The scourge of home gardeners, and the BFF of nursery salespeople! It is estimated that 90% of indoor plant deaths can be attributed to overwatering!
Overwatering can be caused by a number of scenarios; physically overwatering the plant, poor or blocked drainage, and no drainage.
Overwatering generally results when the plant is given more water than it can metabolize within a reasonable period of time, or the physical state of the container prevents excess water from freely draining from the container.
While this article deals mainly with container plants (both indoor and outdoor), similar conditions can also occur in the open ground in the garden.
Symptoms of overwatering (refer to the image gallery above):
Yellowing of the basal leaves is one of the early indicators of overwatering but needs to be taken in context with the overall health and condition of the plant.
Leaf drop, many plants such as Ficus will start to drop/ shed their leaves if they are receiving too much water - this is very clearly noticeable
Wilting of the leaves - read more below
A strong smell of decay coming from the- container/ soil
Overwatering results when:
The plants are watered to frequently and the soil medium does not have enough time to dry sufficiently before it is watered again.
The soil medium is incorrect and contains too much organic matter that simply retains too much water.
The drainage holes in the base of the container are blocked or become constricted by fine particles of soil medium that filter to the base of the pot in the irrigation water.
Salt build-up (scale) in the drainage holes. Dissolved salts (including fertilizer salts) are leached from the pot in excess irrigation water and build up around the edge of the drainage hole, much in the same way as stalagmites and stalactites are formed. This is more prevalent in areas where there is a high rate of evaporation or as the case is indoors in air-conditioned homes.
The plant's roots grow through the drainage holes and block them. This is dependent on the vigor of the plant and the nature of the root system. This is very common in plants with tuberous root systems such as Asparagus.
The container has no drainage holes. Many people buy pots either without drainage holes, or to have the holes blocked before planting into them. The reason is simple, they are afraid that the excess water will leak onto their carpets, floors, or desks.
What happens physically when a plant is overwatered?
To answer this you need to look more closely at what happens to the water when you pour it into the pot. Water percolates through the soil, filling the cavities and pore spaces between the soil as it travels down. The downward movement of the water draws oxygen into the soil along with the water, aerating it. Water is held by cohesive force in thin films around solid particles, and absorbed into fibrous particles like a sponge. When all the space in the soil is occupied by water the soil is said to have reached its “field capacity”, and all excess water that cannot be held simply drains to the bottom of the container via gravity.
The plants then absorb the water and the nutrients dissolved in it through a process known as osmosis. Without going into too much detail this process relies on the fact that the roots of the plant have a thin-walled membrane that allows water and dissolved salts to pass through it. Water is then drawn up the plant stem, and the nutrients are used by the plant, water is finally lost as water vapor and oxygen through small openings on the underside of the leaves in a process known as transpiration.
When a plant is overwatered or the drainage holes have become blocked air is forced from the pores and spaces in the soil medium, and oxygen present in the water is soon used by the plants and microbes in the soil. Without oxygen in the soil, the roots begin to deteriorate, the cells break down and the cell membranes denature, this destroys the plant's ability to absorb water (through osmosis). If the soil medium remains waterlogged the process of decay sets it and the root system is damaged further.
Plants actually absorb most of their oxygen requirements from the soil and not through the leaves!
Two things happen when the root system is damaged; firstly the plant begins to wilt, as it is no longer able to absorb water. Many people take this as a sign of drought and water the plant in an effort to rehydrate it, this merely compounds the problem.
Secondly, as the roots are damaged and the cell membranes are no longer intact, plant fluids containing plant nutrients are drained steadily from the plant back into the soil, this gives rise to the characteristic yellowing of the plant's leaves. As the lower leaves and branches are nearer the roots they are affected first, hence the leaves of overwatered plants are always yellow from the base of the plant first, this progresses up the plant until the entire plant is affected. As the leaves run out of oxygen the tissues start to die and break down, and leaves (and herbaceous stems) turn a creamy yellow and have a mushy texture when rubbed. Note, that this should not be confused with the natural de-foliation process of certain plants, especially varieties that have a single stem, such as Palms, Yucca, and Dracaena where older leaves are shed from the base.
Saprophytic fungal spores that are present in the air and soil then germinate and begin to decompose the dead plant tissue, causing a smell characteristic of dead vegetation. Open wounds in the plant can also allow the entry of parasitic fungi and insects as secondary infections.
How to prevent overwatering
If there are drainage holes in the base of the pot, ensure you don’t block them when planting;
For outdoor containers, make use of a good layer of drainage stone between 8 to 10mm in diameter at the base of the pot. Fill the pot with drainage material to a depth of between 10 and 15% of the container height. This allows water to drain freely from the pot, as well as aids in the aeration of the soil medium.
For indoor or small containers you can simply place a small piece of material, cloth, hessian, or geotextile over the drainage hole(s) to ensure the soil does not plug the hole.
If the pot has no holes have some drilled in it; some indoor containers are "decorative outers", in other words, they should not be planted into directly, they are supposed to have a smaller pot containing the plant which simply fits into the outer. This allows excess water to be removed, always ensure that there is no standing water in the base of the outer/ sleeve pot.
Know how much water is enough. Moisture meters can be purchased cheaply and used as an indicator as to when and how much water to give. Establish a watering schedule, and mark the day on a calendar along with the amount of water to give each plant.
Generally speaking, indoor pot plants need to be watered every 10 to 14 days, however, this depends on the type of plant, its size, the size of the container, the location of the plant, the type of medium used, the amount of fertilizer applied… the list goes on. It is easier to see when a plant is under-watered as opposed to over-watered. Let the plant dry out, the leaves will turn a dull grey-green color (they lose their sheen) and the plant will wilt slightly. Water the plant with a set amount of water, starting with a modest amount. Once the plant recovers see how long it takes to dry out again. This is a good indication of how much water needs to be applied and the duration between watering intervals.
Do not use polymer gels or water-absorbent products if the container is to be kept indoors. Decorative mulches on the top of a container also cause soil moisture to be retained. Adjust your watering accordingly.
Plants in a home that is air-conditioned where the temperature is between 18 and 22 degrees Celsius require less water than those kept at 22 degrees Celsius and above, as their metabolism and growth rate is much lower at cooler temperatures.
Reduce the amount of water given during winter as plants metabolize at a slower rate in cooler temperatures.
A strong odor of decay is a sure sign of drainage problems in a container. Remove the container to a well-ventilated area immediately and check the drainage holes for obstructions.
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