Black locust
Robinia pseudoacacia
also called false acacia, black locust tree
Large and quick, with its troubles mostly up in the leaves — apart from honey fungus, which is not.
- Type
- Tree · Perennial
- Botanical family
- Mixed / other
- Foliage color
- Green
- Flower color
- White
- Sunlight
- Full sun
- Feeding
- All-purpose balanced
- Frost tolerant
- Yes
- Flowering
- June
- Apr–Jun Bud Break
- Jun–Oct Flowering
- Oct–Dec Leaf Fall
- Dec–Dec Winter Dormancy
- Jan–Apr Winter Dormancy
- Plant spacing
- 26 ft 3 in
- In-row spacing
- 26 ft 3 in
- Between rows
- 32 ft 10 in
- Square-foot density
- Not suited to SFG
- Root system
- Woody, shallow, wide and opportunistic
- Working depth
- 12–24 in
- Can reach
- 3.5 ft down where the soil allows
- Spread
- 79–105 ft across, about 3–4× the width of the plant
These roots go looking, and what they find is drains, soakaways, footings, paving and the border next door. Give it real distance from anything you would mind being lifted or blocked: the reach below ground is several times what the crown suggests.
A nitrogen-fixing legume with roots that sucker aggressively over a wide area, and every root fragment left behind makes a new tree. Cutting one down is what starts the thicket.
The habit is Black locust’s own; the figures are what that habit usually comes to at this size, not a measurement of this species. Roots follow the soil either way: a compacted pan, a high water table or heavy clay stops them well short of any of it, and deep open ground lets them go further.
- Cats and dogs
- Recorded toxic
Recorded as toxic to cats and dogs by ASPCA.
The ASPCA Locust row names our genus, Robinia, and records it toxic to dogs and cats. Cornell has our exact species as Black Locust but names only livestock and humans, so the warning rests on the ASPCA row.
- Leaf miners Larvae that tunnel pale wandering trails or blotches inside the leaf itself. Pick off and destroy mined leaves; damage is usually cosmetic.
- Gall mites Robinia gall midge
- Honey fungus A fatal root disease of woody plants: white fungal sheets under the bark and honey-colored toadstools at the base. Remove the plant with as much root and stump as possible.
Beneficial predators that hunt this plant's pests — grow their favourite flowers nearby to recruit them. companion plant
- Parasitic wasps Braconidae & TrichogrammaHunts Leaf miners
Tiny, stingless wasps that lay eggs in or on caterpillars, aphids, and moth eggs — the white cocoons riding a hornworm are their calling card, so leave that caterpillar be. Umbel flowers with shallow nectar keep the adults fuelled.
Sambucus, Cotinus, Gleditsia, Native grasses
Green-leaved trees, like this one.
The Cottage Garden Companion — the gardener's herbarium at cottagegardencompanion.com