How to Grow Walnuts USA

Quick Facts

Factor	Value
Drainage	Deep (at least 5-6ft/1.5-1.8m), well-drained, permeable, fertile, moisture-retentive soil (UC ANR; RHS). RHS separately favours alkaline loam -- a broadly tolerant range from neutral to alkaline rather than a strict conflict with UC ANR's 6-7 figure.
Lifecycle	Deciduous nut tree. Wind-pollinated, monoecious with separate male catkins and female flowers on the same tree, but RHS describes it as only partially self-fertile -- cropping improves with a second, cross-compatible cultivar nearby. UC ANR (Marin Master Gardeners) gives the underlying reason: pollen is often shed by a tree's own catkins when its own female flowers are not yet receptive, so two different varieties planted together set a more reliable crop.
Edible parts	nuts
Documented problems	5 diseases, 3 pests

At a Glance

Diseases

Walnut leaf blotch

low

Named directly by RHS as affecting leaves; RHS does not specify a single named pathogen species so this is recorded at unspecified-fungus level rather than inventing one.

Walnut anthracnose

low

Named directly by RHS alongside walnut leaf blotch, affecting leaves; again no pathogen species is specified by RHS. Controlled the same way -- collecting and disposing of diseased leaves.

Walnut leaf gall

low

Named directly by RHS: pouch-shaped galls on the leaves, described as insignificant to tree health.

Crown gall

moderate

Agrobacterium tumefaciens

Named directly by UC ANR: bacterial growths (Agrobacterium tumefaciens) that reduce nutrient flow through the affected tissue.

Walnut blight

moderate

Xanthomonas arboricola pv. juglandis

Named directly by UC ANR: black lesions on catkins, nuts, leaves or shoots, worse in wet springs.

Pests

Scale insects

low

Diaspididae / Coccidae

Named directly by UC ANR, which recommends a dormant oil spray as treatment.

Codling moth

low

Cydia pomonella

Named directly by RHS as damaging the nuts.

Squirrels

high

Sciurus spp.

Named directly by RHS, which calls squirrels the single biggest threat to a crop and recommends plastic tree guards and raising lower branches to 1.8-2.5m; UC ANR notes trees "can live for centuries" but does not name…

Soil & Growing Conditions

Water: Young trees: 4-12 gallons/day; larger trees: 20-40 gallons/day; water every 3-4 weeks through summer, or drip-irrigate May-October kept away from the trunk (UC ANR). Avoid frost pockets and exposed, windy sites -- both damage foliage and flowers (RHS).

Growth Stages

Stage	BBCH	Notes
Planting and establishment	0	Plant young trees under 3m (10ft) tall during late autumn to winter, spaced 12-18m apart (RHS); UC ANR gives an overlapping 30-60ft spacing figure. Avoid pot-bound stock -- tap root damage is a real risk in young trees (RHS; UC ANR). Dig a hole twice the container's diameter with the root ball sitting about 2in proud of the soil, untangle roots and remove shoots within 6in of the soil, then mulch 3in clear of the trunk (UC ANR). Stake, water in dry spells (RHS). Caution: walnut roots and fallen leaves/husks release a natural growth inhibitor (juglone) that can suppress nearby plants -- do not plant sensitive species close by (UC ANR).
Establishment care: watering and feeding	3	Young trees need 4-12 gallons of water a day, larger trees 20-40 gallons a day; water every 3-4 weeks through summer, or drip-irrigate May-October kept away from the trunk (UC ANR). Feed with compost in the first two years; from year three, apply 5-7lb urea or 90-150lb manure in early spring (UC ANR).
Pruning and training	3	Train with either a central leader, or by removing the leader to restrict size; pinch side shoots at the 5th or 6th leaf; avoid hard pruning (RHS). Prune only between mid-summer and early autumn -- walnut is prone to heavy "bleeding" from cuts made at other times of year (RHS). UC ANR's seasonal task list separately names winter as a time to thin crowded areas; the two sources are not reconciled here and both are given as stated.
Flowering and pollination	6	Wind-pollinated, with separate male catkins and female flowers on the same tree. RHS describes the tree as partially self-fertile but says cropping is improved by cross-pollination -- cv. Broadview is given as an example. UC ANR explains the underlying reason: a tree's own pollen is often shed when its own female flowers are not yet receptive, so two different varieties planted together set a more reliable crop.
Fruiting and harvest	8	Nuts ripen in autumn -- UC ANR gives mid-September to early November -- splitting from their fibrous husk. Harvest daily: very vulnerable to birds and squirrels once ripe (RHS). Remove the fibrous casing with a wire brush before storage and dry in a cool oven at maximum 104°F (40°C) (RHS); UC ANR dries hulled nuts for 3-4 days and stores them airtight, with frozen shelled walnuts keeping up to a year.

Harvest & Storage

When to pick: Nuts ripen and split from their fibrous husk in autumn -- UC ANR gives mid-September to early November. Harvest daily: RHS warns nuts are very vulnerable to birds and squirrels once ripe.

Storage

Remove all traces of the fibrous casing with a wire brush before storage, then dry in a cool oven at maximum 104°F (40°C) (RHS). UC ANR: dry hulled nuts for 3-4 days and store in airtight containers; frozen shelled walnuts keep up to a year.

Sources

1. UC ANR / Marin Master Gardeners - Walnut

   Site: sunniest spot, sheltered from wind, 30-60ft spacing. Soil: deep, well-drained, permeable soil at least 5-6ft deep, pH 6-7. Planting: hole twice container diameter, root ball 2in proud of soil surface, untangle roots and remove shoots within 6in of soil, mulch 3in clear of trunk. Caution: walnut produces a growth inhibitor (juglone) that can negatively affect nearby plants -- do not plant sensitive species close by. Water: young trees 4-12 gallons/day, larger trees 20-40 gallons/day; water every 3-4 weeks through summer or drip-irrigate May-October away from the trunk. Fertilizing: compost in years 1-2; from year 3, 5-7lb urea or 90-150lb manure in early spring. Pollination: wind-pollinated but needs two different varieties planted together, since a tree's own pollen often sheds when its own female flowers are not receptive. Seasonal pruning notes: winter -- thin crowded areas. Harvest: nuts mature mid-September to early November; dry hulled nuts 3-4 days; store in airtight containers, frozen shelled walnuts keep up to a year. Pests/diseases: crown gall (bacterial growths reducing nutrient flow), walnut blight (black lesions on catkins, nuts, leaves or shoots, worse in wet springs), scale insects (treatable with dormant oil spray). Climate: a good choice for Marin's hottest microclimates; trees can live for centuries.

These source is specific to this region. The full list of 2 sources behind this guide (Royal Horticultural Society, UC ANR / Marin Master Gardeners) is in the references of our UK guide to walnuts; they describe the plant itself, which is the same wherever it grows.