07 Feb 2012

What Will Grow Near My Black Walnut Trees?

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A chemical called Juglone occurs naturally in all parts of the members of walnut family (Juglandaceae).  These trees include Butternut, English Walnut, Pecan, Shagbark Hickory,  Bitternut Hickory and Black Walnut. Of these trees, it is generally only the Black Walnut which produces juglone in large enough quantities to cause problems for gardeners.  Many plants are sensitive to this chemical and when grown in the vicinity of Black Walnut trees will exhibit symptoms of foliar yellowing, wilting and eventual death.  The highest concentration of juglone is found in the buds, nuts, hulls and roots of the tree but can also be found in the leaves and stems.  Juglone is not particularly water soluble and so does not readily move throughout the soil.  The effects of juglone tend to be intensified during a drought.   The area within the canopy of a Black Walnut tree is the most toxic to sensitive plants due to the high root density and the presence of decaying leaves and nut hulls.  However, anywhere within the area of root growth may prove fatal to the most sensitive plants.  The roots of a Black Walnut tree may spread up to one and a half times as far as the tree is tall. An established tree in your neighbor’s landscape may affect your garden as well .  Decaying roots continue to release juglone, so sensitivity may continue for several years after the tree is removed.

Gardens should be located away from Black Walnuts whenever possible.  A garden that must be located near these trees requires careful planning. Refer to the list below for suggested plant material to try in the areas around these trees.   Some research has shown that decreased toxicity seems to result from excellent soil drainage.  Any steps taken to improve soil drainage such as amending with organic matter or replacing the soil with a lighter soil type may help.  A raised bed may be possible  if  a layer of landscape fabric is placed under the soil to minimize root penetration into the bed.  Leaf litter and nuts must be removed from such beds.  There are some conflicting thoughts on whether Black Walnut tree leaves can be composted and used as mulch.  It is probably advisable to avoid using even composted leaves, bark or wood chips in the landscape.

Very few plants have been experimentally tests for their tolerance to juglone.  However, the following lists have been taken from various published lists.  Since light conditions, moisture, soil drainage and soil micro-organisms all affect a plant’s ability to grow in the presence of juglone, these lists should be considered only a guide.  Testing in your own landscape may be the only definitive way to know what plants work with your situation.

Juglone Tolerant Plants

  • Begonia
  • Calendula
  • Morning Glory
  • Moon Vine
  • Pansy
  • Viola
  • Crocus
  • Daffodils
  • Grape Hyacinth
  • Hyacinth
  • Tulip
  • Ajuga
  • Astilbe
  • Bee Balm
  • Bellflower
  • Blackberry Lily
  • Candy Lily
  • Chrysanthemum
  • Cinnamon Fern
  • Coral Bells
  • Clematis
  • Cransbill
  • Crested Wood Fern
  • Daylily
  • European Wild Ginger
  • Hollyhock
  • Hosta
  • Jacob’s Ladder
  • Lamb’s Ear
  • Lungwort
  • Pachysandra
  • Phlox
  • Siberian Iris
  • Solomon’s Seal
  • Spiderwort
  • Stonecrop
  • Sweet Woodruff
  • Trillium
  • Virginia Creeper
  • Exbury Azaelea
  • Forsythia
  • Hibiscus
  • Honeysuckle
  • Red Cedar
  • Winged Euonymus
  • Viburnum
  • Black Locust
  • Canadian Hemlock
  • Crapapple (not Hopa)
  • Dogwood
  • Elm
  • Hickories
  • Maples
  • Oaks
  • Redbud
  • Sycamore
  • Parsnips
  • Lima & Snap Beans
  • Beats
  • Corn
  • Onion
  • Kentucky Blue
  •  Fescue
  • Cherry
  • Black Raspberry

Plants Sensitive to Juglone

 

Annuals:  Nicotiania, Petunia

Perennials:  Columbine (Aqilegia),  Peony (Paeonia)

Bulbs:  Autumn Crocus, Narcissus ‘Kng Alfred’, Narcissus ”Ice Follies’, some Tulips

Shrubs:  Azalea, Hydrangea, Lilac, Mugho Pine, Potentilla, Rhododendron, Viburnum ‘Mariessi’, Privet

Trees:  Silver Maple, White Birch, Hopa Crabapple, Loblolly Pine, Scotch Pine

Vegetables:  Cabbage, Eggplant, Pepper, Potato, Tomato

Fruits:  Apple, Blackberry, Blueberry

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