Asiatic Lilies and Juglone Under Walnut Trees

Many amateur gardeners have no idea which invisible substance is at work in their soil.

Between magnificent flowers and deep roots, a quiet chemical battle is taking place: walnut trees release a toxin into the ground that can knock out numerous plants completely. Asiatic lilies are regarded as a hopeful option – but how much of this substance, known as juglone, can they actually tolerate?

What the toxin juglone is all about

Walnut trees may look harmless, but they employ a fairly uncompromising tactic: they release juglone, a naturally occurring toxin. It is produced chiefly by black walnut, butternut and the familiar English walnut.

Juglone occurs in almost every part of the tree, including its roots, bark, leaves and green husks. Rain carries the substance into the soil, while fallen leaves and husks intensify its effects. This creates a “problem zone” for sensitive plants extending roughly 15 to 25 metres from the trunk.

“Juglone blocks cellular energy production in many plants – they wilt, turn yellow and often die within just a few days.”

Clay soils make matters worse because they retain water – and therefore juglone – for longer. Sandy soils or soils with very good drainage release the substance more quickly. Waterlogging is toxic to sensitive species in two ways: their roots remain in juglone-containing water for an extended period.

Which plants suffer most – and which do not

Experts have now documented clearly which plants cannot cope with juglone at all. Those badly affected include:

• Tomatoes, peppers, potatoes and other members of the nightshade family
• Rhododendrons and azaleas
• Many heath family plants

These species usually wilt quickly and do not recover. By contrast, several spring-flowering bulbs, tough perennials and a range of North American woodland species cope remarkably well. Over the course of evolution, they have developed ways of living with juglone.

Asiatic lilies: surprisingly resilient in walnut tree shade

There is encouraging news for lily enthusiasts: Asiatic lilies have a noticeably greater tolerance of juglone than many other garden plants.

“Asiatic lilies are listed by universities as juglone-tolerant ornamental plants and can often grow even within the influence zone of large walnut trees.”

The cells of these lilies appear to be less susceptible to the disruption of respiration. They do not wilt immediately, continue producing flower stems and usually bloom reliably. They are not completely “bulletproof”, however: in heavily affected conditions, they often grow less vigorously, remain shorter or produce fewer flowers than they would in soil without juglone.

How different lily groups respond to juglone

Anyone wishing to minimise risk should begin with a small trial bed containing a few bulbs. If the plants survive for two seasons and flower well, the bed can then be expanded.

The role of soil: drainage determines success or failure

How well Asiatic lilies cope with juglone depends greatly on soil conditions. Above all, water needs to drain away quickly from the root zone.

“The better the soil drains, the faster juglone is diluted around the roots – and the greater the chances of healthy lilies.”

A practical test is to dig a hole around 30 centimetres deep, fill it with water and start timing. If water is still present after 24 hours, the site needs improvement before lilies are planted.

Making soil more suitable in areas affected by juglone

• Work in plenty of mature compost or well-rotted farmyard manure
• Loosen heavy soil with sand or fine grit
• Create slightly raised beds so that water can run off to the sides
• Water regularly but thoroughly, rather than applying small amounts constantly

Organic matter does more than improve soil structure: it also supports soil organisms that may break down juglone more quickly. This noticeably reduces the pressure on the bulbs.

Practical strategies for lily beds beneath walnut trees

Anyone intending to plant Asiatic lilies within the influence zone of a walnut tree should follow a few basic principles.

Choosing the position and planting depth

Lilies perform best in the outer third of the canopy spread, or just beyond it. Juglone concentrations are generally lower there, the soil is easier to improve and competition from tree roots is somewhat reduced.

• Plant lily bulbs 15–20 centimetres deep (Asiatic lilies: often 6–8 inches, which is roughly this depth)
• Keep the soil around the bulbs loose and avoid repeatedly treading it down
• Disturb walnut tree roots only carefully, otherwise the tree will produce even more new growth

Managing walnut leaves and mulch correctly

Walnut tree leaves, green husks and twigs contain juglone and related compounds. As little of this material as possible should be allowed to remain directly in the lily bed.

“Remove walnut leaves from the lily bed as promptly as possible – the less material decomposes in the soil, the lower the juglone burden.”

Suitable mulches include neutral materials such as bark mulch without walnut content, chipped material from other deciduous trees, straw or coarse compost. A layer two to three centimetres deep is enough to retain moisture and stimulate soil life.

Seasonal care: helping lilies withstand stress

Vigorous lilies cope better with mild toxic pressure. Weak, underfed plants are more likely to collapse quickly.

• Feed regularly during the growing period, but do not over-fertilise, for example with a balanced bulb fertiliser
• During dry spells, water less often but deeply
• Monitor plants particularly closely in their first year in position: check the leaves, height and number of flowers
• Lift severely ailing specimens promptly and reconsider the site

Typical signs of stress include sudden wilting despite moist soil, conspicuous yellowing that progresses from the bottom upwards, and severely restricted height growth. In these cases, it is worth checking for compacted soil, waterlogging and the distance to the nearest substantial walnut root.

Resistant lily species as an insider tip

Alongside Asiatic hybrids, there are several species lilies that naturally occur near members of the walnut family. These include North American species such as Lilium canadense and Lilium superbum. Having adapted to juglone over many generations, they are considered robust alternatives by knowledgeable growers.

Those seeking greater variety can combine several lily types and observe over a few years which perform best. Gradually, this creates an individual “best-of mix” of genuinely suitable varieties for a walnut garden.

Suitable bed companions and intelligent combinations

A bed containing only lilies can soon appear static. A better approach is to mix them with other juglone-tolerant perennials and bulbs. Many gardeners report good results with:

• Hostas for partial shade
• Astilbes for colourful summer accents
• Various ferns as calm background planting
• Numerous spring-flowering bulbs, such as daffodils or crocuses

These combinations spread the risk: if one species does fail, the bed as a whole remains attractive. At the same time, the lilies benefit from a more balanced microclimate and better-shaded soil.

Why juglone changes through the year

Juglone is not present in the soil at a constant level. Concentrations generally increase during the walnut tree’s main growing period and after leaf fall. During transitional periods – in early spring or later autumn – the burden is often lower.

This points to two favourable times for new planting: early in the year, as soon as the ground can be worked, or in late autumn, after most leaves have been cleared away. Lily bulbs can then establish roots during a comparatively quieter period before the next wave of toxin arrives.

Assessing the risks realistically – while still creating colourful beds

A walnut tree does not mean accepting a bare “toxic zone”. Gardeners who understand juglone, assess their soil and choose appropriate plants can create lush beds there too. Asiatic lilies offer an interesting middle ground: they are not invulnerable, but they are noticeably more resilient than many traditional bedding perennials.

With trial plantings, effective drainage, careful leaf management and thoughtful variety selection, stable pockets of lilies can be developed step by step beneath or beside walnut trees. The result can be a garden in which the trees’ chemical defence strategies and gardeners’ enthusiasm for flowers coexist perfectly well.