Which flowers are the best source of nectar?

There is growing evidence that both domestic honeybees and wild pollinators are in trouble, and the many wildflowers that depend on them for pollination are also declining. But are pollinator declines driving flower losses or vice versa? Or are other separate factors causing the declines?

Nectar and pollen are the main floral resources for pollinators and lack of food is believed to be one of the major causes of pollinator decline in Great Britain. However, we don’t really know the food value to pollinators of the different flower species. We have reasonable lists of plants that attract pollinators, but not much of the detail is truly evidence based.

We know that a total of 3842 plant species are found in the UK, according to The Countryside Survey (1). Of those 3842 plant species, only 450 cover 99% of UK landscape and more than half of those 450 plants are not rewarding to pollinators at all.

What we have been missing is good evidence on which flowers provide the most value to pollinators and, therefore, which habitats are the best….until now…

The AgriLand project (2), funded by the UK Insect Pollinators Initiative (3), has addressed these questions by surveying both pollinators and wildflowers across the country, and by examining the importance of land use and the structure of the British landscape using historical datasets and national data on some of the most likely causes of declines.

Part of the project, carried out by researchers at Bristol University, set about identifying the top 220 flowering plants in the UK – which ones produce the most pollen and nectar, and which species and habitats contribute the most at the national scale.

The top five most common plants in England, Scotland and Wales were:

• Heather (Calluna vulgaris)
• White clover (Trifolium repens)
• Bilberry (Vaccinium myrtillus)
• Bramble (Rubus fruticosus)
• Creeping buttercup (Ranunculus repens)

The researchers looked for field sites where the targeted plant species grow, sampled 2 populations for each species in 2011 and 2012 and measured pollen and nectar production per flower. They bagged the plants for 24 hours then collected nectar using micro-capillary tubes and measured the volume and sugar concentration per flower. They also collected unopened stamens and extracted, counted and measured the pollen grains.

The top 10 plant species for nectar production in terms of µg of sugar/flower/day were:

1. Himalayan balsam (Impatiens glandulifera)
2. Yellow water iris (Iris pseudacerus)
3. Gladioli (Gladiolus spp.)
4. Common comfrey (Symphytum officinale)
5. Blackberry (Rubus fruticosus agg.)
6. Hedge bindweed (Calystegia sepium)
7. Honeysuckle (Lonicera periclymenum)
8. Sweet pea (Lathyrus latifolius)
9. Foxglove (Digitalis purpurea)
10. Rhododendron (Rhododendron panticum)

But that’s only part of the story. It was important to know and understand which plants would produce the most nectar over a given area or habitat in order to define which provide the most value. So the researchers multiplied the nectar per flower by the number of flowers per floral unit. They then related it to flower abundance and phenology (from existing evidence in literature) and came up with quite a different list:

The top 10 plant species for nectar per unit cover per year (kg of sugar/ha/year) were:

1. Marsh thistle (Cirsium palustre)
2. Grey willow (Salix cinerea.)
3. Common knapweed (Centaurea nigra.)
4. Bell heather  (Erica cinerea)
5. Common comfrey (Symphytum officinale.)
6. Spear thistle (Cirsium vulgare)
7. Ragwort (Senecio jacobea)
8. Common hogweed (Heracleum sphondylium)
9. Common bugloss (Anchusa officinalis)
10. Chives (Allium schoenoprasum)

By then factoring in data on plant species composition and proportional cover in each national habitat from the UK Countryside Survey 2007, the researchers came up with a league table for the nectar productivity of UK habitats (kg of sugars/ha/year):

1. Calcareous grassland
2. Broadleaf woodland
3. Neutral grassland
4. Shrub Heath
5. Improved grass
6. Bracken
7. Fen
8. Acid grass
9. Bog
10. Conifer
11. Arable

The plant species that contribute the most at a national scale were White clover, Marsh thistle and Heather, which together contribute almost 50% of the national nectar provision.

This research highlights the critical importance of the maintenance of our natural grassland habitats and woodlands, but it also confirms the need to improve our arable areas by creating and managing floristically enhanced habitats, like improved grass margins, in a way that Conservation Grade’s Fair to Nature farmers currently do.

It should be noted that non-native plants such as Himalayan balsam, while they may be good nectar sources, are very invasive and are a major problem in habitat conservation, shading out other plant species.

References

(1) The Countryside Survey (http://www.countrysidesurvey.org.uk/reports-2007) provides scientifically reliable evidence about the state or ‘health’ of the UK’s countryside.

(2) The Agriland Project (www.agriland.leeds.ac.uk/) is a partnership between The University of Leeds, University of Reading, University of Bristol, Defra’s Food and Environment Research Agency (FERA), and Natural Environment Research Council’s Centre for Ecology and Hydrology.

(3) The Insect Pollinator Initiative is a range of innovative research projects aimed at understanding and mitigating the biological and environmental factors that adversely affect insect pollinators. (http://www.bbsrc.ac.uk/funding/opportunities/2009/insect-pollinators-initiative.aspx )