The most suitable plants for a Green Roof – our findings

May 17

Introduction

Plant Specialists has done several of these projects - and we have learned a lot from all of them ! Here are our findings and suggestions for what works best on a rooftop green roof project.

Keep in mind – this article only reviews plants we have used in our plantings.

Weight Restrictions

Regardless of the wishes of having the hanging gardens of Babylon on your roof, the biggest issue for a green roof is weight. Buildings by laws will restrict the total weight of soil and medium based on what the structural capacity of the roof is. And it’s not much in most cases.

In some very few cases architects incorporated a large heavy planting capacity into their building parameters. But this is not common. It’s simply put a matter of money. The more reinforcement you need to hold the weight the more steel, concrete and infrastructure you will need. But if you are lucky – then your plant palette gets much broader !

Hence – most green roofs have planting areas which can only accommodate shallow rooting plants.

Exposure

The best plants for a rooftop green roof are typically hardy, drought-tolerant, and low-growing species that can withstand harsh, exposed conditions like high sun and wind. This not only includes the hot dry summer but harsh cold winter winds as well - and keep in mind – every 30 floors up you go is like a lower planting zone.

Irrigation

After all is said and done – the most successful green roof projects we have worked on included an automatic irrigation system. The reduced stress on plants during a summer heat spell made the difference. A summer-based system with a manual turn on/off worked the best. That said - we found that Junipers in particular did not like overly wet soil.

Soil Medium

The substrate is a lightweight, engineered, soilless mix made from shale, clay, gravel. Sometimes blended with compost it is designed to support vegetation while minimizing weight and to promote good drainage. Usually, this medium is placed in depths ranging from 2–6 inches.

Plant Care

Weeds were always the biggest issue. In some plantings weeding was required on a weekly basis. Even plants that creep and cover a lot of surface areas were not exempt from weeds. Your green roof will require maintenance !

The second most pressing issue was nutrition. The irrigation systems washing of the soil, combined with a gravel based substrate and shallow soils, means all the fertilizer you add gets washed away very quickly. We found that a green roof required constant feeding to keep the plants healthy.

Insects and diseases were not a very big issue as the conditions are quite harsh for most pests. That said, Monarda in particular got mildew quite often, and many of the Sedum and Allium got aphids!

Plants

The parameters restricting plant selection are : drought-tolerant, heat-tolerant, wind-tolerant, low-growing, little maintenance, sun-loving plant with a shallow root system.

The list is not that short actually – and you have many choices of plants to use including succulents, sedums, and perennials.

Sedum

We found the creeping stone crop Sedum album grew well. S. spurium was less dense and left many gaps - the weeds were a constant battle. S. ternatum was acceptable but only under irrigation because it did not tolerate drying out.

However, the best for coverage and reliability was S. rupestre “angelina” .

Sempervivum

Lots of choices here – 25 varieties to choose from. Color choices from pale green to bright yellow, orange, burgundy and some in bluish tones. All did pretty well long term. Very slow growers.

Perennials

Dianthus ‘ Rose bouquet” was a strong grower. Gaillardia aristata did well but required a bit more maintenance. Also did not quite hold up to the summer heat. Nepeta – catnip - can get floppy and needs pruning. Not evergreen.