Best Exterior Roof Plants for Green Roof Gardens

Green roof gardens are rapidly becoming a popular way to combine urban living with nature. They offer numerous environmental benefits including improved insulation, reduced stormwater runoff, enhanced air quality, and increased biodiversity. One of the critical components of a successful green roof garden is the choice of plants. Since rooftop environments are exposed to extreme weather conditions such as intense sunlight, high winds, and temperature fluctuations, selecting plants that can tolerate these challenges is essential.

In this article, we will explore the best exterior roof plants for green roof gardens. We’ll consider their adaptability, maintenance needs, aesthetic appeal, and environmental benefits to help you create a thriving rooftop oasis.

Understanding the Environment of a Green Roof

Before diving into specific plants, it’s important to understand the conditions they will face on a green roof:

• Exposure to Sunlight: Roofs typically receive full sun for most of the day.
• Wind: Elevated locations mean more wind, which can dry out plants quickly.
• Temperature Fluctuations: Roof surfaces can get very hot during the day and cool rapidly at night.
• Limited Soil Depth: Many green roofs have shallow substrate layers that restrict root growth.
• Water Retention: Depending on the system, water retention may be limited; drought-tolerant species are often preferred.

Given these factors, it’s clear that hardy, drought-resistant plants with shallow root systems tend to perform best.

Succulents: The Champions of Green Roofs

Succulents are among the most popular choices for green roofs due to their remarkable ability to store water in their leaves and stems. They thrive in dry conditions and require minimal maintenance.

Sedum (Stonecrop)

Sedums are perhaps the quintessential green roof plant. They belong to a large genus with hundreds of species, many of which are perfect for rooftop gardens.

• Advantages:
• Extremely drought-tolerant.
• Able to thrive in shallow soil.
• Variety of colors and textures—from bright greens to reds and purples.
• Low maintenance; they rarely need watering once established.

• Some species can flower beautifully, attracting pollinators.

• Popular Sedum Varieties for Roofs:

• Sedum album (white stonecrop)
• Sedum spurium (two-row stonecrop)
• Sedum reflexum (blue stonecrop)
• Sedum sexangulare

Sempervivum (Hens and Chicks)

Sempervivums are rosette-forming succulents similar to sedums but often with thicker leaves and more pronounced leaf tips.

• Advantages:
• Survive extreme temperatures including frost.
• Form dense mats that help prevent soil erosion.
• Self-propagate easily by producing offsets (“chicks”).
• Require almost no care once planted.

Both Sedums and Sempervivums are ideal for extensive green roofs where substrate depth is limited.

Grasses and Grass-like Plants

Certain grasses tolerate rooftop environments well and provide movement and texture contrast to succulents.

Blue Fescue (Festuca glauca)

Blue Fescue is a compact ornamental grass known for its striking silvery-blue foliage.

• Advantages:
• Drought tolerant once established.
• Adds color contrast against green succulents.
• Requires minimal fertilization.

Prairie Dropseed (Sporobolus heterolepis)

A native North American grass with fine-textured foliage and lovely seed heads.

• Advantages:
• Can handle windy conditions.
• Deep roots stabilize soil.
• Provides seasonal interest through seed plume display.

Grasses generally require slightly deeper soil than succulents but can add biodiversity and aesthetic balance on a rooftop.

Perennial Wildflowers

Adding perennials not only enhances beauty but supports pollinators such as bees and butterflies.

Blanket Flower (Gaillardia aristata)

This hardy wildflower produces bright red and yellow daisy-like blooms throughout summer.

• Advantages:
• Tolerates heat and drought well.
• Attracts pollinators.
• Adds vibrant color to otherwise green roofs.

Black-Eyed Susan (Rudbeckia hirta)

Another pollinator favorite that thrives in full sun and dry conditions.

• Advantages:
• Tough perennial with long bloom period.
• Low maintenance.
• Supports wildlife diversity on the roof garden.

Perennials may require somewhat deeper soil than sedums but can flourish in modular or intensive green roof setups with sufficient substrate depth.

Herbs Suitable for Green Roof Gardens

Herbs not only provide aroma and culinary uses but often possess drought tolerance suitable for rooftop conditions.

Thyme (Thymus vulgaris)

A low-growing herb with fragrant leaves that thrives in sunny locations with well-draining soil.

• Advantages:
• Drought resistant once established.
• Attracts beneficial insects like bees.
• Can form dense mats helping soil retention.

Lavender (Lavandula angustifolia)

Known for its beautiful purple flowers and calming scent, lavender is also suited for green roofs with moderate soil depth.

• Advantages:
• Prefers full sun and well-drained soil.
• Drought tolerant after establishment.
• Adds fragrance and visual interest to rooftop gardens.

Herbs on roofs provide dual benefits—serving ecological functions while offering practical uses for gardeners.

Groundcovers Beyond Succulents

Some non-succulent groundcovers thrive on roofs provided their water needs are met or subsurface irrigation is present.

Creeping Thyme (Thymus serpyllum)

Similar to common thyme but forms an even denser mat; great as a flowering groundcover that attracts pollinators.

Ice Plant (Delosperma cooperi)

A succulent groundcover with bright magenta flowers that tolerates drought well but prefers warmer climates without severe winters.

Considerations When Selecting Plants

When choosing exterior roof plants for your green roof garden, keep these points in mind:

1. Climate Compatibility: Select plants suited to your local climate zone. What thrives in one area may fail in another due to temperature extremes or rainfall differences.
2. Substrate Depth: Match plants to the depth of your growing medium; shallow substrates favor sedums while deeper soils allow grasses and perennials.
3. Maintenance Level: Determine how much time you can dedicate. Sedums require minimal care while perennials might need occasional watering or deadheading.
4. Water Availability: Consider whether your roof has an irrigation system or relies solely on rainfall; drought-tolerant species reduce watering needs.
5. Biodiversity Goals: Incorporate a mix of species to support insects, birds, and beneficial organisms enhancing ecosystem services on your roof garden.
6. Weight Restrictions: Consult structural engineers if unsure about load capacity; plants requiring more soil weight more thus influencing plant choice.

Benefits of Using These Plants on Green Roofs

Choosing appropriate exterior roof plants provides several advantages:

• Energy Savings: Vegetative cover insulates buildings reducing heating/cooling costs.
• Stormwater Management: Plants absorb rainwater reducing runoff burden on urban drainage systems.
• Air Quality Improvement: Photosynthesis filters pollutants while producing oxygen.
• Urban Biodiversity Support: Creates habitats supporting pollinators and birds in city environments.
• Enhanced Aesthetics & Well-being: Green spaces improve mental health by reconnecting people with nature even in urban settings.

Conclusion

Green roof gardens contribute significantly toward sustainable urban development by revitalizing unused spaces into functional ecosystems. The choice of exterior roof plants plays a vital role in ensuring these gardens thrive under harsh rooftop conditions. Succulent species like Sedum and Sempervivum remain top choices due to their resilience, ease of maintenance, and beauty. Adding grasses, wildflowers, herbs, and groundcovers tailored to climate specifics further enriches biodiversity while enhancing visual appeal.

By carefully selecting plants adapted to your green roof’s environment, you will not only create a spectacular rooftop garden but also enjoy all the environmental benefits such spaces have to offer. Whether you aim for an extensive sedum mat or an intensive mixed planting scheme with flowering perennials, the right plant palette makes all the difference in transforming your rooftop into a flourishing green haven.