Climate change projections for the UK predict: wetter winters, drier summers and higher average temperatures across all seasons.
The DRY project has worked with allotment holders and commercial growers to bring together the knowledge and advice in a set of guides.
Being aware of climate change can help you to prepare for likely shifts in average rainfall and temperature on your allotment but also changing patterns of weather extremes.
Summer rainfall may become more intense, which may increase localised flood risk and soil erosion, even though the overall amount of summer rainfall is likely to decrease.
Allotment growers often observe the early stages of drought through their growing. The Drought Risk and You project (DRY) found growers were adapting in creative ways to collect water and reduce water use on their plots. Read more: https://dryutility.info/allotment-information/
What are the implications of possible future climate changes for growing?
What were some water management solutions shared by allotment holders in the DRY project?
How much water does your plot use?
Water consumption across the UK varies depending on soil condition, local climate, what you grow, etc. For example, a grower using mulching and no-dig might use around 1,800 litres per year on their plot. However, another plot holder could use over 8,000 litres per year!
This guide explains how rainwater can be collected and used efficiently.
Rainwater can be harvested from the roofs of sheds, greenhouses or polytunnels. Some sites ask plot owners to collect and use rainwater, if possible.
Swales or shallow gullies can be used to direct rainwater to plants. Water flowing down slopes can be trapped using terraces or French drains (gravel filled trenches). Consider how runoff from paths and other areas might be directed into soil.
A popular (and cheap) option for water storage is the use of plastic barrels (200 litre). Intermediate Bulk Containers (IBCs) offer larger capacity (typically 1000 litres) but are usually more expensive than blue plastic barrels. A key consideration is what they previously contained (avoid anything toxic). IBC can be painted to block light to reduce the growth of algae. Alternatively, shade with plants or wicker panels.
Raising and interconnecting your containers makes filling and emptying easier. Ensure all containers have covers to reduce evaporation and prevent animals or children from falling in. Normally you’ll be able to use about 80% of collected water (as some will be lost in storage due to evaporation).
Green manure. In preparation for the next growing season, fast-growing plants can be sown to cover bare soil and grown over the winter months. These can be turned into the soil (before flowering) to increase the organic matter content of the soil, which in turn increases the water holding capacity. A dense carpet of green makes a perfect environment for slugs and snails, so control measures may be needed.
Drip irrigation (from a raised water container) can be tricky to setup but is very efficient at irrigating the soil above the roots. Linking the water container to the pipework via a battery powered valve allows the plants to be automatically watered before sunrise. This minimises evaporation.
Applying a thick layer of mulch will help to retain moisture by providing protection against sunshine and drying winds. If you have a lot of annual weeds or want to implement ‘no-dig’, it may be beneficial to put down overlapping sheets of wet cardboard before mulch.
Generating enough mulch to cover a whole plot is difficult. It is easier to start with a few raised beds and extend over time. For example, four beds, 1.2m by 1.8m (total area 8.6m2), requires 12 x 70 litre bags of mulch to get a 10cm layer (840 litres of mulch). Established beds require less new mulch year on year, down to 2.5cm per annum.
No-dig needs to be kept in an area that you do not walk on, so space around the bed is required for paths. The paths need to be wide enough to allow space for kneeling while weeding or moving wheelbarrows. It is important to make sure that beds are not too wide or it will be difficult to reach the centre for harvesting.
Try intercropping to create shade and reduce evaporation. If you have a greenhouse, applying shading to the glass will reduce evaporation. It may be possible (subject to your allotment rules) to use trees or bushes to create shade. Weeds compete for moisture in the ground so hoe regularly or cover soil with weed suppressing geo-textile.
Allotment sites vary but normally water is available from two sources: rainwater or mains supply. Plants can grow on either but usually prefer rainwater which is more environmentally sustainable. Some plants (like seedlings) may benefit from mains water to reduce the chance of infection.
Unfortunately, the bulk of rainfall occurs in winter and plants require water mostly over the summer. This creates a dilemma for growers: how to collect and store sufficient rainwater or use the mains supply.
Rainwater can be harvested from artificial structures (such as roofs of sheds, greenhouses or polytunnels) or the landscape (swales, terraces or French drains, runoff from paths and other areas). For details, see guide “Ground level water collection on allotments”. However, storage is much easier if collected above ground level as gravity feed to the container can be used.
How much water your plants require depends on soil condition, local climate, what you grow, etc. With mulching and no-dig, your plot might use around 2,000 litres per year. However, you might use over 8,000 litres per year if you grow lots of leafy crops on open soil.
You can calculate how much water could be collected as follows:
Annual volume of rainwater that could be collected = Annual rainfall* (precipitation) x collection area x 0.8
For example, shed or greenhouse of 2 x 3m (6m2 area) = 0.8m x 6m2 x 0.8 = 3.84 m3 (3,840 litres)
* annual average rainfall for your area can be obtained from the Met Office website: www.metoffice.gov.uk/public/weather/climate
To use as much collected rainwater water as possible requires significant water storage capacity. Normally you will be able to use about 80% of collected water (as some will be lost in storage due to evaporation) so extra capacity is required to compensate for losses.
Collecting rainwater from polytunnels can be challenging. Installing timber rails (e.g. 38 x 63mm) approx. 1 metre off the ground on the inside of the polytunnel allows square section guttering to be screwed onto the outside. To ensure rainwater runs directly into the gutter, silicon sealant can be applied between the gutter and polythene sheet.
An array of IBCs or plastic barrels are often the most practical bulk storage option. Raising and interconnecting your containers is beneficial if connecting them to a drip irrigation system. To reduce the growth of algae in IBCs, they can be shaded with plants or wicker panels. Alternatively, locate them inside a shed which also reduces evaporation.
Drip irrigation (from water containers) can be tricky to set up but is very efficient at irrigating the soil above the roots.
An alternative to containers on each plot is a communal (site) reservoir. This could take the form of a large water tank or pond. For safety, this should be enclosed to keep children out. It should have sufficient capacity to water all the plots. A pond may be easier to fill with rainwater, and be popular with wildlife.
Some plants may require watering little but often, (daily) over the summer. It can be economical to automate this via a capillary watering system. Small pots are placed on capillary matting (often in large trays). The matting absorbs and holds water which can be drawn up from water held in a nearby container. Passive systems are silent and easier to maintain than pump systems.
Drip watering systems use gravity to feed water from elevated containers (e.g. a plastic bottle or bag of 2 to 5 litres capacity) to plants via plastic tubing. The flow rate is regulated by a manual valve. This is particular popular for growbag watering. The flexible setup can be easily re-arranged to accommodate different plants and layouts. Nutrients can be added to the water.
Avoid over-watering which can wash away nutrients (nitrates and phosphate).
The volume of rainwater that can be collected on a plot is proportionate to the collection area. Therefore, ground level water collection usually has the potential to collect significantly more rainwater than water collected from elevated structures (such as the roof of a shed or greenhouse).
In some cases, such as polytunnels, it can be easier to collect rainwater at ground level by adding French drains down the sides of polytunnel.
French drains capture water from the surrounding soil (not just the area immediately above). In other words, moisture moves sideways through soil into the drain. Typically the trench is approx. 60cm depth, and approx. 15cm wide (width of a narrow spade) to accommodate a perforated pipe of 10cm dia. Installing the piping with a slight downwards slope to the discharge will improve flow. French drains can be combined with swales to collect more water. ‘Swales’ are natural or man-made depressions (or ditches), usually grass covered, with shallow-sloping sides.
Advantages of French drains include:
French drains can also be good for drainage and used to slow throughflow and hold water in the soil. Excessive ground moisture (flooding) can harm most plants. Therefore effective drainage can be beneficial to most sites.
In practice, there are two major challenges with water collection at ground level:
If your plot is on a slope, it can be practical to dig out a step to accommodate the water container(s). The plastic sheet (typically a large tarpaulin) is located adjacent to the step. A length of gutter is often used to collect and direct the rainwater from the sheet into the containers. An overflow to an open barrel can be added to handle heavy rainfall.
Another option is using a raised sheet to direct rainwater into a container. The challenge is protecting the sheet from strong winds which can exert considerable forces. Using an Intermediate Bulk Container (IBC) for water storage and keeping it partly filled can provide a secure base for the sheet.
If your plot does not have a structure which can be used for rainwater collection and you do not want to install a French drain or plastic sheeting, you could consider using a stand alone water collector.
On some sites, it may be possible to redirect runoff water from swales or Sustainable Drainage Systems (SuDS) to a storage reservoir or to directly irrigate nearby plots.
The impact of a drainage system on the surrounding plots also needs to be carefully considered. For example, if an overflow directs heavy rainfall off the plot it could result in flooding in a neighbouring plot.
Before deciding which plants to grow, it can be beneficial to consider slope, aspect and soil type of your plot. If your site is on a slope, the top of your plot is usually drier than the bottom. So you might want to put perennial fruit bushes at the damper bottom and early season salad crops at the top where the soil will be warmer and drier.
Sandy soil drains quickly which means it will dry and warm up sooner in the summer. This will be good for plants that will be ready early in the season before the soil dries out. If your plot has heavy clay soil, it will be slow to drain and difficult to cultivate early in the season. Most soil types can be improved by adding organic material. Dig in large quantities of well-rotted garden compost, mushroom compost, and well-rotted farmyard manure.
Work with your site, look at what your neighbours are growing and which weeds come up. For example, if weeds include cow parsley and hogweed, members of the same family such as carrots and parsnips are likely to do well. Experiment with a range of different crops to find out what grows best for you. Read the information on the back of the seed packet.
The following background information on seed germination and dormancy may be helpful when considering plant selection:
Seeds of many common garden plants have been selected to begin growth immediately when supplied with water. Help them to germinate and successfully produce seedlings by watering the drill before planting, so that the seeds are placed in a moist environment.
Seeds of many recently domesticated plants have one or more mechanisms that impose ‘dormancy’. Dormancy is a survival mechanism that allows plants to avoid periods of unfavourable conditions. Plants whose wild ancestors grew in wet conditions like parsnips and parsley contain a growth inhibitor that has to be washed off before they will germinate
Strategies and plant selection for prolonged dry conditions:
(a) Avoid summer drought by growing winter annuals. In areas with mild winters, many plants can be planted in the autumn to grow slowly through the winter to produce an early crop (late spring/early summer) when vegetables in the shops are expensive. This could be done with broad beans, Swiss chard, kale, spring cabbage, chicory, etc. It also works well with many flowers such as calendula, cornflowers and Sweet Williams.
(b) Corn salad, land cress and oriental salad leaves (such as komatsuna, mibuna, mizuna, mustard and rocket) – will provide leaves through the autumn, and winter if covered with a cloche, cold frame or fleece. Sow in early autumn while the soil is warm.
(c) Focus on high value crops. For example, grow on new potatoes (rather than main crop potatoes) which can be harvested early in the summer before the soil becomes dry (over the summer). Although a minimum amount of water is needed for tubers to fill, dry conditions help to prevent infections such as powdery scab of potato.
(d) Many plants can grow with limited water once they have established, for example: kohlrabi, beetroot, parsnip. Observe the plants’ growth and only apply water as necessary to encourage deep root growth and hardiness.
(e) Grow more drought tolerant plants. Some varieties are more drought resistant than others. For example, Cos lettuce requires less water than cabbage lettuce (such as Iceberg or Webbs Wonderful). ‘Cut-and-come again’ salad leaves (like Lollo Rosso) require less water than soft lettuce (such as Little Gems).
(f) Planting perennial vegetables is a way of establishing drought resistant food crops. Some annuals can also be grown as perennials (e.g. Swiss chard, kale, globe artichokes, Babington leeks, cardoons, welsh onions). Plant once and eat always.
(g) Rotation systems group plants according to their watering and nutritional requirements (e.g. growing root veg – carrots, parsnips and beetroots – together and placing leafy crops like lettuce in with brassicas.) This can also save time as some groups are watered less frequently than others.
(h) Space vegetables which are ‘water hungry’ (like Brussels sprouts or courgettes) so their roots have a larger volume of soil to extract moisture.
(i) If planting Mediterranean plants, do so in spring when the soil is warming up. Some of these plants can be susceptible to root-rot if planted in autumn, and become cold and damp over winter.
(j) Most fruit plants are perennial so they can take advantage of water early in the season. They are easy to mulch as they are larger and separated from each other. The disadvantage is some species (like raspberries) like water during the fruit swelling stage.
(k) Growing a smaller number of higher quality plants is better than aiming for maximum yield, and only consuming a smaller proportion of the crop.
(l) Plants use water most efficiently where nutrient levels are adequate so apply sufficient fertiliser.
(m) Remove weeds regularly to prevent them depriving your plants of water.
Consider using vegetables in different ways. For example, traditionally in the UK the tops of beetroots are thrown away but in Greece, they are used as a form of spinach. The young leaves of purple sprouting broccoli can be eaten (as well as the florets). Some vegetables have more intense flavours when watered less.
An example of an alternative drought resistant plant which can be grown without water (once established) is yacon. For more details watch: https://dryutility.info/2019/02/08/ive-got-some-plants-here-that-i-have-not-watered-all-season/
The DRY project worked with the UK Centre for Ecology & Hydrology to model the hydrological impact of possible future climate projections (different ‘What ifs’) for seven different river catchments throughout the country.
In the UK, climate change models are predicting wetter winters and drier summers with higher average temperatures in all seasons. The graph below shows the (global) average surface temperature change against time.
The following example is based on UKCP09 Climate Projections for the River Fowey catchment in Cornwall and gives the most probable average values to show the direction and scale of possible changes.
The table considers how three different levels of greenhouse gas emissions will impact the future climate compared to the 1961-1990 average. Low emissions: a decrease in the rate of greenhouse emissions. Medium emissions: the same rate of emissions as currently. High emissions: four times increase over the current rate of greenhouse emissions.
The following table for the River Fowey catchment shows cloud cover generally decreases with emissions scenarios over time, particularly in summer. Temperature of the coldest night increases over time, particularly in summer.
What future scenarios could mean:
Warmer winters will reduce the number of hard frosts. This means that aphids and other plant-eating insects will not be killed during the winter. Therefore, they may breed more and become active earlier in the cropping season.
Harper Adams University Drought impacts on agricultural and horticultural crops
Royal Horticultural Society advice on vegetables: care during drought
Royal Horticultural Society advice on drought and watering strategy
Royal Horticultural Society advice on weather damage
Royal Horticultural Society advice on drought-resistant gardening
Eden project guide to plants for a changing climate
Eden project guide to saving water at Eden
Allotment and Gardens monthly guidance on growing fruit & vegetable
Sport England guidance on dealing with droughts
STRI Group guidance on green infrastructure
Institute of Groundsmanship guidance on watering during the drought
Environment Agency guide to tackling a summer drought begins now
ADHB Podcast: Managing grass after the drought
ADHB guide Drought – nutrient management
ADHB guide Seasonal water management for potatoes
ADHB guide Drought – burn-down and harvest
Global Food Security Agriculture’s impacts on water availability
Australian Government guide to outdoor water use
Australian online calculator for outdoor water savings
Australian Government guide to rainwater
Australian Government guide to stormwater
Australian Government guide to waterless toilet
California Water Conservation Portal suggestions to conserve water