Planting Green Roofs, Seeds, Plugs or Plants?

The other day I saw a Twitter post asking the question 'What to plant on a green roof; seeds or plugs?"

There were several notable answers and one, from the UK's Green Roof expert, Dusty Gedge, was something to the effect of - 'Both'!

I agree.  A Green Roof should look nice and full on install for many reasons, including:

• Jump starts biodiversity support 
• Creates visual beauty
• Mature plants clean more stormwater
• Cleans the air
• Quickly sequesters Carbon 
• & Much More!

Green Roof supports biodiversity, cleans stormwater

I like to add a third item to the initial planting material - mature plants.

With mature plants, plugs and seeds the green roof will look good quickly and have a second and third wave of growth automatically set to fill in the vacant or bare areas.

Good wildflower seed mixtures are available across the internet and one of my favorite is Prairie Moon Native Plant Seeds.

Plant mature green roof plants across the roof and fill in the gaps with the plugs.  Spread the wildflower seeds across the roof area you wish to grow in as a prairie or wildflower meadow.

Most wildflower seed mixtures contain enough varieties so you can easily meet the 10-20-30 rule for biodiversity, and usually contain tough, hardy plants such as members of the Asteraceae and Poaceae families.

Once established, both the annual and perennial wildflowers will reseed and continue to fill in the rooftop areas.

Utilizing mature plants, plugs and seeds offers a three dimensional quality to the green roof planting plan, one providing beauty over time, habitat support and environmental benefits.

Invasive Plant Species Notes and Green Roofs

I am headed to Orlando this morning to offer a presentation at the Florida Exotic Pest Plant Council's annual symposia.  Details are available here.

Exotic plants are alluring sometimes due to their beauty

Green roof projects are intricately tied to the issue of exotic plants, biodiversity and the use of native species.

Exotic and exotic invasive plant species are problems primarily because the lack of competition, predators, control and the fact they are very persistent in the ecosystem once escaped.

Many exotic invasive species have become introduced into an environment by first being planted in a landscape.

The difference in using an exotic plant species in the ground level landscape and the same exotic plant species on a green roof is exponential in nature.

The area of adjacent influence for the exotic green roof plant is much larger than for the same plant on the ground.  Moreover, the warmth of a green roof system may allow for those species subject to cold damage to survive where they would normally die back, further expanding their range.

Additionally, wind influence on exotic species becomes amplified when those species are part of a green roof project.  Wind can carry seeds, leaves or vegetative matter capable of rooting and spreading.  Blown from a roof, the plant's vegetation can spread across a much broader radius than from the ground level.

Stormwater too becomes a vector for exotic green roof plants, potentially carrying stems or leaves that may re-root downstream vast distances.

There are lots of reasons to use native plant species on a green roof.

As a plant designer I don't always stick with natives, using food, other (hopefully well-behaved plants) landscape plants and flowers in addition to many natives.

Importantly, we must all be cognizant of those plant species we are using.  Green roofs have the potential for becoming an important source of exotic plant species if we as an industry are not careful.

Green Roofs and Crimson Clover - Day Three Leaves

Green Roof Plants Trifolium spp.  Day Three

Sowing Crimson Clover, Trifolium incarnatum, will help keep the green roof really green because Crimson Clover is a nitrogen fixing species - or green manure crop.

Day Three of the sprouting produced a significant increase in the number of initial dicot leaves.  The roof is turning green in three days!

We will be posting daily photos of the clover as it grows on the roof.

As always, email your questions.

Happy Green Roofing, Kevin

Florida Green Roof Plants - Root Architecture Summary

We've been discussing the optimal green roof plant root structure.

There are many adequate types of Florida green roof plant root structures.   The variables different structures become a part of include;

1. Hurricane, tropical storm and wind resistance,
2. Rainfall and water absorption,
3. Plant support,
4. System cohesiveness, and
5. other factors.

One of the major advantages of using a mat with a weave, or trays with an inserted weave for roots to grow into is the overall imparted strength of the system.

 Laterally growing roots will provide greater anchoring during high winds.  This can be an important factor when considering placing a green roof here in Florida -  or anywhere with potential for tropical storms and high winds.

Florida Green Roof Plant - Good Root Architecture - Lateral Growth

 Finally, the most important aspect of installing plants on a Florida Green Roof is - don't go tot he nursery and purchase plants that have been grown in a production nursery, pumped full of fertilizers, used to timely irrigation and replete with tender new growth. Instead, look for those plants with a lateral root system that are hardened off and acclimated to the tough environment you are installing them into.

Roofs are harsh places for plants to survive!  Remember the 5 H's - High desiccating winds, High heat, Hard Freezes, HOT-HOT-HOT!, High humidity, Hurricanes,  and more...

Lateral root growth contributes towards support, enhanced water absorption area, anchoring and a host of other success factors.

Happy Green Roofing!

As always, e-mail me with your questions.  Kevin.