Black Walnut Tolerant Companion Plants 

Black Walnut, Juglans nigra, is allelopathic, which means it secretes substances (generally considered to be juglones, but there has not been enough research to say for sure - it's probably a combination of chemicals, but for the purposes of this article we will say juglones) poisonous to other plants. 

It is a great way of dealing with the competition and can make "companion" planting problematic. 


The harmful effects are strongest on soil that is both wet and low in organic matter like sand or clay and weakest on well-drained, rich soil. After removing a Black Walnut tree, the soil will return to normal again surprisingly quickly. Some studies suggest as little as two months (but I would be inclined to leave it for a year - Ed).  


Below is a list of plants that have no problem at all growing near a Black Walnut. It is not exhaustive, but it's worth noting that the true list of plants that can grow near Black Walnut is much longer than the list of plants that can't. 
Black Walnut is most effective at stifling the growth of new seedlings, but when a human gardener transplants plants under it then that vulnerable phase is skipped.


Trees

  • Most maples apart from silver maple
  • Amelanchier
  • Liquidambar styraciflua
  • Liriodendron tulipifera
  • Birch Betula nigra
  • Sycamore
  • Dogwood
  • Oaks
  • Hawthorn
  • Arborvitae (Thuja occidentalis)
  • Honeylocust (Gleditsia triacanthos)
  • Viburnum 

Annuals

  • Calendula officinalis, pot marigold, 
  • Opomoea, Morning Glory 
  • Viola wittrockiana, Pansy

Bulbs

Herbaceous Perennials

  • Ajuga reptans, Bugle
  • Astilbe
  • Campanula latifolia
  • Dryopteris cristata, Crested Wood Fern
  • Galium odoratum, Sweet Woodruff
  • Geranium sanguineum, Bloody Cranesbill/Geranium
  • Hemerocallis, Daylily
  • Heuchera varieties
  • Hosta fortunei
  • Hosta marginata
  • Hosta undulataIris sibirica
  • Monarda didyma
  • Phlox paniculata
  • Polemonium reptans, Jacobs ladder
  • Primula x polyantha, Primrose
  • Pulmonaria, Lungwort
  • Sedum spectabile, Stonecrop

Shrubs & Climbers


Using Composted Black Walnut & Woodchips

Well-composted material from black walnuts does not pose a problem, but to be on the safe side it is recommended not to use it for seeds and seedlings, or plants that are known to be juglone sensitive. 

You will often see advice saying that black walnut woodchips should be well aged before use as a mulch, but this is being overcautious as the wood itself contains almost no juglone, and the bark only a little, and that is mostly during the growing season - trees are typically felled in winter.
Within a fortnight of being chipped, practically all of it will have decayed.