Ideas for Rain-Garden Shrubs Suited to Hawaiian Landscapes

Hawaii presents a unique challenge and opportunity for rain gardens. Microclimates vary from windward wet slopes to leeward dry plains, soils range from porous volcanic ash to compacted clay, and the island chain has a high responsibility to protect native ecosystems from invasive introductions. This article discusses shrub choices, design principles, planting methods, and maintenance tips for rain gardens in Hawaiian landscapes, with an emphasis on species and practices that perform well under periodic ponding, heavy tropical rains, and island-specific constraints.

Why a rain garden in Hawaii?

Rain gardens are shallow depressions designed to capture and infiltrate stormwater, reduce runoff, trap sediment and pollutants, and create habitat. In Hawaii a properly designed rain garden:

slows and stores intense tropical downpours,
reduces sediment and nutrient loads entering nearshore reefs,
stabilizes slopes and riparian edges,
provides native plant habitat and nectar/fruit for local fauna.

Rain gardens on the islands need to be tolerant of intense rain events, high humidity, salt spray in coastal areas, and the local soil matrix. Shrubs are particularly useful in rain gardens because they offer structure, root mass for stabilization and uptake, seasonal flowering and fruiting for wildlife, and flexibility in height and spacing.

Design principles for Hawaiian rain gardens

Good plant selection starts with solid design. Consider these practical design fundamentals before choosing shrubs.

Site assessment

Determine typical and extreme rainfall for the site and account for storm intensity.
Map microclimates (sun vs. shade, wind exposure, salt spray, ponding duration).
Test soil drainage: dig a 1-foot (30 cm) test hole, add water and see how long it takes to drain. If it takes more than 24 hours, consider an underdrain or amend soil for better infiltration.
Identify the contributing drainage area and calculate expected runoff volume to size the basin. A professional or extension service can help with sizing; a rule of thumb is to size the basin to hold 1/2 to 1 inch (12-25 mm) of runoff across the contributing impervious area for small residential projects.

Rain garden soil media

A well-draining but moisture-retentive media helps plants survive both ponding and dry spells. A common, practical mix:

60% coarse sand or open-graded gravelly sand,
30% screened native topsoil,
10% well-aged compost.

Mix and test infiltration; target 0.5-2 inches per hour (1.3-5 cm/hr) for a functional rain garden in many Hawaiian sites. For heavy clay, increase sand/grit content; for highly porous volcanic soils, increase topsoil and compost to retain moisture.

Ponding depth and planting zones

Shallow ponding depth: 6-12 inches (15-30 cm) is typical. Deeper basins can support sedges and small trees; shrub zones should be planted on the basin sides and berms to avoid permanent saturation.
Create tiers: an inner wet zone for tolerant herbaceous plants, a middle transition for moisture-loving shrubs, and an outer zone for shrubs that tolerate occasional dampness but prefer drier conditions.
Provide an overflow path and consider an underdrain if subsurface layers are impermeable.

Native shrubs suited to Hawaiian rain gardens

Prioritize natives whenever possible. Native shrubs support local ecology and are generally well-adapted to island conditions. Below are several native options that perform well in moist to seasonally wet conditions. Sizes and cultural notes are given so you can place each shrub in the appropriate rain-garden zone.

Dodonaea viscosa (aalii)

Dodonaea viscosa is a versatile native shrub often used for erosion control and hedging.

Mature size: 3-12 ft (1-4 m) depending on variety and site.
Light: full sun to part shade.
Moisture: tolerates a wide moisture range, from drought to periodic wet feet; best placed on middle-to-upper edges of the basin.
Uses: hedge, windbreak, erosion control, good root structure for stabilization.

Myoporum sandwicense (naio)

Naio is a hardy native that can be trained as a shrub or small tree and tolerates coastal exposure.

Mature size: 10-20 ft (3-6 m) as a small tree, but can be pruned to shrub form.
Light: full sun to light shade.
Moisture: tolerant of well-drained to moderately moist soils; place on upper edge or in larger rain gardens where peat-like soils are rare.
Uses: screening, habitat tree-shrub, nectar source for birds.

Scaevola spp. (naupaka)

Scaevola (coastal naupaka and related species) are low-to-medium shrubs with dense branches and attractive flowers.

Mature size: 2-8 ft (0.6-2.5 m) depending on species and cultivar.
Light: full sun, very salt-tolerant.
Moisture: tolerates periodic ponding in coastal sandy soils; best in middle to outer zones for coastal rain gardens.
Uses: erosion control, coastal buffers, pollinator-friendly flowers.

Pipturus albidus (mamaki)

Mamaki is a native understory shrub/small tree valued for habitat and traditional uses.

Mature size: 6-20 ft (2-6 m) depending on site.
Light: prefers part shade to filtered light.
Moisture: enjoys moist to mesic sites and tolerates periodic saturation; suited to wetter rain gardens with partial shade.
Uses: wildlife host plant, medicinal and cultural plantings.

Native Coprosma and other understory shrubs

Various native Coprosma spp. and low-growing endemic shrubs occupy wet forest understories.

Size and habit vary; many species are naturally adapted to wet soils.
Best used in shaded or partially shaded rain gardens at middle-to-lower tiers.
Check with local nurseries to source the appropriate native Coprosma for your island and site.

Non-native, well-behaved shrubs for Hawaiian rain gardens

Where natives are unavailable or additional variety is needed, choose non-native shrubs known to be non-invasive on your island and tolerant of moist conditions. Always check with local invasive species lists before planting.

Tropical hibiscus cultivars (Hibiscus rosa-sinensis): colorful, handle moist soils in full sun to part shade, pruneable to shrub form.
Calliandra haematocephala (powderpuff): tolerates moist soils, provides nectar for birds and pollinators, moderate size 6-12 ft (2-4 m).
Gardenia jasminoides (gardenia) and related gardenias: fragrant flowers, prefer moist, well-drained soil, best in partial shade to full sun with consistent moisture.
Polyscias scutellaria (aralia/polyscias): adaptable shrub for moist, shaded rain garden edges.

Note: Avoid any non-native plants that are listed as invasive nationally or locally. If in doubt, consult the Hawaii Department of Land and Natural Resources or local extension before purchasing.

Shrubs to avoid — common invasive or problematic species

Plant choices should avoid those that cause ecological harm. Common species to exclude from rain-garden plans include:

Miconia calvescens (miconia)
Psidium cattleianum (strawberry guava)
Schinus terebinthifolius (Brazilian pepper)
Clidemia hirta (Koster’s curse)
Hedychium gardnerianum (ginger, invasive in wet forests)
Albizia species that self-seed and escape cultivation

Removing and avoiding these species protects native forest fragments and downstream reef systems.

Practical planting, spacing and establishment tips

Following are concrete steps and tips for planting shrubs in a Hawaiian rain garden.

Excavate the basin to designed depth (6-12 inches typical), creating a level bottom and gentle side slopes.
Install the soil media mix (60% coarse sand, 30% topsoil, 10% compost) to the full planting area and blend with surrounding soils to avoid abrupt transitions.
Plan planting zones: set moisture-tolerant shrubs in the lowest/inner margin and less tolerant shrubs on the outer berms.
Plant shrubs at the same root crown level as in the nursery container. Do not bury the stem deep.
Mulch with 2-3 inches (5-8 cm) of coarse mulch to reduce erosion and moderate soil temperature. Leave 2-3 inches clearance around stems.
Water thoroughly at planting and maintain regular watering during the establishment period (8-12 weeks); after established, many natives will need little supplemental irrigation.
Provide an overflow spillway lined with rock or native groundcover to handle extreme storms.

Spacing guidelines (general):

Small shrubs (3-6 ft mature): space 3-6 ft (1-2 m) apart.
Medium shrubs (6-12 ft): space 5-8 ft (1.5-2.5 m) apart.
Larger shrubs or small trees: space according to anticipated mature canopy to avoid overcrowding and root competition.

Maintenance and long-term stewardship

A rain garden is low-maintenance once established, but a simple maintenance schedule ensures function and plant health.

Year 1: monitor weekly for drainage issues, mulch loss, and plant stress; remove sediment after large storms.
Year 2-5: inspect quarterly, reorder or replace plants that fail, prune to maintain form and prevent obstruction of overflow.
Annual tasks: top-dress mulch, check inlet and outlet clearances, remove debris, and control any invasive weeds arriving from adjacent areas.
Long-term: reduce fertilizer use (native shrubs are adapted to low-nutrient soils and excess nutrients undermine water-quality goals).

Sample planting palette and layout idea

Place the wettest-tolerant plants at the bottom center of the basin and progressively drier-tolerant shrubs outward.

Inner wet zone: sedges, ferns, and low native shrubs tolerant of extended moisture.
Middle transition (moist-loving shrubs): Dodonaea viscosa (compact varieties), Scaevola spp., native Coprosma.
Outer edge/berm (drier-tolerant shrubs): Myoporum sandwicense (trained as shrub), Hibiscus arnottianus in partial shade, Gardenia cultivars.

Alternate clusters of three to five plants of the same species for visual cohesion and easier maintenance, and include a continuous mulch layer with rock spillways for overflow.

Practical takeaways

Prioritize native shrubs adapted to local microclimates; they support wildlife and are best suited for long-term performance.
Match plant moisture and light requirements to the rain garden zones: inner wet zone for the most tolerant species, outer berm for drier-tolerant shrubs.
Use a well-draining engineered soil mix (roughly 60% sand, 30% topsoil, 10% compost) and design for shallow ponding (6-12 inches).
Avoid known invasive species and consult local resources when selecting non-native options.
Plan for a simple maintenance routine: sediment removal, mulch refresh, pruning and invasive monitoring.

Designing a rain garden for Hawaiian landscapes is both an ecological opportunity and a design challenge. Thoughtful plant selection–favoring native, moisture-tolerant shrubs–combined with proper soil media and overflow design will result in functional, attractive stormwater features that stabilize soils, protect reefs, and provide habitat in island settings.