Gardening under a Black Walnut Tree

Winter is a great time to plan changes in your garden, or to plan a new garden.  One of the most restrictive obstacles to have in your garden is a large Black Walnut tree.  These trees, while lovely to look at in summer, have somehow evolved to eliminate all competing vegetation from their environment.  They do this by disseminating a powerful toxin into the earth and onto other plants through their root systems, by way of the leaves, nuts and stems they shed during the year, and also, possibly by shedding their toxins into rain and water that falls onto them and into your garden.

Black Walnut Garden this past July

When I first encountered this enemy, I was unaware of its evil intentions.  I had conceived of a long, lush, undulating garden under the shade of my neighbor’s pretty trees running down the property line as far as I could reasonably maintain it.   A year after planting the first section of this grand vision, I noticed my small Russian Olive and Purple Smoke Bush lingering feebly and failing to thrive, while my bulbs came up smaller and weaker than expected and none of my perennials seemed to spread in the normal way.  The neighbor’s other trees were also becoming feeble and dropping dead in the vicinity of the Black Walnuts, as they grew larger.  I researched this issue and searched Southern Ontario for the one plant I did not have that I could find recommended for this problem:  Virginia Bluebells.  I also moved some large, light green hostas under them which were here when  we bought the property.

Soon it was clear that the Virginia Bluebells were a slow spreader (they eventually disappeared completely), while other plants died a slow and miserable death.  No amount of compost, fertilizer and top-dressing could salavage this site, it appeared, and the weaker the plants became the more difficult it was to keep weeds down.  Meanwhile, to fill in the gaps, I began tossing extras and divisions of things that had overgrown their spots in my other gardens under the evil trees.  Eventually I came up with a garden that works.  These are my recommendations for a thriving (more or less) garden under the seige of a Black Walnut:

Plants that work

• 

  Purple Coneflower thrives under the Black Walnut trees

  Purple Coneflower, Echinacea Purpurea  Of the taller plants I have tried, these are least bothered by the toxins of the Black Walnut.  Mine have spread into a large and expanding mass and have pushed out some of their weaker neighbors.  These plants singlehandedly make my efforts appear successful! Paradoxically, however, their usual friend and neighbor, Black-Eyed Susan (Rudbeckia) will have no part of the Black Walnut garden, growing to less than half its normal height and dying out.

• Grape Hyacinth  This is probably the most prolific grower I have under the Black Walnut trees, and it provides a thick edging of brilliant, early colour.  These plants have spread wantonly to the point where they have almost become pests themselves, and they thrive whether or not you look after the soil.
• Some hostas  As mentioned, an existing large, light green hosta (“Sum and Substance”?) has grown for years around the edges of this garden.  Although it does not grow to the size achieved in other areas of the property, it does not continue to diminish in size and still looks beautiful and healthy each year, covering up the expended Dwarf Hyacinth debris when those are finished their show.  However, my “Big Mama” hosta, a gorgeous blue-grey leaved variety, refuses to thrive.  Each year it gets smaller and weaker and produces desperate, tiny offspring all around it in a feeble attempt to pass on its genes before it dies, I suppose.
• Irises  To my amazement, these beautiful and exotic-looking plants have survived.  I have a Siberian Iris and a very deep, dark purple one that a friend gave me in this
  

  Classic Irises will survive but like fresh soil every so often

  garden, and they have spread and bloomed beautifully year after year in the worst section of the garden.  However, their roots eventually show signs of deterioration and they will need to be dug up, divided and have their soil replaced once in a while; perhaps every five years

• Spiderwort, Tradescantia  This is another plant that so far seems unaffected by its trying setting.  Mine has been under the Black Walnuts for about four or five years and shows no signs of any ill effects.
• Solomon’s Seal  This delightful plant adds a relaxed woodland effect under the trees.  Although it grows larger and darker green in other, less challenging areas of the property, it achieves about one metre in height under the Black Walnuts, and spreads regularly by doubling in number each year.  It will, however, benefit from soil replenishment and the other maintenance suggested below.
• Lily of the Valley  One good toxin deserves another.  I tested extra bits of this resilient, fragrant beauty under the Black Walnut trees a few years ago, and they are spreading happily, much as they do elsewhere.
• Sedum, Purple Emperor  I have a border of these plants at one end of the garden near the failing blue-grey hosta, and they, too, seem to manage from year to year.  Although they do thrive a little better in other locations, this may be due to the fact they are shaded and prefer sun.  Nonetheless, they do not appear to mind the toxins as much as their neighbor, a small purple aster that keeps trying to die no matter what I do.

Sedums and Heuchera Palace Purple succeed in the heavy presence of Black Walnut

• 

  Lilies multiply in the Black Walnut Garden

  White Lily  This plant appears to be an Oriental Lily of some sort.  I planted it with no expectation whatsoever of its survival.  Defiantly, however, it spreads madly and puts on a pretty show every mid-summer, and provides transplants for my planters besides!  I have not tried other lilies so I do not know whether it is unique in its determination.

• Day Lily  While they don’t exactly thrive, they do survive and put on their annual summer performance.  Definitely refresh their soil and divide them in the new soil every so often, and clean up debris from their area.
• Foxglove  My foxgloves survived a long time in the Walnut tree garden, although they eventually did not grow as tall as they started out.  Of course, they are biennial, so they survive by seeding.  As much as I love foxgloves, I do not find they love southern Ontario generally, as they probably prefer less heat and more rain.  Nonetheless, like the Big Mama hosta, they seem to seed much more successfully under the Black Walnuts.  They are definitely worth putting in this situation if you don’t mind them being a little shorter than normal!
• Summer Phlox  I have two varieties of phlox near the dreaded trees.  One is right underneath, and does not grow as tall as it perhaps would elsewhere, but again it is a plant that has persisted year after year and does not seem to be in danger of giving up.
  

  Summer Phlox

  The other is a little further away, and while other plants beside it weaken from the trees’ toxins, this phlox is chest-high and spreading vigorously.  Phlox is worth growing in the sunnier locations near the Walnut trees, because of its brilliant show of flowers in mid- to late summer when there are fewer other things flowering, and, when slightly stunted by the toxins, also fits nicely between shorter and taller perennials, adding variety in height.

I cannot recommend any common feature of plants that will survive.  Some hostas survive and some don’t.  Some acid-loving plants survive, and some die quickly.  Some plants that become weeds elsewhere will die out under the Black Walnuts.  However, having at first thought my meandering border was doomed, I am ultimately pleased with its sleepy beauty and spring-through-frost progression of changing vignettes.

What Won’t Survive  

These are plants that I have found cannot abide the vicinity of a black walnut: 

•  perennial geranium 
•  Nepeta (catmint)
•  hydrangeas
•  lilac 
•  purple smoke bush 
•  ferns  
• Rudbeckia
• Columbine
• Carpathian Bellflower  
• peony
• daffodil
• tulip
• aster

How to Care for the Black Walnut Garden

To start a garden under these trees, you need to dig out the existing soil, which may already be full of toxins, and replace it with fresh topsoil.   This will give your new garden a good start, but it will not prevent the dripping of toxins and dropping of tree debris onto the garden.  Therefore, you will have to clean up the garden thoroughly in spring and fall, raking all bits of the tree up and getting rid of them.  Naturally, you will not want to throw them in your compost either!

In particular, remove the nuts which seem to be the most toxic part of all.  They simply pollute the soil wherever they fall, and the larger the trees grow the more exponentially numerous the nuts become.  In addition, squirrels adore them and bury and shred them all over the property with the result that more Black Walnut trees sprout everywhere and toxic, staining nut debris may even mark up your hardscaping and kill patches of lawn.  The seedlings are very hard to remove once started, because they have a deep tap root that grows rapidly straight down, and if you merely cut a seedling off without the root, it will keep growing back like a dandilion!

Over time, the trees’ roots will grow into your new garden soil, but you will need to top-dress it every two years or so, and mulch the top of it to minimize the impact of the setting and make it harder for tree droppings to get to the soil and root systems of your plants.  Mulch is added around the plants, but not touching them, and it will break down over time and nourish the soil as well.

We were lucky enough to have a lovely, understanding neighbor who encouraged us to prune limbs to our hearts’ desire.  Pruning low-hanging limbs definitely makes a big difference to the health of the garden.

For one of the better articles I have seen on this issue, although the author’s plant recommendations differ from mine, I would suggest:

http://www.icangarden.com/clubs/MarkhamGHS/document.cfm?task=viewdetail&itemid=63&categoryid=0

In the end, it is possible to have both the privacy and beauty provided by the mature Black Walnut trees, as well as the garden under them.  Since all gardens require maintenance of some sort, it is really not that difficult to succeed under the Black Walnut with the right plants and targeted care.

I would love to hear what works for anyone else who has tried designing gardens near Black Walnut trees, so please comment on this post.

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black walnut, toxic tree