Showy Sterile Flowers: A Problem For Pollinators

Guest post by Sarah Gerhardt

It is easy to assume that having plenty of gorgeous flowers in our garden will make it a magnet for bees. Unfortunately, this is not necessarily the case. Many of the popular flowering plant choices in the garden including many delightful looking roses, stunning hydrangeas, large bloomed peonies and showy camellias offer very little food to bees and other pollinators.

In today’s post we will take a look at what sterile flowers are, why they are a problem for pollinators and how we can still support pollinators in the garden.

How does pollination work?

To understand sterile flowers, let us first take a look at how fertile flowers and pollination work. Flowers evolved as adapted leaves for reproduction. They carry female and male reproductive structures. The female part of the flower is the pistil. It has a sticky top (the stigma) that receives the pollen and an ovary at the base which houses one or more undeveloped seeds. The ovary becomes a fruit if the flower is fertilised. Pollen is a dust-like grain produced by the male flower parts, the stamens.

In a complete flower (one that has all the usual parts – see drawing) the outer leaves of the flower, called the sepals, usually serve as protection around the unopened bud. As the flower opens they are shed or curl backwards to reveal the flower leaves, the petals.
Many flowering plants rely on animals such as bees, moths or butterflies to pollinate them which means to transfer the pollen to the stigma. They need to collect the pollen from one flower and transfer it to the stigma of another flower (cross-pollination). Plants use a number of different strategies to attract pollinators to their flowers. Plants that rely on pollinators commonly have large, brightly coloured flowers with ultraviolet patterns that guide the pollinators to the centre. Furthermore, they are often scented and produce nectar which is a vital food source for pollinators.

Some pollinators such as bees, feed on both nectar and pollen. They use the nectar for energy and the pollen for nutrients such as protein. As pollinators fly from flower to flower collecting either nectar or both nectar and pollen they inadvertently transfer pollen from one flower to another as it sticks to them. This way plants and pollinators provide a mutually beneficial service to one another and to the wider environment. For example, the fruit growing industry is reliant on this process as well.

What are sterile flowers and why are they a problem for pollinators?

Many of the flowers the horticultural industry has cultivated over the years have been bred to have bigger, more showy flowers to make them attractive to the garden owner. A lot of them are so-called double flowers. They are cultivars developed from mutations where stamens have been converted into extra petals, giving the flower a fuller, more showy look. These plants are usually sterile or nearly sterile so most of the time they cannot be pollinated and hence will not fruit. As a result, they often remain open for longer than single flowers.

Double flowers are attractive to many garden owners because they look great and last a long time. As a result many of today’s rose cultivars have been bred to be double-flowered. Unfortunately, they have very little to offer to pollinators. There is no pollen or if the cultivars have any functional stamens left, these are often difficult to access under the many petals. If the flower produces nectar, access to the nectaries at the base of the flower is often difficult because of the many petals above them.

A lot of them are so-called double flowers. They are cultivars developed from mutations where stamens have been converted into extra petals, giving the flower a fuller, more showy look. These plants are usually sterile or nearly sterile so most of the time they cannot be pollinated and hence will not fruit. As a result, they often remain open for longer than single flowers.

Double flowers are attractive to many garden owners because they look great and last a long time. As a result many of today’s rose cultivars have been bred to be double-flowered. Unfortunately, they have very little to offer to pollinators. There is no pollen or if the cultivars have any functional stamens left, these are often difficult to access under the many petals. If the flower produces nectar, access to the nectaries at the base of the flower is often difficult because of the many petals above them and is still able to attract pollinators with the sterile flowers.

While wild hydrangeas only have a small number of sterile flowers, many of today’s hydrangea cultivars have been bred to consist almost exclusively of sterile flowers for their looks. As a result they do not offer much to pollinators.

How can we support pollinators in the garden?

Luckily, most flower beds have a good mix of flowers, many of which are not sterile and provide a good food source for pollinators. A flower bed offers a large concentration of flowers in a small space. Even if it includes some sterile cultivars of little benefit to pollinators there are likely to be many flowers that provide accessible pollen and/or nectar. Any flower bed is likely to offer greater benefits to bees, butterflies, moths etc. than a lawn or patio.

However, it is a good idea to be aware of the potential problems that lots of sterile flowers in your garden can cause to pollinator health. If you like plants that typically have double flowers (such as many roses, peonies, carnations and camellias) make sure to include some single-flowered cultivars or wild species in your beds.

In the case of hydrangeas, species such as Hydrangea aspera or H. serrata retain a lot of fertile flowers and are a much better choice for pollinators than the popular mophead hydrangea (Hydrangea macrophylla).

When buying plants avoid those with the abbreviation “fl. pl.” after the scientific name of the plant. It stands for flore pleno, which means “with full flower” and denotes a double-flowered plant cultivar. However, this specification is often not included so it is better to do some research and visually inspect the plant. If you can clearly see stamens in the centre of a flower it will be fertile and of value to pollinators.

The RHS has a helpful list of plants for pollinators for every season:
https://www.rhs.org.uk/science/research/plants-for-pollinators . Most gardeners who have flower beds and do not use pesticides are already doing a good service to pollinators. However, with worldwide bee populations in decline, sterile flowers are an issue that should not be underestimated. Luckily, it is also a problem that can be avoided or mitigated. Increased awareness about it among gardeners and their willingness to both reduce the number of sterile flowers in their garden and increase the amount of fertile pollinator-friendly ones can be of great service to the health of the local bee population and biodiversity as a whole.

About the author: Sarah Gerhardt is a gardener, linguist and punk musician based in Edinburgh. She was head gardener at the Dean Gardens, Edinburgh for 9 years and runs her own gardening business Gerhardt’s Garden Service. Find out more via her Linktree: https://linktr.ee/gerhardtsgardenservice