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Underplanting Black Walnuts

 


Black walnut trees can cause many headaches for gardeners who have these trees in their landscape.  For those of us with these trees, it seems almost impossible to grow anything beneath them.  Black walnuts produce a toxin called juglone, an allelopathic chemical that kills or inhibits the growth of many plants.  Luckily there are many plants that will survive these conditions.  Many native wildflowers thrive in close proximity to the trees.

The first symptom of walnut toxicity is wilting of terminal shoots, often followed by irreversible wilting of much or all of the rest of the plant.  A mild reaction to toxicity may appear as poor flower development or reduced growth.

Gardeners should avoid adding leaves of black walnut to their compost or using them as mulch, because the juglone can leach into the surrounding soil.  Research indicates that for the most part only plants whose roots come into direct contact with black walnut roots are affected by juglone.  Although walnut roots can extend as far as 50 to 60 feet out from the trunk, plants that are close to the base of the tree, where walnut roots are deeper, or that have shallow root systems are sometimes unaffected.  Many plants should be avoided when planting in close proximity to black walnut trees.  Among them are such fruits and vegetables as tomatoes, apples, cabbages, potatoes, and most berries other than black raspberries.  Many ornamentals to avoid include most evergreens, laurels and cotoneasters.  Use caution when planting annuals and perennials. 

The following is a list of perennials found to survive the toxicity levels of juglone and can thrive under black walnut trees.

Ajuga – Bugleweed
Arisawma triphyllum – Jack-in-the-Pulpit
Astilbe – False Spirea
Chrysanthemum – Garden mums
Dryopteris marginalis – Marginal shield fern
Clematis micropetala – Clematis-small flowering
Clematis paniculata – Sweet Autumn
Epimedium – Barrenwort
Erythronium am. – Dogtooth violet
Galium – Sweet Woodruff
Geranium – hardy Cranesbill
Hemerocallis – Daylily
Heuchera – Coral Bells
Heucherella – Mini Bells
Hosta spp. – Paintain Lily
Iris Siberica – Siberian Iris
Mertensia – Virginia Blue Bells
Monarda – Bee Balm
Oenothera – Evening Primrose
Osmunda cinnamomea – Cinnamon Fern
Phlox paniculata – Garden Phlox
Podophyllum peltatum. – Mayapple
Polemonium reptans – Jacob’s Ladder
Polygonatum – Solomon’s Seal
Polystichum across. – Christmas Fern
Pulmonaria – Lungwort
Stachys lanata – Lamb’s Ear
Tiarella – Foamflower
Tradescantia – Spiderwort
Uvularia grand – Bellwort

Bulbs Include
Darwin Tulips
Gregii Tulips
Crocuses
Snowdrops
Grape Hyacinths
Daffodils