Flowers for pollinators
How the various shapes and sizes of flowers appeal to different kinds of insects
It is not a case of 'one size fits all'. By planting a wide range of flowers, always varieities 'close to nature' rather than highly modified hybrids, you will find that many different kinds of pollinator insects will bring life and movement to your garden. On this page I give a very brief summary of various ways in which flowers are adapted to different insects.
From the plants' point of view, they need insects to carry pollen between flowers to help cross-pollination, but this will not happen if the insects eat all the pollen or take it all away to provide for their young. So some kinds of flowers produce copious amounts of pollen so that this situation will not arise. Poppies and willows are examples of this kind of flower.
Others produce small amounts of pollen but in a specific place in the flower so that it will be dusted on to the insect's back and carried to another flower, and the insects do not get the opportunity to eat all of it. Lipped flowers such as those in the pea family (Fabaceae) and Sage family (Lamiaceae) follow this pattern. These lipped flowers usually have a 'landing platform' where a bumblebee or Anthophora bee can land before it enters the flower, as in the Lamium orvala flower pictured above. Hover your mouse pointer over each image for a description.
It is fascinating to sit in a flowery garden on a sunny day and watch how the different kinds of insects interact with different shapes and sizes of flowers. You soon notice that some insects visit a wide range of flowers while others have strong preferences; meanwhile some flowers seem to attract a wide range of pollinating insects while other types of flowers attract a very small range, or perhaps only one type of insect. Clearly flowers and insects have co-evolved in complex and subtle ways.
Open, bowl-shaped flowers generally have a ring of copious pollen in the middle of the flower. Examples include Poppies, Rock Roses (Helianthemum), Potentilla, Buttercups (Ranunculus spp.) and Wild and Shrub Roses. They are 'open access' flowers but they are especially used by honeybees, bumblebees and certain solitary bees to collect pollen on their bodies by running round the inside of the flower in a circle. Bumblebees sometimes also vibrate their wings in such flowers to dislodge pollen, although this is not the same as 'buzz pollination' which I describe below. A number of flowers of this type do not offer nectar, so they are not visited by butterflies or moths. The illustration on the left shows a small Mining Bee (Andrena) collecting pollen from a Buttercup, while on the right a Buff or White tailed bumblebee worker collects pollen from a Welsh Poppy. Note the full 'pollen basket' on her leg.
Other flat, open flowers do offer nectar, usually secreted in nectaries located between the petals. Hardy Geraniums (Cranesbills) are an example of such flowers. A bee will land on the flower and push its tongue between the base of each set of petals in turn to extract nectar, while at the same time it may accumulate some pollen on its coat from the central boss of stamens.
Brush-shaped flowers without petals, such as those of Willows and Sallows, Thalictrums, and Plantains are mainly wind-pollinated but in some cases are also a source of pollen for Bumblebees. pollinated.
Lipped flowers typically belong to the Sage Family (Lamiaceae) or the Pea family (Fabaceae). These are specialised flowers that in many cases have a close relationship with bumblebees or solitary bees. The lip at the front of the flower is the landing platform, the bee then pushes its head into the flower to get at the nectar which is secreted at the back of the flower. At the same time the plant places pollen on to the back of the bee's thorax (neck region). It is very hard for the bee to remove pollen from this spot, and in this way the bee carries pollen from flower to flower and ensures pollination. The Phlomis flower illustrated shows how this is done. Note the stamens poised to place their pollen on the bumblebee.
Members of the Pea family produce a very nutritious pollen that is vital for the growth of some species of Bumblebees; the wild flowers Bird's Foot Trefoil and Red Clover are prime sources and the loss of these two plants from pastures and meadows is thought to have contributed to the decline of several bumblebee species. Broad Beans (illustrated right) and Runner Beans are in this family. If you have plenty of bees in your garden you will get a good crop of beans.
Members of the Daisy family (Asteraceae) are important sources of forage for insects. There are however different types of Daisy flowers that cater for different types of insects. Daisy flowers are in fact compound flower heads made up of hundreds of tiny flowers or 'florets' all tightly packed together. Some daises have a hard yellow button-like centre, consisting of tiny florets. These florets are too small for most bees and tend to attract a limited range of 
pollinating insects. The Inula above has small florets which are mainly used as a nectar source by butterflies such as this Speckled Wood. On the other hand Anthemis tinctoria (pictured right) attracts a wide range of insects, especially small solitary bees. This plant is a good perennial in a sunny well-drained soil and will mildly seed itself around. In the picture on the right a small Mining Bee (Andrena flavipes) is collecting nectar and possibly pollen from Anthemis tinctoria. Note the 'pollen baskets' on her legs.
The banner at the top of this page shows another small solitary bee (probably a Colletes sp.) taking nectar from a tiny floret of Curry Plant (Helichrysum italicum). The image is magnified, the bee is about 6mm long. Many small solitary bees use yellow composites. As well as Anthemis and Helichrysum, Tanacetum (Tansy) will attract them.
Other members of the Daisy family, such as various Asters and perennial sunflowers (Helianthus) have a much 'chunkier' middle with larger central florets. Some of these are excellent forage and attract a good range of insects, providing both pollen and nectar. Among the best in this regard are a late summer perennial Sunflower called Helianthus x laetiflorus 'Lemon Queen' , and various cultivars of Aster novi-angliae (pictured left) and Aster amellus. Gaillardia, Cosmos, and single-flowered forms of Dahlia such as 'Bishop of Llandaff' also fall in this category. (Beware other kinds of Aster by the way, some are very invasive.)
A third group of Asteraceae have flowers that are more brush-like. Many of these are coloured purple and they are excellent forage for insects. Forms of Knapweed (Centaurea), Globe Thistle (Echinops) and Eupatorium (pictured right) are typical examples, attracting bees, hoverflies, and Lepidoptera.
Similar bunched flowers are found in the Teasel family (Dipsacaceae). Favourite types for insects include various kinds of Scabiouses, especially the Small Scabious, 
Scabiosa columbaria, and its very smart cream variety ochroleuca (pictured right, with a Drone Fly), the maroon Knautia macedonica and the Devil's Bit. These attract Lepidoptera as well as bees and hoverflies. All of these can be naturalised in grass and will grow in a lawn, but Devil's Bit flowers later than the other two and is best grown in a separate area. Mowing has to be adjusted accordingly.
There are a number of plant families with small round flowers each with a small tubular centre. These appeal mainly to Lepidoptera (moths and butterflies) which push their long thin tongue into the centre of the flower to drink nectar. Typical examples are members of the Cruciferae (Wallflower family) and Caryophyllaceae (Campion family). Bee Flies and bees also sometimes take advantage of these flowers as a nectar source.
A few flowers, especially Monkshood (Aconitum) and The European Aquilegias seem to have specifically co-evolved with long-tongued Bumblebees such as Bombus hortorum. Monkshoods have deep-throated flowers with nectar at the base of the tube. In 2011 I grew from seed a wild Snapdragon (Antirrhinum braun-blanquetti sourced from Chiltern Seeds,) and was impressed to see how B. hortorum made straight for this flower as soon as they entered the garden. The flower fitted them like a glove, but other insects struggled with it.
The Carrot family (Apiaceae) includes small, flat open flowers in bunches called umbels. In general these present nectar openly and shallowly and appeal to hoverflies, small beetles and small solitary bees, rather than bumblebees or Lepidoptera. Outstanding among these for the pollinator garden are Angelicas (A. archangelica and A. sylvestris). The Sea Hollies (Eryngium) also belong to this family but have slightly larger florets that also appeal to larger kinds of bees.
Finally there are flowers which have evolved to make use of a special kind of pollen-collecting behaviour called 'buzz pollination'. The stamens form a pointed central cone which only releases pollen when Bumblebees visit and vibrate their wings at a certain frequency. Only bumblebees and Anthophora bees can do this; honey bees are unable to buzz-pollinate flowers. Flowers in a number of families have evolved to use this phenomenon. Familiar examples are Tomatoes, Peppers, Borage, and Geranium phaeum.
Exotic plants - not insect-shaped!
In other parts of the world various other types of creatures, especially birds, are important pollinators. This is not the case in Europe, where there are no naturally occurring bird-pollinated flowers. Typical examples of exotic flowers that co-evolved with bird pollinators include Fuchsias, Kniphophias, Phormiums, and Cannas, while the Australian Callistemons co-evolved with a mammal pollinator. Many of these flowers are not accessible at all to our insects as a source of nectar or pollen; others are accessible only to a very small range of insect species (typically honey bees) and so do not sustain pollinator diversity.
The image shows Phormium with the robust infloresescence which serves as a perch for its bird pollinators back home in New Zealand. As well as having tall, robust inflorescences, orange and scarlet flower coloration is typical of bird-pollinated flowers.
Of course I am not suggesting these exotic flowers should be banished from our gardens. We simply need to ensure that we also plant an abundance and diversity of insect-friendly flowers in our gardens, so that the broad range of pollinating insects can find enough to sustain them.
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© Marc Carlton 2011. You may print this page for personal use or for non-commercial, not-for-profit educational purposes. Other reproduction is prohibited without permission.
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