How can I clean up my garden after heavy rain or flooding?

Heavy concentrated rainfall or light but prolonged rainfall over several days can cause inundation, i.e., the temporary flooding of flat surfaces that are normally dry. This happens because the rain does not give the water that has already fallen time to drain away, so it stagnates in place and its level rises. Heavy or continuous rain can also cause a watercourse, canal, etc. to overflow its levees or banks (flooding) and submerge surrounding areas. The flooding of a watercourse can also result from causes other than rainfall, such as rapid snowmelt, tides (in coastal areas), or human intervention.

We hear more and more about floods in the news: generally as a synonym for inundation, but strictly speaking it refers to the effects of overflow due to heavy rainfall, i.e. the flooding of an area and the possible accumulation of sediments, etc. carried by the water. Here we will use the terms inundation and flood as synonyms. In addition to causing loss of life, a flood causes damage to infrastructure, buildings, and crops. In this article, we will discuss the impact of flooding on plant survival and how to save plants and restore your garden.

Floods, overflow, etc.: damage to soil and plants

In addition to the intensity of rainfall, if rainwater accumulates and cannot drain away, this may also depend on the characteristics of the soil and the fact that, in urban areas, the ground is made impermeable by asphalt, concrete paving, and other materials that do not absorb water. In the event of heavy rainfall, this contributes to the overflow of watercourses, even small ones, with possible floods. It also overloads the sewers, which can overflow and cause flooding in turn.

For crops in the countryside and, more generally, for plants, the danger of flooding, and therefore the damage it causes, is linked to the levels reached by the water, how long it remains in that spot, and the season in which it occurs. Due to water stagnation, the soil becomes saturated with multiple negative effects: lack of oxygen, degradation of structure, cooling, formation of toxic substances, loss of nutrients through leaching, and repercussions on organisms living underground. Similarly, for plants, waterlogging causes root asphyxia, slowed or blocked development, disease, and competition with a large number of weeds. In a garden flooded by water that you can see with the naked eye, as we will see, the long-term damage to plants builds up and unfolds over time.

Along with the negative effects caused by water stagnation, a flood also brings those caused by mud. In fact, when the water drains away, the soil and plants are covered with a layer of mud of varying thickness, which soon solidifies, forming a barrier that is impermeable to water, air, and light.

When and how to intervene after a flood

Before intervening, wait for the soil to dry, allowing the water to drain naturally. A motor pump can help to drain the surfaces and speed up the process. In addition, in the event of a flood emergency, a blackout may occur, and a power generator is useful for providing electricity.

In the immediate aftermath, it is best not to walk on the ground or start any work, so as not to pack down the soil—which is less resistant to compaction when wet—and further aggravate the situation. Once it is dry and passable, clean up the area of debris, objects, and various materials that the water may have carried in. Then you can deal with the mud, as we will discuss in the next paragraph.

In the garden, the lawn, annual plants, and vegetables will not survive flooding if they have been submerged for more than 24-48 hours. The same applies to succulents. In gardens and the countryside, species that are not particularly affected by flooding are hydrophilic trees and shrubs, typical of riverbanks, such as elms, poplars, and willows.

As for all other trees and shrubs—including maples, conifers, olive trees, acidophilic plants such as camellias and hydrangeas, rose bushes, and fruit trees—they may suffer more or less serious damage depending on how much water has surrounded or submerged them, and for how long. It is not possible to assess the extent of the flood damage right away: we advise you not to rush and not to act impulsively by removing everything. It is best to wait at least a month or more, patiently observing how the plants react to see if they can survive. It may take several months, even a year or more for fruit trees, for example.

The one thing you can do right away is to check whether the plants show any obvious wounds on the trunk or branches: these are points that facilitate the entry of pathogens such as fungi. To prevent the development of diseases, disinfect the wounds by brushing them with a copper-based product.

We should add that prolonged oxygen deprivation due to waterlogging weakens plants, making them more susceptible to attack by pests and diseases. It is therefore important to monitor them over time and, if necessary, treat them with the most effective pesticide for the symptoms.

To facilitate the recovery of small, delicate plants or those that are particularly dear to you, you may consider transplanting them and potting them up. Once they have recovered, you can replant them in the ground.

How to remove mud from the garden

As soon as the water has drained away, before the mud sets, wash the foliage and bark of submerged trees and shrubs with a fairly strong jet of water. Mud prevents leaves from carrying out the gas exchanges necessary for photosynthesis.

Remove the layer of debris and mud left by the flood from the soil: simply break up and remove the crust with a shovel. Keep away from the roots and trunks of trees and shrubs so as not to damage them.

In vegetable gardens and flower beds, remove the remains of dead plants. If the layer of mud is not very thick, to reseed/transplant, bury the mud by working the soil with a spade, rotary tiller, or motor cultivator. When working the soil, incorporate sand to lighten it, thus balancing the mud, which is actually silt and increases the fine, heavy fraction of the soil. Also incorporate organic matter to improve the soil structure, make it more porous—and therefore more aerated and permeable—and enrich it with nutrients. If, on the other hand, the mud crust is more than 4-5 cm thick, break it up and remove it with a shovel and wheelbarrow. Removing mud crust or tilling the soil also applies to orchards and vineyards in the countryside: you can till along the rows and, taking care of roots and stems, also between the rows.

Finally, to clean mud from outdoor surfaces—sidewalks, driveways, walls, etc. —and the walls of your home, you can use a high-pressure washer.

Once the mud has been removed, wait a few weeks to see if the lawn is able to recover on its own or not. To restore the lawn, you can sow it from scratch: the best time to do this is between late summer and early fall. Start by weeding, then till the soil with a rotary tiller or motor cultivator, distributing organic matter, then proceed with the actual sowing, which should be completed with rolling.

If the lawn seems recoverable, again between September and October, you can try to thicken it by sowing on the existing grass. To reseed, cut the grass low with a lawnmower, collect the clippings, go over it with the scarifier, remove any debris, then proceed with sowing and rolling. However, if most of the lawn needs to be regenerated, it is best to sow everything from scratch.

On the subject of lawns, here you will find a focus on scarifying and an article on how to lay turf, a quick alternative to traditional sowing.

Flood water may be contaminated with harmful substances (pesticides, fertilizers, fuels, etc.) and, if stagnant, can lead to infections, such as mosquito-borne diseases. We therefore recommend wearing protective clothing such as boots and gloves, and clothes that cover your arms and legs.

How to prevent flooding

To protect yourself from flooding and damage caused by standing water, you can use drainage systems with drainage trenches in your garden and in the countryside, which serve to remove or disperse water from an area. In practice, these are sloping trenches filled with gravel or similar material, into which special pre-perforated drainage pipes are usually laid: the water then enters the trench and flows away. Here you will find our article on how to build a drainage channel.

In urban areas, another way to prevent flooding is to avoid or limit the use of impermeable outdoor paving (such as asphalt, concrete, paving with cement joints) for courtyards, driveways, parking spaces, etc. Instead, go for permeable surfaces such as turf, grass pavers, blocks with grass or sand-filled joints, and porous concrete. Permeable surfaces work—i.e., they allow water to infiltrate the soil and not accumulate or run off—where the soil is permeable. Otherwise, if it is not very permeable, drainage trenches must be used.

The permeability of the soil depends on its texture: it is impermeable if rich in silt/clay and low in sand, and vice versa, it is permeable if rich in sand. In order for clay soil to absorb water, it must be managed in such a way that it remains as soft as possible. To this end, we suggest that you:

• Do not carry out overly aggressive tilling, which breaks up the soil structure and creates a compacted layer beneath the tilled soil that hinders water drainage.

• Add organic matter, for example in the form of compost or manure.

• Do not compact the soil when it is damp or wet by walking on it or driving over it with equipment such as transporters.