Analysis: Here are the plants to prioritise for watering, the plants that can survive without extra water and the ones to sacrifice
By Alastair Culham, University of Reading
With hosepipe bans in force, gardeners face some tough choices. When every drop counts, which plants deserve your precious water from the water butt, and which should you leave to fend for themselves?
As someone who has researched how gardeners need to adapt to respond to our changing climate, I can tell you that not all garden plants are created equal when it comes to water needs. Some plants will bounce back from a summer scorching, while others may never recover.
Top plants to prioritise for watering
New woody plants
Any woody plant installed in the last 12-18 months should be your absolute priority. These haven't yet developed the deep root systems needed to find moisture reserves and going without enough water the first year or so after planting could kill them.
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From RTÉ Radio 1's Today with Claire Byrne, gardener Paraic Horkan on how to handle a hosepipe ban in your garden
Water thoroughly and add a deep mulch of wood chips to help the soil hold water. For young trees you can install a watering bag around the trunk but you still need to top it up.
Hydrangeas
Hydrangeas adopt a conservative strategy when it comes to drought. They shut their stomata (leaf pores) rapidly when they sense dry soil, and keep them closed until consistent moisture returns. They often drop their leaves too.
This can mean many weeks without growth, after even a relatively short drought period. So if you want to keep them looking at their best, they need consistent watering. You can cut growth back to reduce water loss, and save the the plant at the cost of flowers.
Moisture loving trees
Japanese maples (Acer palmatum), along with other moisture-loving trees like birch and beech, are prone to serious die-back during summer droughts. Their shallow root systems and large leaves make them particularly vulnerable to water stress. Water and mulch them.
Soft herbaceous plants
Astilbe, dicentra, filipendula, heuchera, primula, trollius and many other soft herbaceous plants require good moisture levels and may not survive prolonged drought.
Shallow-rooted shrubs
Rhododendrons and azaleas are shallow-rooted shrubs particularly susceptible to drought stress, especially the large-leaved evergreen species which are also prone to wind damage when stressed.
Clematis
Many clematis varieties struggle with drought. Since they're often grown for their spectacular flowering displays, maintaining adequate moisture around the roots is crucial, especially for autumn-flowering varieties, or spring-flowering varieties which flower on the previous year's growth.
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From RTÉ Radio 1's Today with Claire Byrne, dealing with water shortages and hosepipe bans
A gravel mulch can help keep the roots cool and damp. However, clematis orientalis, terniflora, and evergreens such as C. cirrhosa can be surprisingly tolerant of a hot dry period.
Ripening vegetables
If you're growing vegetables, prioritise crops approaching harvest and those that split when moisture returns after drought, such as carrots. Runner beans and courgettes need moist soil to keep cropping and potato yields are heavily influenced by water levels.
All the pots
Anything in pots has limited access to soil moisture reserves and will need regular attention. Move containers to shadier spots if possible. Always use a pot saucer to hold water and prevent it draining away.
Plants that can survive without extra water
Research into plant water-stress shows that many common garden plants are surprisingly resilient. Forsythia adopts a risk-taking strategy. It keeps growing and photosynthesising even when soil moisture becomes limited, gambling that it can regrow after damage. This makes it remarkably drought-tolerant. It is also tolerant of heavy pruning which can save it in severe conditions.
Mediterranean shrubs like lavender, rosemary, sage and thyme are naturally adapted to dry conditions. Their grey, hairy or waxy leaves are evolved to conserve moisture. Soil conditions are crucial though. If the plants are deep rooted they will draw water up, but if your soil is shallow or compacted they might well be less drought tolerant.
Sedums, sempervivums and other succulents store water in their fleshy leaves and can survive extended dry periods. RHS research identifies Sedum spectabile as particularly reliable under stress.
Ornamental grasses generally have efficient root systems and many species actually prefer drier conditions once established.
Established shrubs including cistus, phlomis, buddleja, cotoneaster, berberis and viburnum have deep roots and proven track records for drought survival. The RHS report identifies these as garden stalwarts, with high stress resilience.
Some trees, including eucalyptus, bay (Laurus nobilis) and holm oak are remarkably drought tolerant.
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Those to sacrifice
Grass lawns are thirsty and can be left to go dormant. If you have a newly seeded or turfed lawn from this year, some limited watering may be justified. But in general, embrace the golden colour of water-stressed lawns. As long as you don't create too many bare patches from over-use, the green colour and growth will come back when it rains.
Annual bedding plants like busy lizzies and begonias have shallow root systems and high water demands. However, they are only there for one season and are easily replaceable, so prioritise them for watering only if they're particularly important to your garden's summer display and you can spare the water. You could save some by potting them up and enjoying a display that needs less water.
When you do water, research shows that technique is crucial. Water thoroughly but less frequently to encourage deep root growth. Focus water at the base of plants rather than on leaves, and water in early morning or evening to reduce evaporation.
Consider "split-root" watering for established shrubs – water one side of the plant thoroughly, then switch to the other side two to three weeks later. This keeps plants hydrated while chemical signals from the dry side's roots prevent excessive new growth that would increase water demands.
This drought is a taste of our gardening future. The plants struggling most in this year's drought are likely to become increasingly unsuitable for gardens without intensive irrigation. Be willing to swap out plants that suffer in drought for new plants that are more tolerant. Refresh plantings to adapt to the new climate.
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Alastair Culham is Associate Professor of Botany at the University of Reading. This article was originally published by The Conversation.
The views expressed here are those of the author and do not represent or reflect the views of RTÉ
