Plant roots require water and air to function properly. Waterlogging leads to a lack of oxygen and results in poor root function, root disease or root death.
After the deluge
Flood waters deposit silt, debris and, all too often, sewage and oil. Once the waters have receded from a flooded garden, wash down hard surfaces with clean water and collect debris, particularly if it is strewn over delicate-leaved plants. Wear gloves and overalls to minimise contact with pollutants. Pick over plants and remove dead leaves to reduce fungal diseases such as botrytis.
If neccessary, lift the plant and remove the parts of the root system that are affected and replant into fresh compost. Use a pot that is just big enough to accommodate the existing rootball with a little fresh compost. The plant shoot may need to be trimmed back to compensate for the loss of roots.
Edible crops near to harvest are best not eaten: no assurances can be given that root crops will be safe to eat so they should be discarded. Plants eaten raw should be discarded too, and it is prudent to avoid growing salads and other uncooked crops for two years in case pathogenic spores remain in the soil. The following year it should be safe to grow crops that are to be cooked.
Damage prevention
Keep off the soil until it is workable to avoid worsening the conditions. Remove dead or dying shoots from affected plants and apply a dressing of a balanced fertiliser in the spring and mulch over the root area. Foliar feeds during the growing season may help improve leaf colour, and encourage new root growth. Water thoroughly in dry spells as plants will be more susceptible to drought stress following waterlogging.
Long term
Improve soil structure and drainage and relieve compaction by deep digging in the autumn and working organic matter into the soil. Avoid overwatering container plants and ensure they are well crocked and free draining.
Avoid smearing the sides of planting holes on heavy soils, using a fork for digging and breaking down the sides. A drainage system may be required where waterlogging occurs frequently. Alternatively, grow plants in raised beds or plant trees on a slight mound.
Making raised beds from topsoil at least 20cm (8in) deep, held above the surrounding area, provides sufficient depth of well-drained soil for plants to grow. Planting individual trees on raised mounds 25cm (10in) high is also effective.
Flood embankments to keep water out are rarely appropriate in a garden setting. Choose permeable surfaces when laying drives, paths and patios to allow rain to soak in. Roofs converted into living green roofs absorb storm water and release it slowly later.
If there is somewhere for water to go, drainage using perforated pipes covered with gravel can be installed - although this will be disruptive for several months. Or, where appropriate, it may be worth digging out a ditch or seasonal pond at the lowest part of the garden to catch surplus water and let it soak in slowly.
Select plants that are likely to grow well in wet soils. These include:
Shrubs
Cornus alba, C. florida and C. sericea
Hippophae rhamnoides AGM
Hydrangea macrophylla and H. paniculata
Phyllostachys
Physocarpus opulifolius
Salix
Sambucus
Sasa
Taxodium
Tsuga heterophylla
Trees
Amelanchier
Liquidambar
Nyssa sylvatica
alder
hawthorn
mountain ash
willow
Herbaceous perennials
Acorus gramineus
Astilbe
Caltha palustris
Euphorbia griffithii and E. sikkimensis
Filipendula ulmaria
Hosta
Iris ensata AGM, I. laevigata AGM, I. pseudacorus AGM, I. sibirica AGM
Lobelia cardinalis
Lysimachia nummularia and L. punctata
Lythrum salicaria
Mimulus cupreus, M. luteus and M. primuloides
Zantedeschia aethiopica
More information
Watch your water - article from The Garden on water in the garden
Waterlogged lawn action plan - RHS help & advice
Installing drainage - RHS help & advice
Effects of waterlogging on plants - RHS help & advice