Flowers are beautiful to us but they are literally food and drink to insects.
The first flowers of spring seem like a miracle. Most of us, even if we are not gardeners, begin to look forward to the scent and colour of a British summer. In all the excitement, though, it's easy to overlook the flowers themselves - there's more to them than meets the eye!
Flowering plants evolved about 130 million years ago. Some, like hazel, are wind pollinated. Rather than using the wind, others have been in a long-term relationship with all manner of insects, birds, and mammals. These creatures pollinate the plant and their reward is pollen and nectar from its flowers. Just as the plants rely on the animals, many animals have come to rely on these flowers.
Nectar is like an insect energy drink. Pollen is their "meat and two veg".
Pollen is what it's all about as far as flowers are concerned. It's produced from the male part of a flower (the anthers in the diagram below). It's a small packet that carries the male genes of the flower. When it lands on the female part (the stigma) of another flower of the same species, it splits open and the male cell travels down into the flower's ovary. It fertilises the female cells (ovules) - which then develop into seeds. Pollen is microscopically small and incredibly sticky - so sticky that it covers the head/body of any visiting insect. It travels from flower to flower on the bodies of these accidental pollinators.
Each grain of pollen is tiny but each one is packed with protein, fats, vitamins and minerals. Honeybees amongst others, have evolved to collect this amazing source of nutrients. Worker bees get covered in sticky pollen. They comb it off their bodies and pack it into special "baskets" of hairs on their rear legs. They take it back to the hive where they use it to feed the hive's developing larvae. This collected pollen is their only real nutrition. Without pollen, there would be no honeybees - or bumblebees.
But why visit flowers in the first place? Nectar is what it's all about for pollinators. Nectar is sugar syrup - full of energy. A honeybee colony needs about 120kg of nectar a year for it growth and development. Given that one honeybee carries about 30mg per trip, that's a lot of work - and millions of visits to flowers pollinated. Honey is concentrated nectar - the bee's energy supply when there are no flowers.
Flowers have evolved in partnership with their pollinators. They are attracted by colours and scents. Flowers pollinated by bees are often blue/purple - which is the colour bees see best. Pollinators though come in all shapes and sizes. So do flowers that depend on them. Flowers pollinated by night-flying moths may be white/pink and bird-pollinated flowers are typically red. Butterflies are attracted to bright colours including red and purple. Plants have also evolved different flower shapes - butterflies tend to visit flat open flowers whereas bees can access tubular flowers.
Nectar comes from special structures called nectaries. Different plant species have evolved their nectaries from different parts of their flower. Lime trees produce it from the sepals (the leafy part at the base of the flower - in the diagram above) whilst in ivy it comes from a part of the pistil. Broad beans don't even have their nectaries in their flowers but on their stalks!
And this is where human beings start to get involved - and not for the better.
People love flowers and we tend to like them bigger, brighter and frillier than nature intended. So, over the years, plant growers have taken wild species and "improved" them. Sometimes they are just a bit bigger/brighter but sometimes they have complicated petals or long tubular flowers no insects can reach - think big shaggy dahlias and long-flowered salvias. These complicated flowers often hide the parts our pollinators are trying to reach. Sometimes, as in double hellebores, the extra petals have been made from the nectaries. The flowers have none of the nectar our pollinators need. Just as unhelpfully, a plant may have come from a completely different part of the world. Peonies are beautiful but completely alien (useless) to our insects.
None of this is to say "don't plant X or Y" - I'm a fan of big dahlias! Gardens can be a great resource for our pollinators - perhaps the trick is to mix and match what we plant? Better still, if we can plant flowers that work for us and our pollinators, why not? Try simple open flowers rather than doubles. Consider natives as well as peonies from China. There are lots of websites that suggest plants for bees, butterflies even hoverflies and moths - why not check them out?