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Walnut

Juglans regia

Fruit treeDeciduousFrost HardyToxic

The common or Persian walnut is a large deciduous nut tree from south-eastern Europe and Central Asia with a broad, rounded crown and pinnate, aromatic leaves. Its nuts ripen in a green husk that splits open in late September and October.

Overview

Height12–25 m
Crown diameter12–20 m
Root systemTap-rooted
ShadeMedium shade
SunFull Sun
WaterMedium
HardinessUSDA zones 5–9

Seasons

Flowering (Central Europe)Apr, May, Jun
Harvest (Central Europe)Sep, Oct

Planting time for bare-root trees. Container trees can be planted whenever the soil is workable.

Companion Planting

Good Companions

None

Bad Companions

  • Tomato

    Juglans regia forms the juglone precursor in all green parts; leaf litter releases juglone, which inhibits germination and growth of many plants. J. regia produces less juglone than black walnut, and allelopathy is described as weaker than often claimed; listed as sensitive by NC State Extension for J. regia. In vitro, J. regia leaf extract inhibited tomato only at high concentrations.

  • Potato

    Juglans regia forms the juglone precursor in all green parts; leaf litter releases juglone, which inhibits germination and growth of many plants. J. regia produces less juglone than black walnut, and allelopathy is described as weaker than often claimed; listed as sensitive by NC State Extension for J. regia.

  • Bell Pepper

    Juglans regia forms the juglone precursor in all green parts; leaf litter releases juglone, which inhibits germination and growth of many plants. J. regia produces less juglone than black walnut, and allelopathy is described as weaker than often claimed; listed as sensitive by NC State Extension for J. regia.

  • Chili Pepper

    Juglans regia forms the juglone precursor in all green parts; leaf litter releases juglone, which inhibits germination and growth of many plants. J. regia produces less juglone than black walnut, and allelopathy is described as weaker than often claimed; listed as sensitive by NC State Extension for J. regia.

  • Blueberry

    Juglans regia forms the juglone precursor in all green parts; leaf litter releases juglone, which inhibits germination and growth of many plants. J. regia produces less juglone than black walnut, and allelopathy is described as weaker than often claimed; listed as sensitive by NC State Extension for J. regia.

  • Lettuce

    In a laboratory study, aqueous J. regia leaf extract inhibited lettuce germination at all tested concentrations (juglone-type allelopathy).

Only relations with a documented cause are listed.

Toxicity

SeverityModerately toxic
Toxic toDogs

The nuts are edible for humans. Mouldy walnuts (e.g. fallen nuts left on the ground) can contain tremorgenic Penicillium mycotoxins such as penitrem A, causing vomiting, tremors and seizures in dogs.

Planting Tips

LocationFull sun with plenty of space; the tree is shade-intolerant, and young trees and flowers are sensitive to late frost.
SoilDeep, moist, fertile, well-drained loam with good water-holding capacity and aeration; avoids waterlogged or poor soils.
WateringEstablished trees tolerate dry periods (branches may die back in long droughts); water young trees in dry spells.
FertilizingNo specific fertiliser regime verified; site choice (deep, nutrient-rich soil) matters more than feeding.

Care Tip: Do not plant juglone-sensitive vegetables directly under the crown; collect fallen nuts promptly so dogs cannot eat mouldy ones.

Sources