The sheer variety of flowers on plants is almost incomprehensible. Find out all about the classification of monoecious, dioecious or hermaphroditic plants and discover examples for each.
Botanist lingo can sometimes be a bit confusing. You may have come across the terms “monoecious”, “dioecious” and “hermaphrodite” in connection with pollination and fruit bearing. These words describe the distribution of male and female floral organs on plants. But what exactly are monoecious, dioecious and hermaphrodite plants? And why is this something worth knowing for gardeners? Read on to learn all about this topic and get valuable tips for your garden.
By the way: regardless of which of these categories a plant comes under, pollination still occurs by wind, water or by insects. Read more about the different types of pollination in our dedicated article on the subject.
What are monoecious plants?
Monoecious plants bear both purely male and purely female flowers. Male flowers contain stamens that carry pollen. Female flowers can be recognised by the carpels, the so-called pistil, onto whose stigma the pollen is transferred. Monoecious plants can therefore be distinguished by the fact that they produce two different types of flowers.
Tip: the monoecious nature of plants is also known as monoecy, which comes from ancient Greek and loosely translates to something like ‘single’ and ‘house’. This is meant to signify that the male and female are in the same house. So, monoecy simply means that the flowers of both sexes are on the same plant.
However, the male and female flowers of monoecious plants do not always appear at the same time, on the same branch, or even at every stage of the plant. This is because even if a plant develops both types of flowers, inbreeding must be avoided in order to produce healthy offspring. Therefore, many monoecious plants develop the flowers of one sex first, followed by the flowers of the other sex after a period of time (dichogamy).
Alternatively, the flower types can grow spatially separated from each other, for instance by being located in different places on the plant. As a rule, pollen-producing male flowers are found higher up, so that their pollen is blown far away from their own female flowers.
Sometimes, plants only develop both types of flowers once they are old. Producing fruit then becomes the task of the fully developed adult plants while pollen is donated by younger specimens in the area.
Last but not least, a genetic barrier can also prevent self-fertilisation and inbreeding. So-called sterility genes prevent the pollen tube from growing into the ovary of the female flower.
Examples of monoecious plants
Monoecious plants include many of our wild and cultivated plants, as well as some exotic plants:
- Pine (Pinus)
- Larch (Larix)
- Hazel (Corylus avellana)
- Walnut (Juglans regia)
- Alder (Alnus)
- Hornbeam (Carpinus betulus)
- Birch (Betula)
- Most conifers (coniferous trees)
- Sweet chestnuts (Castanea sativa)
- Watermelon (Citrullus lanatus)
- Maize (Zea mays)
- Cucumber (Cucumis sativus)
- Pumpkin (Cucurbita spec.)
- Courgette (Cucurbita pepo convar. giromontiina)
Some plants that were originally dioecious were selectively bred to produce monoecious species. This has the advantage that growers only have to put time and effort into growing the fruiting specimens, not the male plants that do not yield. The common grape vine (Vitis vinifera) and true hemp (Cannabis sativa) are two examples of cultivated monoecious plants.
Monoecious plants still produce flowers, even if fertilisation is not possible. If you simply want to enjoy the ornamental value of beautiful flowers, there is nothing special to consider when planting monoecious plants. However, if you want to harvest fruit from a plant that is monoecious, then there are a few things you need to know beforehand. For instance, you will need to find out whether it can pollinate itself or needs pollen from a foreign, genetically different plant to form healthy seeds.
Monoecious nuts: chestnuts, hazelnuts and walnuts, for example, grow dichogamously, i.e. male and female flowers do not appear at the same time. If there is no suitable pollen donor in the neighbourhood, fruit yield may be low or absent altogether as a result.
Single-seeded vegetables: cucurbits such as cucumber, zucchini and squash are capable of self-pollination. However, cross-pollination can improve fruit sets. Corn is a cross-pollinator, so unless corn is also being grown nearby, those growing it will need to plant two different varieties. Usually a high diversity of varieties has a positive effect on yield.
What are hermaphroditic plants?
Whereas purely female and purely male flowers are formed in the case of separately sexed monoecious plants, hermaphrodite plants have exclusively hermaphroditic flowers. These contain both the male and female floral organs: pistils and stamens.
However, the fact that both organs are so close to each other does not necessarily mean that hermaphrodite flowers can pollinate themselves. In fact, there are some hermaphroditic plants that can pollinate themselves and others that cannot.
If the hermaphroditic plant is capable of self-pollination, pollination often occurs when the flower is closed or when an insect moves around the flower collecting nectar. Pollen is then transferred to its own stigma and fertilises it. However, hermaphroditic plants often avoid inbreeding too, for example through the sterility genes already mentioned or sometimes also in combination with so-called heterostyly. This is fascinating to observe in primroses. There are primrose flowers with short pistils and others with long pistils, promoting cross-fertilisation and inhibiting self-fertilisation.
But self-pollination is also quite possible and is known as autogamy. Autogamous (or self-fruiting) plants can be found in many gardens. For example, tomatoes (Solanum lycopersicum), strawberries (Fragaria x ananassa) and most cherry varieties (Prunus spec.) are capable of pollinating themselves, so they do not require a pollination partner.
Tip: the vast majority of plants, about 95%, are monoecious or even hermaphroditic. By definition, plants that are hermaphrodites are also monoecious, as the male and female flower organs are on the same plant.
Examples of hermaphrodite plants
The majority of our native plants and crops form hermaphroditic flowers:
- Apple trees (Malus spec.)
- Pear trees (Pyrus spec.)
- Cherry trees (Prunus spec.)
- Roses (Rosa)
- Olive trees (Olea europaea)
- Most vegetable plants
- Many ornamental plants, for example daisies (Bellis perennis)
Even with hermaphroditic plants, it is important to know whether or not they can successfully self-pollinate if you want them to produce fruits and seeds. Of course, if it is not the vegetable itself you want, but another part of the plant, there is no need to look for a partner for fertilisation.
Examples of hermaphroditic plants that can pollinate themselves:
- Tomatoes (Solanum lycopersicum)
- Aubergine (Solanum melangena)
- Peppers (Capsicum annuum)
- Many beans (Phaseolus spec.)
- Peas (Pisum sativum)
- Lamb’s lettuce (Valerianella locusta)
- Most strawberries (Fragaria x ananassa)
- All sour cherries and few sweet cherry varieties: e.g. ‘Stella’ and ‘Lapins’
Examples of cross-pollinated hermaphroditic crops:
- Most sweet cherries
- Parsnips (Pastinaca sativa)
- Leeks (Allium porrum)
- Cabbages (Brassicaceae)
Hermaphroditic plants often do not flower for very long. In order for cross-pollination to be successful, the flowering times of the selected varieties must naturally overlap. Therefore, pollination tables are available for many plants to help you find out which varieties are suitable pollinators.
Tip to promote pollination: whether plants are monoecious, dioecious or hermaphroditic, in the vast majority of cases, cross-pollination results in better quality fruit and often a higher yield. Because pollination is very often done by insects, both amateur gardeners and professional fruit and vegetable growers can encourage pollination by attracting insects.
By the way: categorising plants as monoecious, dioecious or hermaphroditic is simply an attempt by botanists to identify a system in the variability of nature. But no botanical system fits reality like a glove and there are, as always, exceptions that prove the rule. Mixed forms often occur. Horse chestnuts (Aesculus hippocastanum) are a good example of this, as they can produce hermaphroditic flowers as well as purely male and purely female flowers. There are also other plants that can do the same.
What are dioecious plants?
Only about 5% of plants are dioecious, which is where each specimen has either male or female flowers. This means that there are purely male and purely female plants. Dioecy has the advantage that self-pollination and inbreeding are impossible. Dioecious plants necessarily require another plant to produce fruits, as one plant can only fertilise or be fertilised by another. This guarantees regular genetic mixing. In dioecious plant species, only the female plants can produce fruit, but the males show abundant pollen production.
Tip: the dioecious nature of plants is also referred to in technical terminology as dioecy, which comes from ancient Greek and loosely translates to something like ‘twice’ and ‘house’. The sexes are therefore in ‘two houses’ or, in other words, on two different plants.
Examples of dioecious plants
Dioecious garden plants:
- Goat willow (Salix caprea)
- Holly (Ilex spec.)
- Ginkgo (Ginkgo biloba) — only males are usually planted
- Seaberry (Hippophae rhamnoides)
- Kiwi (Actinidia deliciosa) — monoecious varieties also exist
- Light carnation (Silence dioica)
- Shrubby cinquefoil (Potentialla futicosa) — only females are usually planted
- Hops (Humulus lupulus)
- Yew (Taxus baccata)
- Skimmia (Skimmia japonica)
- Spinach (Spinacia oleracea)
- Some watermelon varieties
- Asparagus (Asparagus officinalis)
In dioecious plants, both male and female plants must be planted in order for fruit to be produced. So, if you wish to harvest or admire the fruits of your gardening labour, you must plant both. Often, a few male plants produce enough pollen to fertilise many females.
We often want plants to produce their fruit. Who would want to go without the red berries of the yew, holly or skimmia, not to mention tasty seaberries or kiwi? However, with some plants, it is better to avoid fertilisation altogether. Ginkgo females, for instance, produce very unpleasant smelling fruits. Therefore, people only tend to plant male ginkgo trees.
In some of the plants presented, only a few varieties have been bred to be monoecious or to be capable of self-pollination in order to simplify cultivation.