We’ve all heard of “root rot,” a general term for when the roots of your plant turn black and wither away, killing your plant. But there actually isn’t one reason for root rot. Below, you’ll learn what happens when water doesn’t drain well from your soil. I cover three reasons soil drainage is key to having a thriving garden. With your new knowledge found below, hopefully you’ll never experience root rot again.

What is drainage?

Drainage is term that means how quickly water can flow through your soil.

Too dense: If you have excessively dense soil (e.g., clay), you’ll notice the soil doesn’t drain at all; water “sits” in the clay, saturating the clay with nowhere to go.

Too loose: If you have excessively loose soil (e.g., gravel, mulch, or course sand/pebbles), you’ll notice water drains too quickly. With this type of soil, water immediately passes through the soil without saturating the soil for the plants to grow.

Just right: Ideally, you want about 25% moisture in your soil with a combination of coarse and fine soil medium. In sum, you want enough density to hold water for a couple days, but not much more. As you’ll read below, the key is having tiny air pockets in the soil, but not so much water just falls through it.

Reason #1: Drainage flushes away plant waste

Like all living organisms, plants produce waste as they grow (they poo, sort of). If you don’t have proper drainage where the roots grow, the plant will sit in its own waste and die. Though the waste of animals is much more complex than the waste of plants, both alike cannot survive living in their own feces.

OK, Ryan.. But isn’t oxygen itself the major waste product of a plant?!?!

Yes, of course. But it isn’t the only waste product a plant produces. Plants excrete the excess nutrients they absorb, salts, and perhaps most important, carbon dioxide.

Unlike we were taught in elementary school, plants don’t exclusively absorb carbon dioxide and emit oxygen. In reality, they also absorb oxygen and emit carbon dioxide. It sounds a little confusing, but it’s true.

Here’s what happens. At nighttime especially, plants absorb oxygen in order to convert glucose to useable energy that allows them to grow their cells, emitting carbon dioxide as a byproduct of such cell production (much like humans do). When that carbon dioxide penetrates the soil and isn’t flushed away, the saturation of carbon dioxide in the soil can kill the roots, thus killing your plant.

In sum: flush away plant waste with properly drained soil.

Reason #2: Drainage prevents plants from “drowning”

As mentioned above, plants need oxygen to convert carbohydrates (sugars are starches) into energy to produce cells. How?

To recap: plants use a process of photosynthesis — they combine light + carbon dioxide + water into sugar, which produces oxygen as a byproduct. Now that the plant has the sugar it needs for energy, it needs to metabolize those carbohydrates to have cell growth. Oxygen is needed to metabolize such carbohydrates. This process produces carbon dioxide as its byproduct. And the cycle goes on and on...

By having tiny air pockets in your soil, even microscopic ones, your plants’ roots can respire, absorbing the oxygen they need to “breathe.” Now you can understand why planting in clay isn’t a good idea. Yes, the clay retains moisture for the roots, but it completely suffocates your roots, preventing them from having precious exposure to oxygen.

On the other hand, extremely loose soil is not good either. It will provide air pockets for your roots, but it won’t retain moisture, desiccating your plant.

In sum: drainage allows your roots to absorb oxygen from the air.

(Water plants can get their oxygen from water itself. Lilies are an example).

Reason #3: Drainage Helps Prevent Fungi Growth

Fungi (singular: fungus) thrive in moist, warm, stagnant conditions. Such fungi are usually transmitted via soil, cross-contamination, or air travel. They can stay dormant for years.

Fun fact: The Irish Potato Famine was actually caused by a plant-eating water fungus called Phytophthora, killing around 1 million people from 1846-1851!

There is not one single fungi that causes root rot or plant degradation, but all of them can be mitigated through proper drainage. Some species of fungi live exclusively on organic matter, such as plant roots. Those fungi, if untreated, will completely kill your roots, and ultimately your plant.

The two common types of plant fungi are Phylum Ascomycota and Phylum Basidiomycota, and you can see their symptoms here. Since fungi need to stay stagnant to grow on roots, having a continual flow of water around them will limit -- or even eliminate -- their ability to attach and grow on roots. Think of drainage as a way plants can wash themselves. Having proper drainage helps clean your plants, ridding them of pesky pathogens.

In sum: Drainage helps wash away pathogens and fungi that may be living in your soil.

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