Macadamias are rainforest trees, native to a small patch of lowland subtropical forest in southeast Queensland, Australia.1Europe PMC / Biological Conservation. Predicting loss and fragmentation of habitat of the vulnerable subtropical rainforest tree Macadamia integrifolia with models developed from compiled ecological data They do best in climates that copy that home: mild, humid, rarely frosty and reliably wet. A tree that merely survives and a tree that crops well need different things, because flowers and nuts are far more sensitive to weather than the trunk and leaves.
The Temperature Range Where Macadamias Grow Best
Macadamia leaves grow and make food fastest at about 15–25 °C.2Department of Primary Industries and Regional Development. Young trees Young trees put on the most height, trunk thickness and leaf area anywhere from about 15 to 30 °C.3Scientia Horticulturae. The effect of temperature on growth and dry-matter production of macadamia At a steady 10 °C, though, young trees stop growing altogether.4Scientia Horticulturae. The effect of temperature on growth and dry-matter production of macadamia A long cool winter therefore means a long pause in growth each year.
Heat is the other limit. At a constant 30 °C, fresh flushes of leaves yellow and then brown and die.5Scientia Horticulturae. The effect of temperature on growth and dry-matter production of macadamia At 35 °C, buds burst open in clusters, scab over and dry out.6Scientia Horticulturae. The effect of temperature on growth and dry-matter production of macadamia
The best growing regions average about 16–21 °C across the year, and in those regions the coldest month’s nights usually stay around 6–10 °C.7PubMed Central. Global climate change and Macadamia habitat suitability: MaxEnt-based prediction under future scenarios To compare your own climate, look up your area’s average annual temperature and coldest-month night lows from a weather service. A yearly average near 16–21 °C with cool but frost-free winter nights is a strong match. Then count how many summer days pass 30 °C; many of them point to a hotter-than-ideal site.
Keep in mind that the damaging heat figures above were steady, day-and-night temperatures. A garden that hits 32 °C in the afternoon but cools off at night is less harsh. Prolonged heatwaves with warm nights come closest to the damaging conditions.
How Much Frost a Macadamia Can Take
Established macadamias can survive brief cold down to about −4 °C (24 °F).8California Rare Fruit Growers. Macadamia They handle light freezing near −1 °C (30 °F) for short periods.9TopTropicals.com. Macadamia sp. (Macadamia nut) | Top Tropicals Plant Encyclopedia The rough-shelled species, Macadamia tetraphylla, seems slightly more cold tolerant.10California Rare Fruit Growers. Macadamia
Flowers and young trees are the weak point. Flower clusters are usually killed at about −2 °C (28 °F),11California Rare Fruit Growers. Macadamia so a late frost during flowering can wipe out that year’s crop even when the tree itself is fine. Young trees can be killed by a light frost and need protection.12California Rare Fruit Growers. Macadamia13TopTropicals.com. Macadamia sp. (Macadamia nut) | Top Tropicals Plant Encyclopedia
On frost nights, a young tree does best with a few simple measures. Plant on a slope, where cold air drains away, or near a north-facing wall in the southern hemisphere (south-facing in the north), which gives back warmth stored during the day. Cover the tree with frost cloth reaching the ground to trap warmth from the soil, and water the soil the day before, since moist soil holds and releases more heat. Take the covers off in the morning so the tree doesn’t overheat.
If frost is common where you live, ask a local nursery for tetraphylla or hybrid types. Avoid low spots, where cold air pools.
The Night Temperatures That Make Macadamias Flower
Flowering happens in two temperature steps. Warm nights around 19–20 °C get flower buds started, and cooler nights averaging about 10–12 °C afterwards push those buds to emerge.14Scientia Horticulturae. Effects of night temperature on floral initiation and raceme development in macadamia15The University of Queensland. Axillary bud behaviour in Macadamia integrifolia and its hybrids Reverse the order, with cool nights followed by warm ones, and the tree produces almost no flowers.16The University of Queensland. Axillary bud behaviour in Macadamia integrifolia and its hybrids Only-warm or only-cool nights give a moderate show.17The University of Queensland. Axillary bud behaviour in Macadamia integrifolia and its hybrids
Once buds form, accumulated warmth, hot days of 26 °C or more and total sunshine govern how fast they reach bloom.18University of Hawaii. Modeling Floral Phenology of Macadamia Nut (Macadamia integrifolia) A warm, sunny late winter brings flowers on sooner.
This explains why some trees in tropical lowlands flower poorly. Places with warm nights all year miss the cool-night step, so the tree may still flower but less heavily. Slightly higher or cooler sites in the tropics often flower better.
In a normal year, flowers appear as long hanging strings in late winter to spring. A cold snap timed with flowering can undo it, which is where the frost limit for flower clusters comes back in.
Heat and Weather While the Nuts Fill
The kernel, the edible nut meat inside the shell, fills with oil fastest at about 25 °C.19Scientia Horticulturae. Effects of temperature during latter stages of nut development on growth and quality of macadamia nuts At that temperature nuts reached top-grade quality, with 72% or more oil.20Scientia Horticulturae. Effects of temperature during latter stages of nut development on growth and quality of macadamia nuts At 15 °C and 35 °C the nut meat grew less and held less oil.21Scientia Horticulturae. Effects of temperature during latter stages of nut development on growth and quality of macadamia nuts At 35 °C it actually lost weight, and most fell below top grade.22Scientia Horticulturae. Effects of temperature during latter stages of nut development on growth and quality of macadamia nuts
Wet seasons lower quality too. Higher rainfall during the growing season means fewer good nuts, more immature ones and more insect damage.23Journal of Agriculture and Food Research. Reduced macadamia nut quality is linked to wetter growing seasons but mitigated at higher elevations Cooler temperatures at higher elevations softened that effect.24Journal of Agriculture and Food Research. Reduced macadamia nut quality is linked to wetter growing seasons but mitigated at higher elevations
At home, poor-quality nuts show up as shriveled or small nut meat inside a full-sized shell. A common home check is to drop shelled nuts in water: good ones float, and poorly filled ones sink. After a wet season you may also find more nuts with insect holes.
Late-summer heat can be eased a little. Deep watering during hot spells keeps the tree working, and mulch keeps roots cooler.
Rainfall and Watering Needs
The best macadamia regions get about 1,050–2,050 mm of rain a year, and their driest month still gets roughly 30–100 mm.25PubMed Central. Global climate change and Macadamia habitat suitability: MaxEnt-based prediction under future scenarios Rain in the driest month is the single biggest factor in whether an area suits macadamias.26PubMed Central. Climate suitability predictions for the cultivation of macadamia (Macadamia integrifolia) in Malawi using climate change scenarios
Dry spells hurt most at flowering and oil filling. Spring often warms up before the rains come, stressing trees during flowering and nut set, which makes it a key time to water.27Water Research Commission. Water Use of Macadamia Orchards Volume 2 Even mild drought while nuts fill cuts the tree’s food-making and lowers yield and quality.28Australian Journal of Agricultural Research. Macadamia responses to mild water stress at different phenological stages
That is why most growing areas rely on irrigation. Most South African macadamia orchards need it to cope with dry periods.29Elsevier (Scientia Horticulturae). Effects of environmental drivers and irrigation on yields of macadamia orchards along an altitudinal gradient in South Africa Micro-sprinklers are popular partly because their spray cools the trees in heat.30Elsevier (Scientia Horticulturae). Effects of environmental drivers and irrigation on yields of macadamia orchards along an altitudinal gradient in South Africa Watering by evaporation estimates alone can mean over-watering, because trees use less water in dry air.31Water Research Commission. Water Use of Macadamia Orchards Volume 2
For a home tree, water deeply and less often so roots grow down. Check the soil a few centimetres below the surface before watering. Give extra attention from flowering to nut set, and again as nuts fill. Avoid waterlogging, since soggy soil harms roots. A tree short of water gives signals: new leaves wilt or droop in the afternoon, flowers or young nuts drop heavily, and nuts come out light or poorly filled at harvest.
Humidity and Dry Air
When the air gets hot and dry, macadamias close the tiny pores on their leaves to stop drying out.32Acta Horticulturae. Ecophysiology and Water Use of Macadamia33Scientia Horticulturae. Adopting and evaluating a simple model for macadamia tree transpiration in periodically water-scarce subtropical regions That makes them careful water users, from about 0.6 mm a day in winter to 1.3 mm in summer.34Scientia Horticulturae. Adopting and evaluating a simple model for macadamia tree transpiration in periodically water-scarce subtropical regions Closed pores also mean the leaves make less food, so growth slows.35Acta Horticulturae. Ecophysiology and Water Use of Macadamia Food-making peaks in moderately humid air and drops off as air gets drier.36Scientia Horticulturae. The seasonal regulation of gas exchange and water relations of field grown macadamia
Watering cannot fully fix this, because the pores close in response to the air. Shelter and mulch soften the effect by keeping the air near the tree moister, and coastal and humid inland gardens have an advantage here.
Wind and Sunlight
Strong, persistent wind slows young trees and reduces later flowering and nut set.37The Macadamia South Africa. Planning and designing effective windbreaks – The Macadamia South Africa Windbreaks help mostly in the first four years, until trees grow together.38Department of Employment, Economic Development and Innovation. Macadamia grower’s handbook In constantly windy places, shelter also keeps the air moister around the trees.39The Macadamia South Africa. Planning and designing effective windbreaks – The Macadamia South Africa
In a home garden, plant behind a fence, building or hedge on the windward side. A stake for the first year or two steadies a young tree.
Sun matters just as much. Bigger canopies on young trees give earlier yields, likely because they catch more light.40HortScience. Phenotypic Association of Multi-scale Architectural Traits with Canopy Volume and Yield: Moving Toward High-density Systems for Macadamia In crowded orchards, though, yields kept rising only until the trees caught about 94% of the light.41Australian Journal of Agricultural Research. The relationship between orchard crowding, light interception, and productivity in macadamia
Macadamias grow into large trees, often 10 metres or more. Avoid planting where buildings or big trees shade them much of the day. Pruning to open a crowded canopy lets light reach inner branches.
Climate and Disease Risk
Flower blight and nut diseases grow fastest at about 23 °C.42Revista Cultivar. Milder temperatures lead to a greater occurrence of macadamia diseases Humidity and dry-air levels predict when flower blights break out.43PubMed. Influence of climatic factors on dry flower, grey and green mould diseases of macadamia flowers in Australia A branch dieback fungus spreads spores after rain when humidity is over 70% and temperatures are 20–27 °C.44Plant Pathology. Seasonal dynamics of inoculum of Botryosphaeriaceae in macadamia orchards in Australia Another dieback fungus prefers heat near 30 °C.45Revista Cultivar. Milder temperatures lead to a greater occurrence of macadamia diseases
Watch for browning or mouldy flower strings after wet, mild spells, and for dying branch tips after summer rain. Prune out dead wood and keep the canopy open for airflow. A local extension service can advise on treatments during wet flowering seasons.
Choosing Trees for a Dry Climate
A grafted macadamia is two plants joined: the roots and lower trunk come from the rootstock, while the branches, leaves and nuts come from a chosen nut variety on top. Because the rootstock supplies the root system, it largely decides how well the tree can pull water from soil during a dry spell, even though the nuts come from the variety above. Rootstocks differ widely in drought tolerance. After 30 days of drought, rootstocks O.C and H2 showed the least leaf damage, while QA5 was damaged most, with whole plants yellowing.46Environmental and Experimental Botany. Morphological and physiological responses of 14 macadamia rootstocks to drought stress and a comprehensive evaluation of drought resistance If you garden somewhere dry, ask your nursery which rootstock a grafted tree is on.
For a marginal climate, grafted trees combine a reliable nut variety with a chosen root system. Seedling trees take longer to fruit and vary in nut quality.