Bees and other pollinating insects prefer gardens planted with flowers that are native to the UK and the northern hemisphere, a study has found.
But exotic plants provide nectar and pollen food for bees and hoverflies at times of the year when native flowers are thinner on the ground, while certain foreign blooms can prove a hit with some species, the Royal Horticultural Society research showed.
The researchers, writing in the Journal of Applied Ecology, said the best advice for gardeners keen to help wildlife was to plant a variety of flowering plants, focusing on native and northern hemisphere species but with a selection from further afield.
With around half of gardeners in the UK actively encouraging wildlife into their gardens, the findings will help nature-lovers make their plots more attractive to bees, hoverflies and other insects
Garden border-like plots were planted with three different sets of bulbs, perennials, shrubs, climbers, grass and ferns that were either native to the UK, near-native from the northern hemisphere, or exotic species from the southern hemisphere.
The plots were monitored for the abundance of flowers and the number of pollinating insect visitors over a four year period for the study, which was supported by the Widllife Gardening Forum.
More flowers - wherever they were from - meant more visits from pollinators, but a greater number of insects were recorded on the plots with native and near-native species than the exotic ones, with 40% fewer visits to the beds with flowers from far away.
Short-tongued bumblebee species were found in larger numbers on the native and near-native plants, hoverflies favoured native flowers and honeybees preferred the near-native plots, the study found.
Long-tongued bumblebees and solitary bees were found on all three sets of plants in around the same numbers, but in the exotic plot a third of visits by solitary bees were to one type of plant, the Eryngium agavifolium Griseb, a variety of sea holly.
There were fewer visits to exotic plants in early summer compared to the other plots, but relatively more later in the season when the non-native species were flowering more than the UK and northern hemisphere blooms.
RHS experts said the results showed that gardeners who wanted to encourage and support pollinating insects should plant a mix of flowers from a wide range of geographical regions.
While there should be an emphasis on plants native to the UK and the northern hemisphere, southern hemisphere plants such as Lobelia tupa and Verbena bonariensis can play an important role.
Southern hemisphere plants tend to flower later, extending the flowering season and providing much needed food for bugs and bees after other species have gone to seed.
Lead researcher Dr Andrew Salisbury said: "The UK's 1,500 species of pollinator are thought to be under increasing pressure due to the loss of habitat and food sources.
"As more traditional habitats have been reduced the role of gardens as havens for pollinators and other wildlife is growing in importance."
But he said the role native and non-native plants play in helping wildlife had been unclear and confusing before now.
"Now for the first time, gardeners can access robust, evidence-based information on the most effective planting strategy they can adopt if they wish to attract and support pollinators.
"These findings will help gardeners to confidently pack their borders, window boxes and allotments with flowers without getting hung up on the idea that they are somehow doing the 'wrong thing' if the plants are not all UK natives," he said.