The English Walnut- creating a planting poser…

Whilst on holiday in Suffolk, recently, one of my friends, Richard (who lives in Bristol), posed an interesting question:

‘I have a Walnut Tree in the garden and have been trying to grow some plants in a raised area underneath it, against a wall. This area only gets early morning sun during the middle of the year. I believe that Walnuts deposit some sort of poison in the ground which affects the plants? Over 20 years I’ve managed to establish a small selection of plants through trial and error (mainly the latter); geraniums give some cover and with a ‘Chelsea Chop’ may give a second flowering, but only towards the lighter edge of the bed; Ivy seems to do well; I can squeeze out some summer colour by planting some annual begonias but these need a lot of watering. I’ve also tried several different ferns, but none have been a success to date. Any thoughts about planting the area, especially towards the back, would be welcome.’

Not having come across this issue before I did a bit of research and also sought some advice from the RHS Member’s Advice Service. There are two main species of Walnut tree; Black Walnut (Juglans nigra) – most common in North America (and native to eastern areas), and the English Walnut (Juglans regia), the type most often found in the U.K. Walnuts -especially Juglans nigra– contain a chemical called “juglone” which can be poisonous (or allelopathic) to other plants. Juglone appears to act as a respiration inhibitor which deprives sensitive plants of needed energy for metabolic activity.  For gardeners this means that many plants growing in the vicinity of a black walnut tree will either be killed or will struggle to live, with yellowing, wilting leaves.

Leaves and nuts on the Black Walnut

Juglone is found in all parts of the black walnut tree, but it is most concentrated in the flower buds, nut hulls and roots. Unfortunately the roots of a black walnut can extend 3-4 times the diameter of the tree’s canopy, so the area affected is quite wide. Toxicity is further dependent on the soil’s texture and drainage. All walnuts – including the English Walnut- produce some juglone, as do the walnut relatives bitternut hickory, hickory, pecan and shagbark. However the amount of juglone produced is insignificant, compared to the black walnut, and the effect on other plants is minimal, if any.

Tomatoes seem to be the most sensitive to growing under black walnuts. However juglone sensitivity is also dependent on other growing conditions and what will or won’t grow under one black walnut tree may be fine under another. However, here is a compiled list of flowers and vegetables that are considered extremely sensitive to juglone.

Richards’ problem patch

Geraniums surviving near the lighter canopy edge…

Turning back to Richard’s question, Guy Barter, Chief Horticultural Adviser at the RHS, says:

‘In theory English walnuts secret Juglone and we would advise that it is not underplanted – in any case they cast such a heavy shade that few plants will thrive underneath a walnut. Of course roots spread out beyond the canopy but interestingly we seldom encounter difficulties in the UK. All the same it may be well to avoid apples and tomatoes near a walnut tree, or at least make some experimental plantings in the first instance. In the USA the light levels are very much higher and underplanting options greater and it is not surprising that problems arise.’

Not very encouraging, eh, Richard? You seem to have had some success with plants nearer the lighter edge of his planting area, and if you want to persist with trying to plant up this and the darker inner area, you might want to start by improving the growing conditions; maybe you can prune the tree a little to raise the crown and let in more light? Having a raised bed underneath is good because you can deepen the soil above the tree roots; and maybe you can also try to replace the topsoil which could over time become affected by juglone? It might also be a good idea to add organic matter each year and at the same time remove all the walnut tree leaves and litter and dispose of these – it’s probably best to avoid composting it.

As to planting, for the inner area (assuming that you can find planting pockets large enough around any tree roots), I’d try out a few small to medium – sized shrubs and other plants that tolerate heavy shade and dryish soil – in effect trying to create an ‘understorey’ with plants of varying heights and in a combination to provide something of interest all-year round. If you go down this route then try to get healthy, well-established plants (2-3 years old and/or in 3 litre pots). As Guy suggests, you’ll have to ‘trial’ these to see if they’re up to the task! My recommended ‘planting scheme’ would include, for the darkest area (and maybe including some Ivy as ground cover):

• Lonicera pileata– spreading evergreen shrub with small creamy flowers in spring followed by purple berries; height 60cm (2ft), spread 2.5m (8ft)
• Hypericum calycinum (‘Rose of Sharon’) – spreading shrubs with yellow flowers from mid summer. Height 60cm (2ft), spread indefinite
• Vinca minor (lesser periwinkle) – mat-forming shrub with trailing shoots and violet-blue flowers from mid-spring to autumn. Height 10-20cm (4-8in), spread indefinite

Lonicera pileata

If you’d like to increase the variety of your planting around the edge (and lighter) area, you could try in addition to your geraniums:

• Cotoneatser simonsii– deciduous or semi-evergreen with good autumn leaf colour; small pink flowers in summer followed by bright orange-red fruit. Height 2.5m (8ft), spread 2m (6½ft)
• Mahonia aquifolium– evergreen with yellow blooms in spring. Height 90cm (3ft); spread indefinite.
• Sarcococca hookeriana var. humilis (Christmas box)- evergreen with scented flowers in winter. Height 60cm (2ft); spread 1.2m (4ft)

Cotoneaster simonsii

Mahonia aquifolium

…and a couple of herbaceous perennials:

• Bergenia– evergreen leaves may colour in winter; white, pink or red flowers in spring. Height 20-45cm (8-18in), spread 45-63cm (18-25in)
• Tellima grandiflora – semi-evergreen, with greenish white flowers from spring to mid-summer. Self-seeds freely. Height 40cm (16in), spread 25cm (10in)

Tellima grandiflora

…and maybe one or two patches of bulbs/tubers?

• Anemone nemorosa– white flowers from spring to early summer. Height 7.5-15cm (3-6in), spread more than 30cm (1ft)
• Cyclamen hederifolium– attractively marked foliage and pink to maroon flowers in autumn, seeds freely. Height 10-13cm (4-5in), spread 15cm (6in)
• Galanthus nivalis (snowdrop) – white flowers in late winter. Height and spread 10cm (4in)
• Iris foetidissima (stinking iris) – blue flowers in late spring and orange berries in autumn/winter. Height and spread 40cm (16in)

Iris foetidissima- flower

…and fruit

If this all seems rather too much hassle, then you could always cover the bed with a landscape fabric and then put a layer of aggregate or other mulch- wood bark, slate chippings etc. This covering could be punctured at intervals for clumps of bulbs (see above) to give you some (simpler) interest throughout the year- and maybe end the years of heart ache trying to get things to work that just don’t stand much of a chance!

Sources and further information:

Black walnuts at About.com

RHS guide to planting under trees

Old School Gardener

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