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Sometimes we may be inclined to add small succulents to a green roof planting scheme because we know they may be tolerant to just about every environmental factor encountered on a roof.
Agave are drought, salt, wind, heat and hurricane tolerant, but they will add weight to a green roof!
Succulents can be deceiving with respect to size and weight, especially to those who have not worked long term with these plants and seen their size at maturity. What may be a tiny, perfectly sized immature succulent may turn into a very large and heavy specimen with age.
Potential Florida Green Roof plant or potential roof problem?
Root architecture too is another important consideration for the green roof designer to take into account.
Some succulent plants develop swollen root tubers for water and nutrient storage. Additional water means heavier weight. Moreover, some of these roots grow in a very aggressive fashion and can damage an underlying single ply membrane or asphalt shingle roof.
Potential green roof plants have also have aggressive root systems that can damage single ply or shingles
While some small succulents, like sedum may never grow six feet tall, others can. Though the succulents and cacti you plant may do wonderfully for the first several years, over time they may outgrow the roof's planting bed.
Problems arise when these overgrown plants present weight issues with respect to the loading capabilities of a supporting roof structure. An extensive green roof with smaller plants may weight twenty to thirty pounds per square foot or 100 - 150 kg/sm. The same extensive green roof with 5' to 6' agaves may weigh three or four times the original weight.
Wind resistant can be another important factor with large plants on a rooftop, especially in hurricane impacted areas.
When you increase a roof's live load fourfold you may cause structural failure issues.
Treat some succulents as potential large shrubs or small trees. Remember, just because a small succulent looks just right for your green roof doesn't mean it will stay that way.
Today brought the monthly watering of a 1/4" H2O for the Alliums. Within a matter of hours they were bright green.
We've been working with Resurrection fern also - and I love this plant. Resurrection fern, Polypodium polypodioides, was the first fern in space - going up on a 1997 Space Shuttle Mission to see if the roots would absorb water in a space capsule.
Both of these plants, the Allium and Resurrection fern have unique root characteristics.
We call plant root structure by the name - 'Root Architecture'.
Green roof design has unique root structure and root architecture requirements.
Unless you have a huge potable water or well water source and are going to pump all that water up on a roof to keep plants up there watered, then your green roof plants need to be somewhat drought tolerant.
Certain root architecture patterns support plant acclimation to drought conditions better than others.
Remember, Florida's rainfalls usually are short, afternoon events of 1/2" or less and because rain generally occurs between the hotter months of the year - June - September, there is a tendency for it to evaporate quickly.
Except for hurricanes and tropical storms, rain events in Florida are usually over relatively quickly.
Meaning green roof plants have to scramble to grab the rain water.
Also recall, most green roof plants do not like wet roots (wet feet) so the soil must be well drained.
Proper green roof plant root architecture is crucial for providing a Florida extensive green roof plant with the advantages needed to survive a Florida vegetated roof.
Examine the diagram below showing the root architecture of a green roof plant raised in a one gallon standard nursery container and then a green roof plant raised on a green root mat.
The plant raised on the mat possesses 8 times the amount of Root-Rain surface contact area as the same size plant grown in a nursery container.
Green Roof Plant Root Architecture - Florida Extensive Green Roofs - MetroVerde
So when the afternoon 1/2" rainfall (13mm) event occurs and every drop is important - the green roof plant with the appropriate root architecture will sequester the most water.
More stormwater is captured, runoff is reduced, plants acquire necessary water volumes, plants have less of a tendency to uproot in high winds, and more.
Green Roof Plant Root Architecture is important to the success of a green roof.
A while back we looked at green roof plant root architecture and the benefits of green roof plants possessing a broad, shallow root structure rather than a narrow, deep root structure. Click here to review the blog post information.
The diagram is posted for review here.
Green Roof Plant Root Architecture Comparison - MetroVerde
Real life Green Roof Plant photos of the two types of root architecture are posted below:
Root Architecture - Poor Green Roof Formation
The photo above typifies the normal nursery container plant root structure - narrow and deep. This is not a good root formation or architecture to begin a green roof with. Note the following photos of a plant grown on a mat. I've cut the mat out of a green roof that had a 2" engineered soil layer. The plant had grown on the roof for approximately 3 years or longer. The green roof system was non-irrigated.
Green Roof Plant - Root Architecture 1 - MetroVerde
Green Roof Plant - Root Architecture 2 - MetroVerde
Green Roof Plant - Root Architecture 3 - MetroVerde
Green Roof Plant - Root Architecture 4 - MetroVerde
When I removed the mat section I cut out about twice the diameter of the plant crown or habit (form diameter) thinking I'd capture all the root mass. Upon further examination the plant had extended its roots over triple the area I'd cut.
Note how the roots have grown and woven themselves into the mat - providing an anchor against high winds and allowing for use vertically.
The root architecture of plants grown in mats exemplifies the benefits of growing green roof plants in mats. Roots do not like to be bound up with limiting walls. It you do not use mats, then use large tray systems.
With time, green roof plants with access to a large, monolithic and non-sectioned growing space will eventually 'move' to the best spot for their particular needs - on the roof.
The same basic principle applies to living walls. A monolithic fabric with no limiting sectional structure will allow the plant to choose the best spot for successful, long tern growth.
As you can see, roots do not like to be constrained. Though all roofs have a perimeter and roots must eventually reach a boundary, allowing the plants to reach out for nutrients and water offers many benefits!
Tomato plant roots reaching out horizontally in the Green Roof hurricane weave
Having grown plants in pots for decades I can say with no uncertainty root bound plants are prone to disease and do not preform well in the long term.
Pot bound roots spiral around and around looking for room to grow
Green Roof trays must be large to prevent root binding patterns
Normal root architecture is best achieved by open growing systems on Green Roofs
To me, a photo is worth a thousand words and it is easy for me to see how roots want to stretch out. Of course we've discussed the many benefits of horizontal root architecture in previous posts. An example of a shallow green roof application can be found here.
Let your green roof roots grow. If you use a tray system make sure the trays allow for root crossover through adequate openings.
Roots need oxygen too. When roots grow too deep or are strangled by other roots due to a lack of horizontal growing room the plants can suffer. Providing adequate horizontal growing room ensures good plant development, unless you are using tap root plants (not too many of these are appropriate for green roofs).
Finding the right depth for green roof soil media is so critical in hot and dry climates for when the rains come the roots must be in place where they can take full advantage of any and all rainfall.
This design maxim is why we use shallow soil media depths of 100mm or less, usually ranging in the 50mm thicknesses.
Green Roof plant roots will spread horizontally once they reach an impenetrable root barrier or the bottom of the green roof system. If your green roof system is deep, then many of your green roof plant roots will travel downward, vertically. If your green roof system is shallow, then the roots will spread out horizontally.
Most plants in nature have a horizontal root architecture. I see large oak trees sometimes blown over and their entire root mass is no more than 60cm or so deep (1-2').
Here in Jacksonville, Florida we have been experiencing a severe drought this year and for several years past. Our weather service has classified the local vicinity as being under a severe drought. I understand from speaking to others across the south, some areas, such as Houston has not received any significant rain in four months.
Yet for the last two days we've had strong intermittent thunderstorms and rain.
The flower pots were filled with water and buckets overflowed. However this morning when I was working in the garden I noticed the soil below 20mm or so (an inch approximately) was completely dry! I had expected this, having seen rain time and time again when it comes, barely penetrate the ground surface layer.
Shallow, extensive green roof systems encourage horizontal root architecture. When the rains do come here, I need the green roof plants installed on our nature irrigated green roofs to be ready to adsorb as much of the rainfall as possible, wasting little if any. They cannot exist deep down, hidden in the dry.
With a deeper green roof soil media, even if the media is well drained, I still see almost complete dryness just below the surface after a typical thundershower here.
Shallow green roof soil media can promote horizontal root architecture, and in turn capture more rainfall for use in the plant's photosynthetic processes.
Additionally, shallow root architecture can facilitate enhanced nitrogen and nutrient uptake. Nutrient uptake is important for several reasons, including of course plant nutritional requirements but also the removal of available nutrients from stormwater. Shallow root architecture cleans stormwater and allows for optimal plant growth.
The root architecture model, SimRoot developed by Penn State is shown in the below video clip to illustrate how nitrogen is taken up in the upper margins of the plant root architecture system. Note how the root hairs take up the most nitrogen and water.
I've heard many arguments for deeper soil media systems. Yet they are heavy, expensive and require so much more water wasted. Rather than build heavy stormwater ponds on top of buildings, put them under parking lots for a much more reasonable cost.
There are many benefits to shallow green roof systems. Water efficiency and nutrient uptake are just two.
