather and the switch into energy conservation survival mode when extreme and damaging weather arrives.

C4 plants, since the plant's photosynthesis process is protected deeper under the surface of the leaf, are more resilient to drought and extreme temperatures.

Therefore, under certain conditions and when placed in certain roof locations, make very good green roof plants.

Realize however, green roof plant selection is modeled on many inputs, C3, C4 or CAM type just being one factor.

However the green roof plant designer must understand the difference in the photosynthesis processes taking place in the plants being specified on any green roof.  

There are many interesting discussions about  C4  and C3 plants and climate change.  We will reserve a future discussion on insights into whether C3 or C4 or CAM plants will have an evolutionary advantage in the future with increased levels of CO2.  Surprisingly - no - not so surprisingly, 'Mother Nature' always steps in and provides adaptation mechanisms.  We will discuss these adaptations both C3 and C4 plants implement to survive either higher or lower levels of CO2.

Having moved through some heavy technical facts on C3, C4 and CAM plants we hope the discussion has not been too boring.

The real photosynthesis processes are much more complicated than we've talked about in these articles.  Understanding the basic differences between C3, C4 and CAM plants will allow the green roof plant designer to greatly increase survival opportunities for their green roof design.

As always, email your comments or questions.

Happy Green Roofing!

Kevin