What to plant under a walnut tree?

Why isn't the walnut a good companion plant?

The cultivation under a walnut tree, a large tree in the Juglandaceae family, can pose problems. And there are several reasons for this. First, its foliage is alternate, rather oval, a glossy green, comprising 5 to 9 leaflets, and is particularly dense. The shade beneath its crown is therefore particularly thick. Next, the walnut benefits from a fairly substantial root system: its main stem is vertical and taproot-like, but the secondary roots grow obliquely. They can extend up to 15 to 18 m from the end of the tree.

Finally, it is mainly juglone, a chemical compound, that is implicated. Indeed, it has been scientifically proven that juglone has the power to inhibit germination and respiration of many plants that cannot grow in its neighbourhood. Concretely, this incompatibility manifests as foliage that yellows and withers, signalling the plant’s death. In fact, walnut trees secrete this chemical compound, making them toxic to counter and eliminate competition from other vegetation. It is, in a way, a survival instinct that drives it to retain water and nutrients in the soil.

Planting plants under a walnut tree presents constraints, but it is possible

This allelopathic compound is present in all parts of the walnut tree, namely the foliage, roots, buds, but also the walnut shells. Juglone is present in varying concentrations and varies by walnut species. The black walnut (Juglans nigra) and the butternut (Juglans cinerea) contain a lot, the common walnut (Juglans regia) and the pecan (Carya illinoinensis) somewhat less. This toxic substance persists in the soil for up to a year after the felling of a walnut.

To grow under a walnut, plants must therefore withstand its three components. This does, however, reduce the possibilities… Already note that you can eliminate azaleas and rhododendrons, cotoneasters, the silver maple (Acer saccharinum), kalmia, peonies, Potentilla, rhubarb, but also tomatoes, eggplants, peppers…

Shrubs and climbing plants that can grow under a walnut tree

Some shrubs are sensitive to juglone; others tolerate it well. Nevertheless, they must also tolerate semi-shade or shade. Here is a list of shrubs that can be planted under a walnut tree:

• • The black elder (Sambucus nigra)
  • The daphne (Daphne)
  • The mock orange (Philadelphus)
  • The viburnums (Viburnum)
  • The summer heathers (Calluna)
  • The amelanchier (Amelanchier)
  • The Canadian redbud (Cercis canadensis)
  • The Canadian hemlock (Tsuga canadensis)
  • The hawthorn (Crataegnus)
  • The forsythia
  • The barberry (Berberis)
  • The hamamelis
  • The hydrangea (Hydrangea)
  • The millepertuis (Hypericum)

Some shrubs that thrive under a walnut tree

Also consider climbing plants that tolerate juglone and cope with a bit of shade: the clematis, the climbing hydrangea or ivy: ivy.

Perennials and flowering bulbs that tolerate juglone

Imagine a pretty border of flowering perennials planted beneath your walnut tree. Do you think it’s impossible? Think again, because many perennial plants thrive under the canopy of a walnut tree. Not only because they are tolerant of juglone, but also because they can bloom in partial shade, or even in full shade. Thus, examples include:

• • The creeping bugle (Ajuga reptans)
  • The ‘daylily (Hemerocallis)
  • The liriope (Liriope muscari)
  • The astilbe
  • The anemone
  • The lobelia (Lobelia)
  • The primroses (Primula)
  • Theaspérule odorante (Gallium odoratum)
  • Thelungwort (Pulmonaria)
  • Themeadow rue (Thalictrum)
  • Theelfs’ plant (Epimedium)
  • Themarguerites (Leucathemum)
  • Theasters
  • The campanules (Campanula)

Some perennials to grow under a walnut tree

To accompany these colourful flowering displays that will brighten the shade under your walnut tree, it is also possible to grow plants chosen for their foliage: ferns, heucheras, and hostas are essential.

And to brighten the area under a walnut tree from spring, bulbs are a must. Few fear juglone, so treat yourself to the narcissi, the grape hyacinths, the snowdrops, the scillas, the Chionodoxa, the cyclamens and the crocuses that will thrive beneath the walnut tree.

Why not growing a vegetable garden under a walnut tree?

If you really have no other option, it is feasible to set up a vegetable garden not far from a walnut tree. But bear in mind that some vegetables will never be able to be grown there because of the lack of sun. To start with sun-loving vegetables such as tomatoes, aubergines, peppers and chillies. Nevertheless, with a few precautions, creating a small vegetable garden remains possible. The best solution lies in above-ground gardening in raised beds or square vegetable beds, which will be isolated from the soil by geotextile fabric. Likewise, you should be vigilant in autumn by removing fallen leaves and walnuts that have fallen.

To grow vegetables beneath the vegetable garden, it is recommended to adopt square vegetable beds

Once this vegetable garden is installed, you will be able to grow beetroot, various squashes, carrots and parsnips, beans, onions and sweetcorn. Or mint on the herb side.