Are Trees Male or Female? The Science of Tree Sex

Most trees are neither male nor female. They carry both pollen-making and seed-making parts on the same plant. A real minority, though, come as separate male and female individuals, much like animals. Ginkgo, yew, holly, willow, poplar, mulberry and date palm are all among them. In at least one tree, sex isn’t even fixed for life and shifts with the tree’s health.1PubMed Central. Time for a change: patterns of sex expression, health and mortality in a sex-changing tree

The Three Ways Trees Divide Up Sex

Many trees, such as apples and cherries, have “perfect” flowers, each holding both the pollen-making stamens and the seed-making pistil. Others, like oaks, birches and walnuts, have separate male and female flowers on the same tree. On an oak, dangling catkins shed pollen while tiny separate flowers become acorns. One oak or walnut can produce acorns or nuts without any partner tree nearby.

That’s why “male oak” or “female oak” makes no sense: every oak is both, and catkins are just the pollen half. Asking a tree’s sex only has a real answer for species that split the sexes.

Trees with only male or only female individuals are called dioecious, a word you’ll see on nursery tags and plant guides. A holly is a good example: only some bushes ever carry red berries, because male trees make pollen and never fruit. Still, only about 5 to 6% of flowering plant species work this way.2PubMed. The relative and absolute frequencies of angiosperm sexual systems: dioecy, monoecy, gynodioecy, and an updated online database3PubMed. The relative and absolute frequencies of angiosperm sexual systems: dioecy, monoecy, gynodioecy, and an updated online database

Among conifers and their relatives, separate sexes are the norm. About 65% of conifers and related cone-bearing species have separate male and female trees.4Basic and Applied Ecology. Sexual systems in gymnosperms: A review Every species in the ginkgo and cycad families is separate-sex, and most yews and podocarps are too, around 94 to 95% of species.5Basic and Applied Ecology. Sexual systems in gymnosperms: A review The cypress family is the exception, with only about 30% of species separate-sex.6Basic and Applied Ecology. Sexual systems in gymnosperms: A review With a separate-sex species, a lone female with no male anywhere in pollen range won’t set viable seed, however healthy she is.

Common Trees That Come in Male and Female

To find out whether your own tree is a separate-sex type, look up the species name on the nursery tag or with a plant identification app. Then search the species name plus “dioecious” to confirm. If you’re unsure of the species, a local extension office can help. Keep in mind that some varieties are sold specifically as male or female clones.

What you do next depends on whether you want fruit. For berries from a holly, you need at least one female and a male of the same species within pollen range, and a single male can serve several females. If mess is the problem, the usual route is a male cultivar of a species like ginkgo, which makes pollen but none of the smelly seeds. A mature female you dislike can’t be changed by care, so you can live with her or replace her.

How to Tell a Male Tree From a Female

Flowering season is the time to check. Male flowers or cones are usually pollen-laden and fall off after shedding, while female ones swell into fruit or seed. A tree that has bloomed for years without ever fruiting may be male, though an unpollinated female can also fail to fruit, so check the flowers themselves. Young trees may take several years to flower, so a sapling can’t be sexed by looking.

A simple spring check works well. Note when the tree flowers and photograph the flowers close up. Tap a branch: a puff of yellow dust suggests male, though pollen from nearby trees can settle on any branch. Check back in early summer for small developing fruit or seeds, then compare what you see with photos for your species from a reliable plant guide.

Up close, the differences are specific. On a male willow, the catkins look fuzzy and turn yellow with pollen, while female catkins are greener and stay on to produce the cottony seed. On a female holly, look at the center of a flower for a small green bulb-like ovary, the future berry; male flowers show pollen-bearing stamens instead. On yew and juniper, male cones are small and release a cloud of pollen in spring, while females carry the seed that becomes the fleshy red or blue structure. Because the flowers are small, a hand lens or a close phone photo makes the difference easy to see.

A female may still not fruit. Hollies and similar females need a male of the same species nearby, and without pollen the flowers drop off unfertilized. A late frost or young age can also stop fruiting. Buying a known female plant avoids the guessing.

Outside flowering, sex is often impossible to see. In a drone-photo test on poplars, a computer identified tree sex only about 52% of the time.10Copernicus Publications. SEX DISCRIMINATION OF INDIVIDUAL TREES USING UAV IMAGERY From the outside, a male and female tree can look nearly identical. Date palm growers use DNA tests on seedlings because sexing them early saves years of wasted growing.11PubMed Central. Molecular Identification of Sex in Phoenix dactylifera Using Inter Simple Sequence Repeat Markers

Trees That Change Sex

In striped maple, sex is a moving target. Over four years, more than half of the trees changed sex, and about a quarter of those switched at least twice.12PubMed Central. Time for a change: patterns of sex expression, health and mortality in a sex-changing tree Healthy trees were more likely to flower male, and trees in declining health more likely to flower female.13PubMed Central. Time for a change: patterns of sex expression, health and mortality in a sex-changing tree Three in four trees that died flowered female just before death.14PubMed Central. Time for a change: patterns of sex expression, health and mortality in a sex-changing tree

Injury can flip the switch too. Stripping all the leaves made a striped maple about 4.5 times likelier to flower female.15PubMed Central. When the going gets tough, the tough turn female: injury and sex expression in a sex‐changing tree No heavily pruned male trees were still flowering male two years later.16PubMed Central. When the going gets tough, the tough turn female: injury and sex expression in a sex‐changing tree The tree can decide whether buds become male or female as late as mid-spring,17Trees. Down to the wire: late season changes in sex expression in a sexually labile tree species, Acer pensylvanicum (Sapindaceae) so a hard pruning or storm damage could change what a tree like this produces next season.

Other trees stagger their sexes. Some walnuts and maples flower with male and female phases at different times on one tree, with no overlap.18PubMed Central. Flowering Phenology and Wind-pollination Efficacy of Heterodichogamous Juglans mandshurica (Juglandaceae) Each tree avoids pollinating itself and mates with neighbours on the opposite schedule.19Plant Ecology. Analysis of the disassortative mating pattern in a heterodichogamous plant, Acer mono Maxim. using microsatellite markers

For most yard trees, this is not a worry. Sex change is well documented only in a few species like striped maple, and a ginkgo or holly is generally expected to keep its sex for life. Unexpected fruit on a “male” tree is more often a mislabelled plant.

What Decides a Tree’s Sex

Poplars have one master switch. A single gene acts as an on-off switch: on makes a female, off makes a male.20PubMed. A single gene underlies the dynamic evolution of poplar sex determination Male poplars carry extra gene pieces on their Y chromosome that silence that switch,21PubMed. A single gene underlies the dynamic evolution of poplar sex determination and the same switch works across the poplar family.22PubMed. A single gene underlies the dynamic evolution of poplar sex determination

Sex chromosomes have arisen separately in different trees. Willows and poplars are close relatives, yet their sex chromosomes formed from different chromosomes.23PubMed Central. Different autosomes evolved into sex chromosomes in the sister genera of Salix and Populus Date palm and an unrelated palm evolved very similar sex genes independently.24bioRxiv. Convergent evolution of sex chromosomes in two palm species, Phoenix dactylifera and Kerriodoxa elegans

Hormones also steer flowers toward one sex. In plants like cucumber, one hormone pushes flowers female while another pushes them male.25PubMed Central. Transcriptomic Analysis Implies That GA Regulates Sex Expression via Ethylene-Dependent and Ethylene-Independent Pathways in Cucumber (Cucumis sativus L.) In some walnuts, hormones play a leading role in whether buds become male or female.26PeerJ. Transcriptome-based analysis of the hormone regulation mechanism of gender differentiation in Juglans mandshurica Maxim

How Male and Female Trees Live Differently

Making fruit costs females far more. In one tropical nutmeg tree, males spent more on flowers, but females spent about four times more on reproduction overall because of fruit.27American Journal of Botany. FLOWERING, SEX RATIOS, POLLEN‐OVULE RATIOS, FRUIT SET, AND REPRODUCTIVE EFFORT OF A DIOECIOUS TREE, MYRISTICA INSIPIDA (MYRISTICACEAE), IN TWO DIFFERENT RAIN FOREST COMMUNITIES Seeds and fruit need sugars, water and nutrients over months, whereas pollen is cheap per grain but made in huge quantities. Males pay too: in wind-pollinated plants, making pollen also slows male growth.28PubMed. The dynamics of resource allocation and costs of reproduction in a sexually dimorphic, wind-pollinated dioecious plant

Males and females favour different spots. Because females need more water for fruit, they may do best where soil is moist, while males occupy drier ground. Female yews are more common on wetter sites, while males spread more widely.29Forest Ecology and Management. Sex-related spatial segregation along environmental gradients in the dioecious conifer, Taxus baccata A Patagonian conifer showed the split on a single hillside, with more males on sunny slopes and more females on shady, moister ones.30Écoscience. Sex-related spatial segregation and growth in a dioecious conifer along environmental gradients in northwestern Patagonia

Drought hits females harder in some trees. Under drought, female poplars lost more of their leaf defences than males, so they suffered more leaf-chewing damage.31Canadian Journal of Forest Research. Sexual differences in growth and defence of <i>Populus yunnanensis</i> under drought stress In yews, females’ higher water needs may limit where the species can grow.32Plant Ecology. Do secondary sexual dimorphism and female intolerance to drought influence the sex ratio and extinction risk of Taxus baccata? A population without enough females can’t reproduce well.

For your own trees, a female fruiting tree may need more watering in dry spells, and heavy fruiting years can slow her growth. Check stressed females more closely for pests during drought.

Do Male Trees Make Allergies Worse

Some writers argue that cities plant mostly male trees to avoid messy fruit, flooding the air with pollen. But no objective data yet show it causes rising pollen allergies.33Office for Science and Society. Is “Botanical Sexism” Really to Blame for Increased Pollen Allergies in Urban Cities? In New York City, sex bias in planting was unlikely to matter much for pollen exposure,34Urban Forestry & Urban Greening. The effects of tree planting on allergenic pollen production in New York City because the main separate-sex pollen source there, mulberry, mostly grows wild on fence lines and vacant lots.35Urban Forestry & Urban Greening. The effects of tree planting on allergenic pollen production in New York City Many of the biggest pollen producers, like oaks and birches, have both sexes on each tree, so they can’t be “male trees” at all.

If you’re allergy-prone, a female separate-sex tree makes no pollen of her own, though she can catch pollen from nearby males. The trade is mess: fruit, seeds or cotton to clean up instead of dust in the air. Weigh that for each species rather than assuming male is always worse, and ask a local allergist which trees peak in your area.

Could Climate Change Shift the Balance of Sexes

Drier conditions may favour males. Male cottonwoods and boxelders are more common on dry ground and less sensitive to drought than females.36PubMed. Does sexual dimorphism predispose dioecious riparian trees to sex ratio imbalances under climate change? Because both grow along rivers, a drier climate could favor males there first, and future climates could produce populations with too few females to reproduce well.37PubMed. Does sexual dimorphism predispose dioecious riparian trees to sex ratio imbalances under climate change?

The forecast stays uncertain. Warming in one mulberry experiment made males and females help rather than compete with each other,38Journal of Plant Ecology. Warming reverses intersexual interactions: a shift from competition to facilitation in the dioecious mulberry tree which cuts against a simple male takeover.