41 plants sold in nurseries that suppress whatever you plant beneath them

Black walnut is famous for it. Most gardeners could not name a second. USDA flags 57 plants as allelopathic — they release compounds that chemically suppress their neighbours — and 41 of them are routinely sold. The label never mentions it, and the effect is usually blamed on the shade.

Published 11 September 2026 · Understory Guide · USDA PLANTS data, catalogue version 2026-09-11

What allelopathy actually is

Allelopathy is chemical interference: a plant releases substances through its roots, leaf litter or decaying tissue that inhibit germination or growth in other plants nearby. It is not competition for light or water. A plant can be allelopathic and still leave plenty of both.

This is why it goes unnoticed. When nothing establishes under a tree, the obvious explanation is shade and root competition — and both are usually true as well. The chemical layer on top is invisible, so the gardener concludes they picked the wrong shade plant and tries another one, which also fails.

USDA records this as a flat yes/no field, Known Allelopath. That is a real limitation and we will come back to it: the data tells you that a plant does this, never how strongly or against what.

The numbers

• 57 plants in our catalogue are flagged allelopathic by USDA
• 41 are routinely available from nurseries
• 34 are native to North America — this is not an invasive-species story
• 9 carry a USDA invasive or noxious status as well
• By habit: 30 trees, 10 grasses, 9 shrubs, 7 perennials

Trees dominate, which compounds the problem: a tree is the one plant you site permanently and then plant underneath for the next thirty years.

The ones that will surprise you

Black walnut and butternut are the textbook cases and both are here. The list also contains plants nobody warns you about:

• Sugar maple — among the most planted shade trees in North America, and the reason a great many "shade garden under a maple" projects fail.
• Eastern redcedar, common juniper, western juniper, Utah juniper — four junipers, routinely sold as screening.
• Chokecherry, great laurel and kinnikinnick — all staples of native-plant gardening.
• Tall fescue and Bermudagrass — two of the most widely sown lawn grasses, which matters if you are trying to establish a wildflower bed in former turf.
• Alfalfa and sweetclover — both frequently recommended as cover crops and green manure, where suppressing the next crop is precisely the outcome you do not want.

All 41, as sold

Grouped by growth habit. Plants that also carry a USDA invasive or noxious status are marked.

Trees

1. 
   black mangrove Avicennia germinansZone 8+ · 30 ft · native
2. 
   black walnut Juglans nigraZone 4+ · 100 ft · native
3. 
   butternut Juglans cinereaZone 3+ · 80 ft · native
4. 
   California laurel Umbellularia californicaZone 5+ · 80 ft · native
5. 
   chokecherry Prunus virginianaZone 2+ · 25 ft · native · ⚠ also USDA invasive/noxious
6. 
   eastern redcedar Juniperus virginianaZone 2+ · 50 ft · native
7. 
   great laurel Rhododendron maximumZone 5+ · 25 ft · native
8. 
   little walnut Juglans microcarpaZone 6+ · 20 ft · native
9. 
   netleaf hackberry Celtis laevigataZone 2+ · 20 ft · native
10. 
    sugar maple Acer saccharumZone 2+ · 100 ft · native
11. 
    sugarberry Celtis laevigataZone 4+ · 80 ft · native
12. 
    twoneedle pinyon Pinus edulisZone 3+ · 50 ft · native
13. 
    Utah juniper Juniperus osteospermaZone 4+ · 26 ft · native
14. 
    western juniper Juniperus occidentalisZone 3+ · 33 ft · native
15. 
    Chinaberrytree Melia azedarachZone 8+ · 50 ft · introduced · ⚠ also USDA invasive/noxious
16. 
    English walnut Juglans regiaZone 6+ · 60 ft · introduced
17. 
    river redgum Eucalyptus camaldulensisZone 8+ · 120 ft · introduced
18. 
    swampmahogany Eucalyptus robustaZone 8+ · 160 ft · introduced
19. 
    Sydney bluegum Eucalyptus salignaZone 9+ · 160 ft · introduced
20. 
    Tasmanian bluegum Eucalyptus globulusZone 8+ · 160 ft · introduced
21. 
    tree of heaven Ailanthus altissimaZone 5+ · 55 ft · introduced · ⚠ also USDA invasive/noxious
22. 
    white leadtree Leucaena leucocephalaZone 9+ · 20 ft · introduced
23. 
    white willow Salix albaZone 3+ · 70 ft · introduced

Shrubs

1. 
   bog Labrador tea Ledum groenlandicumZone 3+ · 3 ft · native
2. 
   common juniper Juniperus communisZone 2+ · 10 ft · native
3. 
   greenleaf manzanita Arctostaphylos patulaZone 4+ · 6 ft · native
4. 
   kinnikinnick Arctostaphylos uva-ursiZone 3+ · 0.5 ft · native
5. 
   rhodora Rhododendron canadenseZone 4+ · 18 ft · native
6. 
   shadscale saltbush Atriplex confertifoliaZone 3+ · 3 ft · native

Grasses

1. 
   Bermudagrass Cynodon dactylonZone 9+ · 1.5 ft · introduced · ⚠ also USDA invasive/noxious
2. 
   cereal rye Secale cerealeZone 2+ · 3.5 ft · introduced
3. 
   grain sorghum Sorghum bicolorZone 11+ · 8 ft · introduced
4. 
   Johnsongrass Sorghum halepenseZone 5+ · 3.5 ft · introduced · ⚠ also USDA invasive/noxious
5. 
   Sudangrass Sorghum bicolorZone 12+ · 2.5 ft · introduced
6. 
   tall fescue Schedonorus arundinaceusZone 7+ · 4 ft · introduced · ⚠ also USDA invasive/noxious
7. 
   timothy Phleum pratenseZone 2+ · 3 ft · introduced · ⚠ also USDA invasive/noxious

Perennials

1. 
   Jack in the pulpit Arisaema triphyllumZone 5+ · 1.5 ft · native
2. 
   Tilesius' wormwood Artemisia tilesiiZone 1+ · 36 ft · native
3. 
   alfalfa Medicago sativaZone 2+ · 2 ft · introduced
4. 
   sweetclover Melilotus officinalisZone 3+ · 6 ft · introduced · ⚠ also USDA invasive/noxious
5. 
   white sagebrush Artemisia ludovicianaZone 3+ · 3 ft · introduced

What we do with this data

The Guild Builder excludes every one of these from every companion layer it suggests, along with the 114 plants carrying an invasive status — 162 plants in total that it will never recommend as a neighbour. If you pick a flagged plant as your target, it tells you rather than silently substituting something else. Your choice stays yours; you just get to make it knowing.

Every affected plant profile carries the warning too, quoted rather than inferred.

Related

• Guild Builder — it already excludes all 41
• 528 native plants no nursery sells
• Dry shade: 179 plants can take it, 87 you can actually buy

Written and edited by the Understory Guide team. We publish the query behind every number. Corrections: contact us.