Soils become waterlogged when water is unable to drain away. This leaves no air spaces in the saturated soil, and plant roots literally drown. Waterlogging is common on naturally poorly drained soils or when heavy soils are compacted.

Short-lived flash floods after a downpour seldom harm most plants. It is prolonged, saturated soil that cause the most damage as the oxygen is used up by the plant roots and soil microorganisms.

Flooding from overflowing drains can add the complication of sewage and waste water, especially if growing edible crops.

Symptoms of waterlogging are not easy to tell from other disorders but look for the following;

• The first symptoms appear on the leaves. This includes yellowing or decay between the veins, resulting in soft areas at the base or centre of the leaf. There may be dark areas along the midrib, and areas within the leaf go brown, especially on evergreen leaves
• The plant may also look like it is short of water, even wilting
• A root sample will show blue-black roots, a typical sign of waterlogging that may be accompanied by a sour, rotting smell. Roots may rot away completely, with few remaining. Damaged roots will be blackened and the bark may peel away
• Shoots may die back due to a lack of moisture (the roots cannot supply water to the leaves) and bark peels off the shoots easily
• Herbaceous plants may fail to sprout in spring, or leaves may open and then die
• Plants may be stunted, or even die
• Some plants suffer from a condition called oedema

Some of the symptoms are easily confused with water stress (too little water). This is because a waterlogged plant actually is water stressed. The roots are short of oxygen and cannot absorb any water or nutrients to move around the plant.

Excess water causes problems for plants in a number of ways;

• Waterlogging limits oxygen supply to the roots and prevents carbon dioxide from diffusing away. Root function is reduced or stops and the roots start to die off, allowing the invasion of rots and decay organisms. This has a subsequent effect on the visible parts of the plant, as the leaves and stems are unable to obtain enough water and nutrients
• In cold, winter soils, roots and soil microorganisms respire little, so waterlogging is much less damaging than during warm seasons, when roots respire freely and demand more oxygen. Few plants can survive summer waterlogging, unless they have special roots adapted to such conditions. Willows and marginal aquatic plants such as flag irises are examples of these
• Waterlogged soils may be compacted or have a naturally dense texture lacking drainage channels. This means that the soil remains wet after rain
• Hard landscaping does not allow rainfall to drain into the soil below. If you hard landscape part of your garden, more surface runoff has to be absorbed by a smaller area of soil which risks waterlogging

Short term

• Where did the excess water come from?  Heavy rain, groundwater flooding or from overflowing drains? Contact your water company if you believe it has come from overflowing drains that are outside of your garden
• After flooding, wash down hard surfaces and collect up debris to prevent drains blocking, soil surfaces being covered, and pollutants or contaminants lingering in the garden. Wear gloves and overalls to minimise contact with pollutants
• Keep off the soil until it is workable, to avoid compacting it and worsening the conditions
• Remove damaged shoots from affected plants
• After flooding, 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 disease spores remain in the soil. However, the following year after flooding, it should be safe to grow crops that are to be cooked
• Apply organic matter to improve the soil structure or a balanced fertiliser in the spring, mulching over the root area after application
• Foliar feeds during the growing season may help improve leaf colour, and encourage new root growth
• Water thoroughly in dry spells after a waterlogged period, as plants will be more susceptible to drought stress

Long term

• Improve soil structure and drainage through cultivation
• Avoid smearing the sides of planting holes on heavy soils – or prick the sides of the hole with a fork before planting
• Consider planting trees on a slight mound
• Grow plants in raised beds
• 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
• If there is somewhere for water to go, drainage can be installed. 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
• Choose trees and plants that are well suited to wetter soils
• Install some water butts. They will reduce the runoff into the drains but only if there is space for them to collect water. Use water from the water butts to wash cars and paths in the winter and water your garden in the summer
• Consider creating a space for the water if it floods regularly, such as a rain garden or bioswale