We all know how it works with animals, but can trees be either male or female? Or do they have their male and female reproductive parts on the same tree? In this article I’ll break down for you the answer to the question whether trees can be male or female and also give you some common examples you can see for yourself, out in the countryside.
The short answer to the question is… sometimes.
It’s a question I had to ask myself when doing research for creating my FREE Tree Identification Online Course. Having scoured the books and spent extensive time in the field looking at the evidence, I’ve discovered that here in the UK we have a number of native species which have separate male and female trees, although they are in the minority.
WORD OF THE DAY: DIOECIOUS
Trees which do have separate male and female individuals are referred to as being dioecious (pronounced dye-e-schuss), which is defined in the dictionary as “having the male and female reproductive organs, especially flowers, on different individuals.”
On the flip side, trees and plants that include the organs or flowers or both sexes on the same individual are called monoecious. The majority of native trees fall into this category, including trees like oak, birch, beech and hazel.
In the image above you can see that the birch (Betula spp.) has female flowers (small, green and upright catkins) as well as male flowers (long pendulous and speckled catkins) on the same branch. Meanwhile hazel (Corylus avellana) has very different female and male parts on the same tree.
An easy way to remember which parts you’d expect to see on a dioecious tree is that with male trees they produce the seed (in this case pollen) which then fertilises the female trees which then grow the fruit, seed, nut or berry – sound familiar?
LOOKING AT EXAMPLES
So, where might you have seen a dioecious tree whilst out on your Sunday walk? Let’s talk about some examples.
Have you ever wondered why you don’t always see red berries on a holly (Ilex aquifolium) in winter? This is because only the female trees bear the fruit. It’s worth noting that there are other factors at play as well, such as whether a holly tree is getting too much shade (making it sterile) or it has no male nearby to pollinate it, so not every holly you see without berries will be a male, but EVERY holly you see with berries is a female.
Both male and female holly trees have flowers, appearing in May, but there are subtle differences for which you might need a hand lens, so the easiest way to tell is in berry season.
WILLOWS & POPLARS
Once the early spring comes around there’s another two families of native trees which announce their gender, loud and proud, for all the world to see. These are the willows and the poplars. Both families of trees are closely related.
In late February and through March the willows explode with fluffy, dangling catkins. Think of the ‘pussy willow’ (Salix caprea) named after its young male catkins which look and feel like fluffy cats paws.
Look at the yellow pollen, clearly visible on the male flowers on the left picture. Remember, this pollen is needed to fertilise the female flowers on another tree.
Meanwhile the poplars, follow suit with their own catkins. Like willows these catkins are easy to spot in early Spring as they are out before the trees come into leaf, so they really stand out. Look at the example in the picture below, we have both White Poplar (Populus alba) and Black Poplar (Populus nigra) with their striking crimson male flowers. In both cases the female flowers (also catkins) are green rather than red, so easy to tell apart once you know this.
A TREE THAT BREAKS THE RULE
The ash (Fraxinus excelsior) is our third most common tree and it’s the rebel of the pack. Bending the rules – there’s always one. Look closer and in spring you could see both male and female flowers on the same tree (though on different branches) but you can also find ash trees which are exclusively male or female.
In winter, once the leaves have fallen you can often spot an ash a mile off, distinctive by its seeds, called “keys” hanging in dense bunches all in the crown. They really stand out in the overall silhouette of the tree, so look out for it. You’ll have to wait until spring though to answer the question of whether it’s male or female… or both! If you want to see pictures of both the male and female flowers of the ash, you can find these in the photo galleries of my Tree ID Course, it’s free to sign up.
So, if you’re planning some tree planting yourself and you want to include any of the species mentioned here you do need both a male and a female in order to let nature work its magic. However, if you are planting trees simply for ornament and you don’t want to encourage a young forest sprouting up around you (although personally that sounds quite nice to me) then it doesn’t matter, the tree will be perfectly healthy on it’s own.
BOYS & GIRLS – THE DEFINITIVE LIST
Here’s a complete list of all the native tree species which are dioecious (male or female)
Ash (sometimes), Aspen, Black Poplar, Buckthorn, Grey Poplar, Holly, Juniper, Sea Buckthorn, White Poplar, Willows and Yew
If you found this interesting and want to know more about the trees around you, then you can start building your tree ID skills right now by signing up to my FREE introductory online course Kickstart Your Tree ID Skills.
When you sign up to this free mini-course you’ll be identifying common trees with video tutorials and photo galleries at your fingertips. Start your journey to becoming a fully fledged Tree Expert today. The course includes Tree ID Cheat Sheets which you can download and take outdoors with you.
“I’ve been frustrated for so long trying to learn my trees myself and haven’t gotten far. This course answered everything and has seriously upped my game.” Dr. Patrick Alexander
Happy tree hunting folks.