tree

Idaho Locust

Robinia x margaretta 'Casque Rouge'

The Story

Plant Bio

Robinia x margaretta is a deciduous hybrid tree of North American parentage, valued for its open, upright habit and tolerance of poor, dry soils. The species features pinnately compound leaves that emerge with a bronze tint before turning green, and produces hanging clusters of pea-like flowers in late spring.

'Casque Rouge' is distinguished by its dense, pendulous racemes of deep pink to purplish-red flowers that measure up to eight inches long. This cultivar is also notably smoother and mostly thornless compared to wild locust trees, making it safer for garden use.

Uses & Benefits:

Produces striking, pendulous racemes of deep pink flowers and attractive bronze-to-green pinnate foliage.

Fixes atmospheric nitrogen, allowing it to thrive in nutrient-poor soils where other trees struggle.

Nectar-rich flowers provide an abundant food source for native bees, pollinators, and hummingbirds.

Essentials

At a Glance

Plant Type

Tree

Height

315"

Width

157"

Light

Full Sun

Zones

5, 6, 7, 8

Water

Low to Moderate

Humidity

Moderate

Lifespan

Moderate

Native To

North America

Toxicity

Toxic to Humans and Pets

Bloom period

Mid spring – Early summer

In flower about 3 months of the year

Planting

How to Grow

Select a planting site in full sun with excellent drainage to prevent root rot.
Dig a hole twice as wide as the root ball but no deeper, ensuring the graft union remains above the soil line.
Backfill with native soil without adding heavy organic amendments, as this tree adapts well to lean soils.
Water deeply to settle the soil, then apply a layer of mulch to conserve moisture while keeping it clear of the trunk.

Pro Tip

Prune only in mid-to-late summer to prevent the heavy sap bleeding that occurs in spring, and promptly cut away root suckers at the base to prevent unwanted thickets.

Keep It Thriving

Care Guide

Do

Do prune in mid-to-late summer to avoid excessive sap bleeding.
Do monitor for and cut away root suckers at the soil line in autumn.
Do plant in highly permeable soil to ensure dry winter roots.

Don't

Don't apply high-nitrogen fertilizers, as this tree fixes its own nitrogen.
Don't prune heavily into old, leafless wood where new shoots may fail to form.
Don't plant in waterlogged or poorly drained clay soils.

Seasonal Care Calendar

Task	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Fertilize
Pest Check
Prune
Spring Cleanup

Watch Out For

Pests & Diseases

Common Pests

Common Diseases

In the Garden

Landscape Uses & Companions

Landscape Uses

Specimen TreeShade TreeUrban GardenFocal Point

Companion Plants

Cercis canadensis Nepeta faassenii Salvia nemorosa

Multiply

Propagation Methods

Good to Know

Common Questions

Idaho Locust questions

What zones can Idaho Locust grow in?

Idaho Locust is hardy in USDA Zones 5–8. Inside that range it survives winter in the ground; outside it, grow it as an annual or a container plant you protect.

Is Idaho Locust deer resistant?

Not reliably. Deer may browse Idaho Locust, especially tender spring growth — plan on repellents or fencing where pressure is high.

When does Idaho Locust bloom?

Idaho Locust typically blooms in mid spring through early summer. Exact timing shifts a week or two with your zone — Sow's bloom calendar maps it to your garden.

Does Idaho Locust need full sun?

Idaho Locust does best in full sun (6+ hours).

Is Idaho Locust drought tolerant?

Yes — once established (usually after the first full season), Idaho Locust handles dry spells with minimal watering. Water deeply its first year to build the root system that makes that possible.