Tree · Proteaceae
Macadamia
Macadamia integrifolia
Give it warmth, deep soil, and a pollination partner, and the macadamia sets long racemes and hard-shelled nuts.
Full sun
Every 7–14 days
USDA 9–11
30–50 ft · 9.1–15.2 m
Overview
The macadamia is a large evergreen of the Proteaceae family, and up close it is all broad, dense canopy. The tree fills out to 30 to 50 feet (9 to 15 m), its evergreen crown heavy and wide. Through winter and spring it drapes long racemes of tiny white to pinkish flowers along slender, hanging strands, and from late summer into winter those give way to round, hard-shelled nuts that ripen and drop to the ground.
It is a tree of the subtropical forests of southeast Queensland, at home on warm, frost-free ground in USDA zones 9 to 11. It stands in full sun, roots into deep and rich acidic soil, and takes at least 50 inches of rain or irrigation a year. One detail of its makeup sets it apart from most fruit trees: as a Proteaceae, its tissues cannot take much phosphorus, so heavy feeding scorches the very new growth it should protect.
The nut itself is what the whole tree builds toward, encased in a tough, woody shell that guards a rich, pale kernel. A grafted tree takes about 2 to 5 years to bring the first of them, and most cultivars need a second compatible partner nearby before the racemes set a full crop. The kernel is one of the richest nuts there is, and yet that same nut is dangerous to dogs.
Care
The short version: full sun, deep rich acidic soil, and a frost-free spot out of strong wind. Keep the tree evenly watered, feed it lightly with a low-phosphorus blend, and plant a second cultivar nearby so the flower racemes set nuts.
Light
The tree crops best in full sun, its broad canopy tolerating a little partial sun. The more light those wide, evergreen leaves catch, the more flowers and nuts the crown can carry.
Water
The tree wants consistent moisture and at least 50 inches of rain or irrigation a year, with a deep soak during dry spells every 7 to 14 days. An established tree handles some drought, but nut set and the fullness of each kernel drop the moment it goes thirsty.
Soil
It roots into deep, rich, well-drained acidic loam at a pH of 5.0 to 6.5. Depth counts as much as fertility, since this tall tree drives down a substantial, spreading root system.
Temperature
The tree favors subtropical warmth and holds up between about 32°F and 95°F (0°C to 35°C). Frost bruises the tissue, and sustained heat past the top of that range browns the leaf margins and stalls the nuts.
Humidity
Average humidity of 50% to 70% suits it, though damp, still air among the flower racemes invites blossom blight, so open airflow through the canopy matters more than the number.
Feeding
The tree feeds through spring and summer during active growth, on a balanced blend low in phosphorus. As a Proteaceae, macadamia is sensitive to phosphorus, and too much yellows the tender new leaves and kills the tips.
Pruning
A young tree trains to a single central leader, with low, crossing, or dead branches cut out in late winter. Once the crown matures, it needs only light corrective cuts to hold its broad shape.
Repotting
This is a tree for open ground, so repotting barely applies. A young grafted tree is potted up only until field planting, then set out where its roots can spread.
Size & spacing
360–600 in (914–1524 cm) tall
When to plant
The macadamia runs on warmth and cannot take frost. A grafted tree goes into the ground in spring once the cold has passed. The slender flower racemes, fragrant and white to pinkish, hang through winter and spring, and the hard-shelled nuts ripen and drop 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 macadamia trouble writes itself across the leaves, the flower racemes, or the nuts, and the cause is usually feeding, weather, or a missing pollination partner. Look closely at what the tree shows and act early. A young tree given the basics rarely takes on the rest.
New leaves yellow with dying tips
When the tender new leaves pale to yellow and their tips brown and die, the cause is excess phosphorus, which macadamias and the rest of the Proteaceae cannot process as most plants do. A soil or leaf test confirms it. Stop the high-phosphorus feed, switch to a balanced low-phosphorus blend, and flush the soil with deep watering where the level runs high.
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
Long racemes that bloom full yet leave the branches nearly bare of nuts point to a lone tree or a self-sterile cultivar, and poor cross-pollination is the core of it. Heat above 95°F (35°C) through flowering and nut fill deepens the loss. Plant a second compatible cultivar nearby, keep bees moving over the flowers, and hold steady moisture with wind shelter while the nuts swell.
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
Blossom blight is a fungus that settles into the long racemes when the air stays humid and the flowers stay wet. The pale strands discolor to brown and fall apart in muggy weather. Open the canopy with proper spacing and pruning, and spray an approved fungicide at bloom where it returns year after year.
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
Sustained heat above 95°F (35°C) and hot, drying winds scorch the leaf margins to a dry brown, worst on the most exposed foliage at the edge of the crown. Raise a windbreak or shelter belt and keep the soil moist through the heat. A frost-free site that does not bake keeps the browning in check.
Fix:
- Provide wind protection or shelter belts
- Maintain soil moisture during heat
- Site in a frost-free but not excessively hot location
Toxicity
That rich, pale kernel is a fine snack for people, but the same nut is a serious danger to dogs. Even a few of the hard-shelled nuts can leave a dog weak, shaky, and unable to stand within hours, and the reason stays unknown. Keep every nut, raw or roasted or fallen among the leaf litter, well out of a dog's reach.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds
Safe for
People
Cats Caution
No poisoning has been recorded in cats, though the nut is not proven safe for them either. Keep the kernels off-limits and call a vet if a cat eats one.
Dogs Severe
The dog is the real danger. Even a few nuts can leave a dog weak in the hind legs and unable to walk, with vomiting, a high fever, muscle tremors, an unsteady wobble, and a flattened mood, usually within 12 hours.
Horses Caution
Macadamia toxicity has not been studied in horses, so the nuts and any windfall are best kept out of reach until more is known.
Rabbits Caution
There is no record of how rabbits react, and a rich, fatty kernel suits them poorly regardless, so it should not be offered.
Livestock Caution
Cattle, goats, and sheep have not been tested for this toxin, so fence them from a fruiting tree so they cannot graze on the dropped nuts.
Birds Caution
The effect on birds is unknown, and the high fat content does them no favors, so keep the kernels away from poultry and pet birds alike.
People Edible
For people the roasted kernel is a popular, perfectly safe snack. The toxin troubles only dogs, not the humans sharing the house.
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 macadamia comes down to two species and the hybrids and cultivars bred from them, and much of the difference shows in the shell and the new growth. These are the names worth knowing before you plant.
Smooth-shell (Macadamia integrifolia)
Its shell is smooth and rounded, and it is the main commercial type grown across Hawaii.
Rough-shell (Macadamia tetraphylla)
A separate species, its shell pebbled and rough and its new growth often flushed pink, prized as a parent in several hybrids.
'Beaumont'
A pink-flowered cross of integrifolia and tetraphylla whose racemes have made it a favorite for backyard plantings.
'Kau' (HAES 344)
A Hawaiian smooth-shell cultivar the University of Hawaii selected for its high yield and full, fine kernel.
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
There are two ways to start a macadamia, and only one gives reliable nuts. Grafting is how orchards do it; seed is for rootstock, breeding, and gardeners with time to spare.
GraftingModerate
Warm months once seedling rootstocks are about pencil thick · Grafts take in a few months; trees begin bearing in about 2-5 years
Grafting gives trees true to the cultivar. The work is done in the warm months once the seedling rootstocks thicken to about pencil width, and the union knits within a few months while the tree comes into bearing in about 2 to 5 years. Match scion to rootstock with care, and never join a smooth-shell to a rough-shell type by mistake.
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
Seed suits rootstock or breeding rather than predictable nuts. Sow fresh, mature nuts, still heavy in their woody shells, in late winter to spring, and they sprout in 1 to 2 months. The catch is patience and chance, since seedlings vary and can take 7 to 12 years to fruit.
Myths
A few macadamia nuts are harmless to dogs.
Even a small handful of the kernels can set off macadamia poisoning, with signs appearing inside 12 hours. Any amount is a vet call, not a wait-and-see.
A single macadamia tree will produce a full crop on its own.
Cultivars run from self-fertile to nearly self-sterile, so one tree leaves the racemes half empty. A second compatible cultivar lifts both nut set and yield, which is why orchards never rely on one tree.
Seed-grown macadamias come true to the parent and fruit quickly.
Seedlings vary and can take 7 to 12 years to bear their first nuts. Orchards plant grafted trees instead, which stay true to the cultivar and begin fruiting in about 2 to 5 years.