How drought is affecting plants and pollinators: expert analysis

What happens to the relationship between plants and pollinators during a drought?

All plants need water, and the lack of water affects plants in a range of ways that are critical for their flowers. Drought stressed plants redirect growth to their roots, to find more water, and may close their stomatal pores – this prevents any further water loss but also limits access to the carbon dioxide needed for photosynthesis. As a result of these two changes, drought stressed plants often make fewer flowers. 

The flowers that are produced can also look and smell different when plants are droughted. Evidence from buckwheat and some other species shows that the range of compounds that make up the perfume of a flower can change under stressed conditions, and we know that pigment production and flower colour can change too. Flowers can also be smaller, and may not last very long, wilting after a few hours.

This all makes them less attractive to pollinators, so reduces their chance of setting seed and fruit. But it is compounded by the effects on their production of the rewards that pollinators depend on – drought-stressed pumpkin flowers make less nectar than normal, and there is evidence from several species that the amount of sugar in that nectar can be less and the number of pollen grains – which pollinators use as a protein source – can also be reduced. 

Altogether, drought stressed flowers are less attractive to pollinators and less healthy themselves – so producing less seed and fruit.

How hotter and drier summers will bring change over the next 20 years for pollinators and plant interactions in UK gardens

Over the long term, none of this is good news. Flowers that make less rewards for pollinators this year mean that those pollinators can’t provision their colonies or feed their offspring – so next year there will be fewer pollinators. Fewer pollinators exacerbate the problem of reduced fruit and seed set due to drought, and the two become linked in a negative cycle. In the long term we are likely to see smaller populations of wild flowering plants and smaller populations of insect pollinators, with the pollinators also facing challenges in finding nest sites in baked ground – local extinctions are possible.

What traits make plants more resilient to dry conditions while still attracting pollinators? 

There are a few things to think about when considering what traits might result in more resilient flowers. The major factor is the plant itself – how drought tolerant is it? Something with some succulence – such as a stonecrop – which has the opportunity to store some water, has a natural buffer against drought. And then longer lived woody plants that have the chance to get their roots down deep are also in a better position to keep making flowers and nectar – things like lavender are good for this.

As to the flowers themselves – smaller flowers will generally cope better with drought than larger ones, because there’s less surface area for water to evaporate from. And tube shaped flowers that conceal the nectar will experience less evaporation than open flat flowers – again lavender is good here, as are daisies, while poppies will struggle.

Evidence that drought changes which pollinators visit flowers, and which species cope best?

There’s plenty of evidence to show that pollinators do prefer flowers that aren’t drought stressed to those that are. Side by side experimental studies show the bees, in particular, choose flowers that aren’t drought stressed – probably because those changes to scent, shape and colour we discussed earlier alert them to likely reductions in nectar and pollen. 

What we can do to help plants and pollinators during a drought

The good news is that there’s lots we can all do to help pollinators during a drought, and of course helping the pollinators helps our plants too. 

First, think about what you are growing – some succulent plants, some drought tolerant plants, some plants with deep roots. Even in a small garden or in a few pots you can include some lavender and some other Mediterranean plants. In addition to lavender and stonecrops, plants that have done well this year in the Botanic Garden include: Nepetas (catmints); Ceratostigma plumbaginoides (blue leadwort); Verbena bonariensis (purple top vervain); Cistus (rock rose); Salvia (especially Salvia yangii, Russian sage); and Oenothera lindheimeri (white or pink gaura). 

Next, make sure you water those plants – use a small watering can, water in the early morning or evening to avoid water evaporating from the soil, and make sure you don’t waterlog the roots. 

And third – put out some water for the pollinators too – a shallow dish, like an old plant pot saucer, with a few pebbles in for them to land on, is ideal. They’ll use it to cool themselves down, and to dilute overly sticky nectar back in the hive. 

If you have a bigger garden other ideas are to install some shade – use old logs to produce a shady patch and keep some taller weeds or grasses to give shady ground cover. If you have an insect hotel, think about which direction it faces and whether you can install some shade over that too.

The role Botanic Gardens play in helping us understand and respond to the combined challenges of drought, climate change, biodiversity loss and pollinator decline

Botanic Gardens have an enormous role to play. Across the global network of botanic gardens, institutions like ours are collectively growing perhaps half of the planet’s 400,000 plant species. In Cambridge alone, we cultivate around 8,000 species. Around the world, researchers are investigating the genetic basis of drought tolerance across a range of species, aiming to understand why some plants cope better than others and ultimately to develop plants and crops that can withstand drought and other climate extremes.

Botanic Gardens are also home to invaluable long-term records. In Cambridge, for example, we have more than 100 years of daily weather data, alongside a long-term phenology project that is revealing how plants respond to climate change, including which species are flowering earlier and how this may affect their relationships with pollinators.

Here at the Cambridge University Botanic Garden, we are undertaking projects to help us protect plants and pollinators against droughts to come. We are currently developing an irrigation network that will allow us to rely on harvested rainwater rather than borehole and mains water.

Together, Botanic Gardens are exploring how plant pollinator interactions are changing, which species continue to thrive and flower in hotter, drier conditions, and how these findings can be shared. By bringing this knowledge together over the coming years, we will be better placed to develop solutions that help plants and pollinators adapt to climate change.

Further reading on plants coping with climate change

References: 

CC Rering, JG Franco, KM Yeater and RE Mallinger. 2020. Drought stress alters floral volatiles and reduces floral rewards, pollinator activity, and seed set in a global plant. Ecosphere 11(9):e03254. 10.1002/ecs2.3254

Barman, M, Tenhaken, R and Dötterl, S (2024), Negative and sex-specific effects of drought on flower production, resources and pollinator visitation, but not on floral scent in monoecious Cucurbita pepo. New Phytol, 244: 1013-1023. https://doi.org/10.1111/nph.20016