Residential gardens are a valuable habitat for insect pollinators worldwide, but differences in individual gardening practices substantially affect their floral composition. It is important to understand how the floral resource supply of gardens varies in both space and time so we can develop evidence-based management recommendations to support pollinator conservation in towns and cities.

We surveyed 59 residential gardens in the city of Bristol, UK, at monthly intervals from March to October. For each of 472 garden surveys, we combined floral abundances with nectar sugar data to quantify the nectar production of each garden, investigating the magnitude, temporal stability, and diversity and composition of garden nectar supplies.

We found that individual gardens differ markedly in the quantity of nectar sugar they supply (from 2 g to 1662 g), and nectar production is higher in more affluent neighbourhoods, but not in larger gardens. Nectar supply peaks in July (mid-summer), when more plant taxa are in flower, but temporal patterns vary among individual gardens. At larger spatial scales, temporal variability averages out through the portfolio effect, meaning insect pollinators foraging across many gardens in urban landscapes have access to a relatively stable and continuous supply of nectar through the year.

Turnover in species composition among gardens leads to an extremely high overall plant richness, with 636 taxa recorded flowering. The nectar supply is dominated by non-natives, which provide 91% of all nectar sugar, while shrubs are the main plant life form contributing to nectar production (58%). Two thirds of nectar sugar is only available to relatively specialised pollinators, leaving just one third that is accessible to all.

Synthesis and applications. By measuring nectar supply in residential gardens, our study demonstrates that pollinator-friendly management, affecting garden quality, is more important than the size of a garden, giving every gardener an opportunity to contribute to pollinator conservation in urban areas. For gardeners interested in increasing the value of their land to foraging pollinators we recommend planting nectar-rich shrubs with complementary flowering periods and prioritising flowers with an open structure in late summer and autumn.

Data: Flower counts, garden level data and temporal stability simulations

[N.B. See 'Related Works' for data on nectar production of individual plant taxa (available from 01 August 2022)]

Flower_count_data_raw.csv - floral counts obtained from garden surveys in 2019.

• 'garden_code' = a unique code for each of the 59 gardens based upon the 'region' in Bristol the garden was found.
• For each survey visit, ‘date’, ‘month’, survey ‘start_time’, survey ‘end_time’ and survey ‘time_taken’ are recorded.
• For each plant taxon in flower (‘taxon’), it is recorded in the ‘front_or_back_garden’, ‘genus’ is given separately, it is recorded as ‘multi-petal’ or not and the floral unit definition (‘fu_definition’) is given. In addition, the number of separate plants is recorded where it is known (‘num_plants’) as well as the ‘growth_form’, an indication of if it appears to be ‘weed_or_planted’, the number of floral units (‘num_floral_units’), its ‘location’ in the garden and a ‘sampled_order_for_sorting’ column based on the order each row was initially recorded in the field.

Garden_level_data.xlsx – garden-level summarised statistics on nectar and plant richness as well as garden area and income information.

• 'garden_code' = a unique code for each of the 59 gardens based upon the 'region' in Bristol the garden was found.
• Each region belongs to either ‘income_band’ 1, 2 or 3 and median annual household income per region is given in GBP (‘median_income_gbp’). For each garden, area is given in square metres (‘garden_area_sqm’).
• In tab 1 (‘Annual’), estimated total nectar sugar in grams produced from 01 March to 31 October is recorded as ‘annual_nectar_sugar_g’, as well as the ‘coefficient_of_variation’ in monthly nectar production and the total richness of plants in flower recorded across the year (‘annual_plant_taxon_richness’).
• In tab 2 (‘Daily’), ‘survey_month’, ‘survey_date’, ‘days_from_start_march’, daily nectar sugar production in grams on the survey visit (‘daily_nectar_sugar_g’) and plant richness recorded flowering on the survey visit (‘surveyed_plant_taxon_richness’) are recorded.

Temporal_stability_simulations.csv – results of temporal stability simulations in nectar supply.

• For each simulated ‘number_of_gardens’ from 1-100, coefficient of variation in their aggregated monthly nectar supply was calculated (‘simulated_coefficient_of_variation’). This was iterated 1000 times, giving 100000 rows of data.