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What Is Overpotting and Why Is It Bad for Your Plants?

potted red primula flower sitting in large pot

Overpotting is a term used to describe the negative effects on plant health when a plant is grown in a pot that is way too big for it.

While it might seem like a good idea to give a plant plenty of future growing space, the mismatch between the size of the plant and the pot it’s planted in can actually do more harm than good.

The potting mix in a pot will dry naturally through evaporation, but that’s quite a slow process, especially where large pots are concerned, and it’s even slower in winter with cooler temperatures, high humidity and frequent rain.

Water is taken up from the potting mix much faster by a plant’s root system, which siphons water up to meet the plant’s moisture requirements. The more leaves a plant has, the more extensive its root system is, and the more water it can take up.

Small plants only have small root systems though, which have a very limited capacity to take up the excess water in the large volume of potting mix (potting medium) in an overly large pot or container. As a consequence, the potting mix stays too wet for an extended period of time, becoming waterlogged and rotting the roots of the plant. The end result of overpotting is the same as overwatering a plant.

How Overpotting and Overwatering Harms Plants

Most plants require a wet/dry cycle, where their soil or potting mix is allowed to dry to the point where it’s just slightly moist between waterings.

Healthy soils are porous, they have a loose structure, which is comprised of around 25% air. By no coincidence, good quality potting mix also has a porosity of around 25%, providing good drainage along with sufficient moisture holding capacity.

Plant roots require oxygen from the air to reach them. When water moves down through the soil, it creates a vacuum behind it which pulls the air down into the soil to roots.

If the growing medium stays too wet for too long, the roots don’t get any air in the waterlogged anaerobic (without free oxygen) environment, and they drown, die off, turn black and eventually rot away.

Only aquatic plants (water plants) can grow happily in anaerobic mud. Some terrestrial (land-based) plants have adapted to tolerate temporary, but not permanent, flooding. These plants usually grow along river or creek banks where the water level can rise for a period, or in tropical rainforests that are subjected to heavy torrential rains where soils can become boggy from time to time.

What Are the Symptoms of Overpotting

When a plant is overpotted, overwatered, or if its pot is left sitting too long in in a saucer of water, it may show the following symptoms.

When the potting mix/growing medium of indoor plants stays too wet for too long, there may be a rise in the number of fungus gnats, the small, annoying tiny little flies present around overwatered indoor plants, whose larvae feed on the fungus that breaks down wet rotting potting mix.

Why Do Plants Wilt and Droop When There Is Excess Water?

Plants take up water from their roots and carbon dioxide from the air, and use the energy of the sun to produce carbohydrates and sugars in a process known as photosynthesis

The purpose of plant roots is to take up nutrients from the soil, and also to take up water that is lost from the pores on the undersides of the leaves that open up for the plant to breathe. The process by which water moves through a plant, and its evaporation from the aerial (above-ground) parts, such as the leaves, stems and flowers, is known as transpiration.

The amount of plant roots matches the amount of leaves they have to support. If roots are damaged, they can no longer supply enough water to support all the leaves on the plant, so some of the leaves may wilt, yellow, brown, and drop, resembling the symptoms of a lack of water.

That’s because the plant is actually short of water, it just doesn’t have enough roots to take up the required amount water to replace what the leaves are losing, even if the growing medium (soil or potting mix) is soaking wet. In such instances, adding more water just makes the problem worse.

How to Avoid Overpotting

There are a few things gardeners can do to avoid overpotting:

1. Repot plants into the next pot size only

When repotting plants, avoid potting them in an overly large or overly deep pot. The general rule in gardening for repotting plants into larger pots is to only repot them into the next pot size up. A plant in a 10cm (4″) pot goes into a 15cm (6″) pot, which then goes into a 20cm (8″) pot and so on.

Choose a pot that is approximately 2.5-5.0cm (1-2″) wider than the original pot, and of similar depth or a bit deeper, but don’t overdo it.

2. Repot evergreen plants during their growing seasons

Select the right time of year to repot, in early in spring or autumn, when the weather is mild, and plants are actively growing and able to extend their root systems deeper into the pot to take excess water up.

In winter, most plants are dormant, and it’s the traditional time to transplant deciduous plants and trees (those that lose their leaves in autumn-winter).

On the other hand, evergreen plants won’t lose their leaves in winter, but won’t be actively growing either. Their root systems will not grow further until the weather warms up.

We don’t repot in the heat of summer, as recently repotted plants take time to re-establish themselves, and usually can’t establish strong root systems fast enough to cope with sudden wind and temperature extremes. That’s why we choose the milder seasons, which are less stressful for the plants.

3. Make the necessary changes when using tall pots

Overpotting is a problem in large pots that hold a huge volume of potting mix, but also in very tall or deep pots, especially if the plant’s roots don’t grow too deep, as many plants are shallow rooted.

If we want to use a tall pot for aesthetic reasons, it doesn’t have to be to the detriment of the plant. There is no necessity to use the whole depth of the pot.

Fill the bottom half with a light but strong material that won’t collapse in time, such as scoria, which is a red-brown volcanic rock filled with air spaces. Cover it with a layer of shade cloth or geotextile fabric, which will prevent the potting mix falling into the layer below and will not rot away. Then fill the pot with potting mix.

Don’t ever use a layer polystyrene foam pieces at the bottom of a tall pot under the potting mix, it will eventually break down and make a real mess, and the plant will sink down in the pot. It will also create a very top-heavy pot that is more prone to be knocked over accidentally or blown over by the wind, which is a really bad idea.

Another solution is to put the plant in a regular sized plastic pot, and then sit that on an inverted plastic or terracotta pot inside the decorative tall pot to make up the height.

How Do You Rescue Overpotted Plants?

In an overpotted plant, there are two issues which need to be addressed:

To remedy the problem of too much water, we can:

Stop watering the plant to allow the potting mix to dry out a little. This is a more viable solution on the warmer seasons, when the plant transpires more, and evaporation is increased. If using self-watering pots, empty the water reservoir underneath the pot. If the plant pot is sitting in a saucer, empty any standing water.

Add more plants into the pot, either more of the same plants, or other different plants, into the pot so there are more root systems taking up water. Consider growing short-lived annual flowers that attract beneficial insects, or fast-growing, short harvest period vegetables such as lettuce, if appropriate, to gain additional benefits while addressing the water issue.

Repot into a smaller pot with fresh potting mix if the potting mix is breaking down, and the plant roots are rotting away, or the plant looks unhealthy. Don’t tease out, cut or wash the roots, as this causes unnecessary transplant shock. Choose a pot that is just larger than the inner rootball. Water the repotted plant with seaweed extract, as this contains natural root growth stimulants which will aid the plant’s recovery and assist with transplant shock.

To remedy the problem of insufficient roots to support the leaf canopy:

Pruning off some of the excess leaves or branches, as the plant will eventually drop them itself anyway. Select the unhealthy-looking leaves to remove, and do not remove any more than one-third of the total leaves, as this will weaken the plant.

Allow the plant time to recover, and be patient, at the plant will gradually regrow its damaged root system, and then be able to put on new growth of leaves and branches. When new leaves begin to emerge, it’s a positive sign of recovery, and that the rescue efforts were successful.

More articles on Garden Pests, Diseases and Problems

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