Tree · Proteaceae
Macadamia
Macadamia integrifolia
Match its needs for warmth, deep soil, and a pollination partner, and the macadamia sets a rich crop.
Full sun
Every 7–14 days
USDA 9–11
30–50 ft · 9.1–15.2 m
Overview
Macadamia integrifolia is a large evergreen of the Proteaceae family, native to the subtropical forests of southeast Queensland. At maturity it forms a broad, heavy-canopied tree of 30 to 50 feet (9 to 15 m) and bears long racemes of white to pinkish flowers through winter and spring, with the hard-shelled nuts ripening and dropping from late summer into winter.
Its cultural range sits in frost-free USDA zones 9 to 11, and its performance follows subtropical origins: full sun for photosynthesis and flower initiation, deep rich acidic soil for the rooting volume it needs, and at least 50 inches of annual rain or irrigation to sustain nut fill. The defining trait is phosphorus sensitivity. As a member of the Proteaceae, macadamia evolved on low-phosphorus soils and forms proteoid roots that scavenge it efficiently, so conventional heavy feeding overloads the plant and causes injury.
Establishment is slow because the tree invests early years in structure before yield. A grafted tree begins bearing in about 2 to 5 years, and because most cultivars are partly self-sterile, a second compatible cultivar nearby raises fertilization and crop set. The payoff is a dependable supply of a rich nut, though the same kernel that feeds people is documented as toxic to dogs.
Care
The short version: full sun, deep rich acidic soil, and a frost-free position sheltered from wind. Maintain even moisture, feed sparingly with a low-phosphorus formulation, and plant a second cultivar nearby so cross-pollination sets the crop.
Light
Site the tree in full sun, since light drives the flowering and nut load it can carry, though it tolerates a degree of partial sun. The more radiation the canopy intercepts, the greater its cropping potential.
Water
Maintain consistent moisture, backed by at least 50 inches of rain or irrigation a year, and water deeply during dry spells every 7 to 14 days. An established tree withstands some drought, but water stress reduces both nut set and kernel quality because the developing nuts draw heavily on soil moisture.
Soil
Grow it in deep, rich, well-drained acidic loam at a pH of 5.0 to 6.5. Depth matters as much as fertility, because the tree develops a substantial root system that needs the volume.
Temperature
Its comfortable range runs from about 32°F to 95°F (0°C to 35°C), consistent with subtropical origins. Frost injures the tissue, and sustained heat above the upper limit scorches foliage and disrupts nut development.
Humidity
It performs at average humidity of 50% to 70%. During bloom, damp and stagnant air favors blossom blight, so canopy airflow protects the flowers more than the humidity reading alone.
Feeding
Fertilize through spring and summer during active growth, using a balanced blend low in phosphorus. Because macadamia is a phosphorus-sensitive Proteaceae, excess phosphorus is toxic and shows as yellowing new leaves and dying tips.
Pruning
Establish structure by training a young tree to a single central leader, and remove low, crossing, or dead branches in late winter. A mature tree requires only light corrective cuts.
Repotting
The species is a field tree, so repotting is largely irrelevant. Pot up a young grafted tree only until field planting, then establish it in the ground.
Size & spacing
360–600 in (914–1524 cm) tall
When to plant
Because the tree is subtropical and frost-intolerant, timing follows temperature. Plant a grafted tree in spring once frost risk has passed, expect the fragrant racemes through winter and spring, and harvest the nuts as they ripen and abscise from late summer into winter.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Late winter to spring if growing from seed, though grafted trees are strongly preferred
- Plant: Spring, after all danger of frost has passed
- Harvest: Fragrant white to pinkish flowers on long racemes in winter to spring; nuts ripen and drop from late summer through winter
Hardiness
USDA hardiness zones: 9–11
Temperature: down to -7°C
Troubleshooting
Most disorders present in the foliage, the flowers, or the nut crop, and they trace back to nutrition, weather stress, or inadequate pollination. Read the symptom against the likely cause below and intervene early. A young tree grown on sound fundamentals seldom develops the more serious problems.
New leaves yellow with dying tips
Chlorotic new leaves with dying tips indicate excess phosphorus, which macadamias and the rest of the Proteaceae cannot metabolize as most plants do. Confirm with a soil or leaf tissue test. Discontinue the high-phosphorus fertilizer, move to a balanced low-phosphorus blend, and leach the profile with deep watering where levels are excessive.
Fix:
- Stop applying high-phosphorus fertilizer
- Switch to a balanced, low-phosphorus fertilizer
- Leach the soil with deep watering if phosphorus is excessive
Nuts drop early and few hold on the tree
Abundant bloom followed by heavy drop points to poor cross-pollination, typically from a single or self-sterile cultivar, and heat above 95°F (35°C) during flowering and nut fill compounds it. Interplant a second compatible cultivar, sustain bee activity for fertilization, and maintain steady moisture with wind shelter through nut development.
Fix:
- Plant a second compatible cultivar nearby
- Support bee activity for pollination
- Provide steady moisture and wind protection during nut development
Flower racemes brown and collapse
Discolored, disintegrating racemes signal blossom blight, a fungal infection favored by humid, wet conditions at bloom. It advances when the flowers stay wet in muggy weather. Improve air circulation through spacing and pruning, and apply an approved fungicide at bloom where the disease recurs annually.
Fix:
- Improve air circulation through proper spacing and pruning
- Apply an approved fungicide at bloom if blight is recurrent
Burnt edges on sun- and wind-hit leaves
Marginal leaf scorch reflects sustained heat above 95°F (35°C) and hot, drying winds, concentrated on the most exposed foliage where transpirational demand is highest. Establish a windbreak or shelter belt and keep the soil moist through heat events. Selecting a frost-free but not excessively hot site limits recurrence.
Fix:
- Provide wind protection or shelter belts
- Maintain soil moisture during heat
- Site in a frost-free but not excessively hot location
Toxicity
The kernel is a safe food for people, but the nut is a documented toxin to dogs. Ingestion of even a few nuts can produce weakness, tremors, and an inability to stand within hours, and the toxic mechanism remains unidentified. Keep every nut, raw or roasted or fallen, entirely out of a dog's reach.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds
Safe for
People
Cats Caution
No feline poisoning has been documented, but the nut has not been shown safe for cats either. Treat it as off-limits and consult a veterinarian if a cat ingests one.
Dogs Severe
This is the primary hazard. A few nuts can leave a dog weak in the hind legs and unable to walk, with vomiting, hyperthermia, muscle tremors, ataxia, and depression developing within 12 hours.
Horses Caution
Macadamia toxicity is unstudied in horses, so the nuts and any windfall should remain inaccessible until data exist.
Rabbits Caution
No record describes the reaction in rabbits, and a rich, high-fat nut is unsuitable for them regardless. It should not be offered.
Livestock Caution
Cattle, goats, and sheep have not been tested for this toxin. Exclude them from a fruiting tree so they cannot graze dropped nuts.
Birds Caution
The effect on birds is unknown, and the high fat content is not beneficial, so keep the nuts from poultry and pet birds alike.
People Edible
The roasted kernel is a common, safe food for people. The toxin affects dogs specifically, not the humans in the household.
Keep fallen nuts, raw or roasted nuts, and any nut-containing foods away from dogs; even small amounts can trigger signs.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
United Kingdom
Animal PoisonLine: 01202 509000 (fee applies)
Australia
Animal Poisons Helpline: 1300 869 738
Varieties
The commercial and garden material derives from two species and the hybrids and cultivars selected from them. These are the names worth understanding before planting.
Smooth-shell (Macadamia integrifolia)
It bears a smooth, rounded shell and is the principal commercial type grown across Hawaii.
Rough-shell (Macadamia tetraphylla)
A distinct species identified by its pebbled, rough shell and frequent pink new growth, valued as a parent in several hybrids.
'Beaumont'
A pink-flowered hybrid of integrifolia and tetraphylla that has become a favorite for backyard plantings.
'Kau' (HAES 344)
A Hawaiian smooth-shell cultivar the University of Hawaii selected for high yield and good kernel quality.
Companions
What grows around a macadamia matters less than what grows beside it. A second cultivar is the partner that counts, while thirsty turf under the canopy only gets in the way.
Plant near
second compatible macadamia cultivar: Cross-pollination between cultivars improves nut set and yield.
Keep apart
shallow-rooted turf grass under the canopy: Competes for surface moisture and nutrients and complicates harvesting fallen nuts.
How to propagate
Two methods start a macadamia, and only vegetative propagation preserves cultivar identity. Grafting is standard in orchards; seed serves for rootstock, breeding, and gardeners willing to wait.
GraftingModerate
Warm months once seedling rootstocks are about pencil thick · Grafts take in a few months; trees begin bearing in about 2-5 years
Graft to preserve cultivar identity, since seedlings do not come true. Perform the work in the warm months once seedling rootstocks reach pencil thickness; the union knits within a few months, and the tree enters bearing in about 2 to 5 years. Match scion to rootstock deliberately, and never combine a smooth-shell with a rough-shell type.
SeedModerate
Sow fresh, mature nuts in late winter to spring · Germinates in 1-2 months; seedlings may take 7-12 years to fruit and are variable
Use seed for rootstock or breeding rather than predictable nut production. Sow fresh, mature nuts in late winter to spring and expect germination in 1 to 2 months. The limitation is genetic variability and time, as seedlings differ from the parent and may take 7 to 12 years to fruit.
Myths
A few macadamia nuts are harmless to dogs.
Even a small quantity can trigger macadamia poisoning, with clinical signs appearing within 12 hours. Any amount warrants a veterinary call rather than observation.
A single macadamia tree will produce a full crop on its own.
Cultivars span the range from self-fertile to nearly self-sterile, so a lone tree is unreliable. A second compatible cultivar raises both nut set and yield, which is why orchards are never single-cultivar plantings.
Seed-grown macadamias come true to the parent and fruit quickly.
Seedlings segregate and may take 7 to 12 years to bear. Orchards therefore plant grafted trees, which remain true to the cultivar and enter production in about 2 to 5 years.