Juglone-Tolerant Plants for Gardening Near Black Walnut Trees

What Would You like to Learn About Today?

While black walnut (Juglans nigra) trees are beloved for their rich history, they have accrued a problematic reputation among gardeners for a few reasons. They develop an extensive root system, cast a dense shade, and, most notably, they produce a toxic chemical called juglone. Or, really, they produce the precursor called hydrojuglone. Hydrojuglone is found in high concentrations in the roots and hulls of black walnuts, and as it makes contact with the soil, it is transformed into juglone. Why is this problematic? Long story short: juglone is toxic enough to kill surrounding plants. Researchers aren’t quite sure how it happens, but they believe it interferes with photosynthesis, respiration, and water uptake.

There are two ways juglone spreads to nearby plants. Large, fleshy nuts develop early to mid-fall and drop to the ground. As the outer shells (or hulls) break down, they release high concentrations of juglone. The second is allelopathy – which is practically chemical warfare for plants. Their roots exude chemicals (in this case, juglone) into the soil for nearby plants to uptake.

Is growing near a black walnut tree a death sentence? No, it’s absolutely not. For how much it’s discussed, there surprisingly hasn’t been much research on it. The scientific community can’t conclusively agree on whether allelopathic juglone can be spread in high enough concentrations to kill nearby plants. Plus, many symptoms from juglone poisoning overlap with drought stress, so it may be misdiagnosed in the garden. Gardening near (or even under) walnut trees is still possible, so don’t resent your trees just yet. We’ve compiled a list of plants showing juglone tolerance in the landscape to help.

Juglone-tolerant shrubs

  1. Arborvitae (Thuja)
  2. Barberry (Berberis)
  3. Beautyberry (Callicarpa)
  4. Clematis
  5. Deutzia
  6. Dogwood (Cornus)
  7. Elderberry (Sambucus)
  8. Forsythia
  9. Honeysuckle (Lonicera)
  10. Juniper (Juniperus)
  11. Mockorange (Philadelphus)
  12. Ninebark (Physocarpus)
  13. Rose of Sharon (Hibiscus)
  14. Smooth Hydrangea (Hydrangea arborescens)
  15. St. John’s Wort (Hypericum)
  16. Viburnum
  17. Winterberry Holly (Ilex verticillata)
  18. Blue Holly (Ilex x meserveae)
Philadelphus Illuminati blooming in the garden

Pictured above: Illuminati collection

Honorine Jobert Japanese anemone has white flowers.

Juglone-tolerant perennials

  1. Anemone
  2. Aster
  3. Astilbe
  4. Barrenwort (Epimedium)
  5. Bee Balm (Monarda)
  6. Bellflower (Campanula)
  7. Bleeding Heart (Dicentra)
  8. Bugleweed (Ajuga)
  9. Cardinal Flower (Lobelia)
  10. Coral Bells (Heuchera)
  11. Cranesbill (Geranium)
  12. Daylily (Hemerocallis)
  13. Coneflower (Echinacea)
  14. Ferns
  15. Goatsbeard (Aruncus)
  16. Hosta
  17. Iris
  18. Jacob’s Ladder (Polemonium)
  19. Lenten Rose (Helleborus)
  20. Lilyturf (Liriope)
  21. Lungwort (Pulmonaria)
  22. Milkweed (Asclepias)
  23. Pachysandra
  24. Phlox
  25. Primrose (Primula)
  26. Sedge (Carex)
  27. Shasta Daisy (Leucanthemum)
  28. Speedwell (Veronica)
  29. Stonecrop (Sedum)
  30. Sweet Woodruff (Galium)
  31. Violet (Viola)
  32. Yarrow (Achillea)

What are the symptoms of juglone toxicity?

The main symptoms of juglone toxicity are yellow leaves, wilting, and stunted growth. If plants are especially sensitive, symptoms can appear days after planting, while other plants may take years to show signs. It’s believed that juglone blocks the vasculature (or water-uptake pathways), which is why juglone toxicity can look similar to drought stress.

With that being said, many plants suffering from drought stress may be misdiagnosed as juglone-sensitive. Black walnut trees have an aggressive root system, making it hard for younger plants to compete for water and nutrients. Plus, the dense leaf cover can prevent rainfall from really saturating the soil beneath the canopy. Irrigating is the best way to ensure your newly planted perennials and shrubs can grow under walnut trees.

Which plants are most sensitive to juglone?

Shrubs like lilacs, azaleas, rhododendrons, yews, and hydrangeas (besides smooth hydrangea) are juglone-sensitive, along with perennials like peonies, columbine, and lilies. Many vegetables like tomatoes, asparagus, peppers, and potatoes have also exhibited juglone sensitivity. There isn’t a definitive list of sensitive plants, but those listed have grown poorly under walnut trees, as observed by horticulturists and gardeners.

What precautions should I take when gardening near walnut trees?​

Unless you’re growing plants with high sensitivity to juglone, the biggest threat is falling fruits and leaves. Aside from the roots, the hulls of the fruits contain the highest concentration of hydrojuglone. The leaves have a lower concentration, but their sheer quantity can do some damage. Simply remove the fallen leaves and fruits before decomposing to ensure your plants aren’t affected.

Fallen walnut hulls on the ground around the tree

Is it safe to use walnut mulch and leaf compost?

This is a surprisingly a hot topic in the gardening community. The leaves of walnut trees do contain juglone, but the juglone breaks down as the leaves do. Allow your walnut leaves to compost for a month or so before using them in the garden. The wood has little to no juglone, which means it is safe for gardeners to use as mulch. Similarly, any juglone in the wood is broken down after exposure to air and water, so your mulch should be completely safe by the time it reaches your garden.

Written by: Miranda Niemiec, click here to read bio.

Questions, comments, or concerns? Let us know! We have an experienced, knowledgeable staff ready to make sure your garden turns out perfect. Or check out our other blogs, here.