Botany Made Easy

A Healthy Root Microbiome

Key to Strong Immunities & Health

Exotic hibiscus 'Mirage' with glossy green leaves, lots of well-developed flowers ~
How can we help our hibiscus maintain this kind of health?
They need the help of other organisms!
Microbiome is one of those new words that we're all hearing a lot these days. But what is it? Is it really important, or is it just another scientific fad?

A microbiome is a group of tiny living organisms that live with a larger living thing. Humans and other animals have vast microbiomes of bacteria, viruses, fungi and other microbes in our digestive tracts, all over our skin, in our mouths, respiratory systems, eyes, urinary tracts and more. We work hard to keep the right "probiotics" in our bodies to keep our microbiomes as healthy as possible.

But it's not just animals who depend on a healthy microbiome to survive - plants do too! Like animals, plants have microbes living all over, and even inside, their leaves, stalks, roots, flowers and fruits. But because plants stay in one spot all their lives, they are able to build a special microbiome in the soil all around the outside of their roots, what we call the rhizosphere. This root microbiome, like the microbiome in the human gut, has huge numbers of microbes living in it with amazing diversity. Each plant has as many as tens of thousands of different microbes living around its roots when the microbiome is healthy. This makes the root microbiome by far the most important microbiome for the health of the plant. We take probiotics and eat healthy foods to keep our human gut microbiome healthy. But how do we keep the root microbiome healthy for our hibiscus and why is it so important?

Why do Plants Need a Healthy Microbiome?

A healthy root microbiome is a plant's best line of defense against disease. For example, if you have grown hibiscus for a few years, you may have heard of the dreaded "wilt disease." A hibiscus plant is healthy and green one day, then suddenly starts wilting the next day, and within a few days completely collapses into a wilted mess and dies. It's rare, but it is the one disease that can be fatal for hibiscus. When a hibiscus dies of wilt disease, a poor or dying microbiome is usually to blame. Wilt disease occurs when a bad microbe enters the roots and moves through the whole root system, completely infecting it, clogging it up, and shutting it down. But these nasty root pathogens are easily pushed away by a healthy microbiome! The beneficial microbes and the plant's own root chemicals drive the bad microbes away, and create a protective halo around all the plants' roots like a guardian angel. It is only when a microbiome is underdeveloped, or has deteriorated due to poor conditions, that it is unable to create this protection. The plant roots will try to fight off the pathogens alone, but if conditions are not absolutely perfect, they may be unable to.

A healthy microbiome does many other things for plants too. The beneficial microbes help plants take up more nutrients from the soil and even from fertilizers. They help the plants transport the nutrients through the plant better. They help keep all parts of the plant stronger and healthier, reinforcing all the other microbiomes in and around the top part of the plant, so pathogens can't find a place to enter the plant anywhere. Like healthy gut microbes in humans that greatly improve our overall nutrition and health, healthy soil microbes in plants greatly increase overall plant nutrition and health on many levels - probably more than we know!

What Makes a Healthy Microbiome?

The microbiome in the soil around the roots of a plant, amazingly, is developed by the plant itself! The roots pick and choose which microbes they want near them. They exude certain types of chemicals to attract the microbes they want and they exude opposite types of chemicals to drive away microbes they don't want in their microbiome. When the roots are successful in creating a healthy microbiome, the plant is able to resist disease, grow vigorously, and bloom beautifully. But how can we help our hibiscus plants be successful at building a healthy microbiome?

Strong healthy roots like these will
build a strong, healthy microbiome
if given soil that with microbes
from compost or worm castings.
First, it takes a healthy plant to build a healthy microbiome. So we can start by giving our hibiscus plants all the good care that they need: good nutrition, control of pests, a constant supply of sun and water in healthy doses. When a plant starts going downhill, it's easy for the microbiome to deteriorate too. So after stressful events in the life of our hibiscus plants, it's important to give the plant the extra boost of nutrition and support it needs to shorten the effects of the stress and get the plant quickly back up to optimum health. Extreme heat, drought, heavy rain or overwatering, extreme cold, pest attacks, any of these things can stress a plant enough that it will need extra support from a stress-reducing product like our growth enhancer or houseplant formula. If a plant bounces back quickly enough from stress, any damage to both plant and microbiome will be minimized, so the more quickly we help our plants recover, the less impact there will be on the future of the plant.

The other part of a healthy microbiome is healthy soil. We all know that healthy soil has a good, rich, dark, loamy color. It needs to have the dual properties of holding water well while also draining well, a balancing act that can be hard to achieve. All of this is easier when hibiscus grow in the ground where there is natural drainage and roots have room to spread out. Healthy soil is more challenging when hibiscus have to be grown in pots where roots are cramped, and the soil is what it is without any more available resources than what is put into the pot. We can use an excellent potting mix that starts the plant off with good drainage and a perfect mix of ingredients, but over time the soil in the pot will need some help.

But there is one more thing necessary for plants to build a healthy microbiome: microbes! These have to be present in the soil. If a plant is growing outside in the ground, it is possible that all the necessary microbes will be present, and the plant can slowly attract the ones it needs and repel the ones that could harm it. Not all gardens have a great array of microbes in their soil though, particularly gardens where a lot of herbicides have been used or where too many plants have been grown for too many years without compost or some other kind of biological matter being added to the soil.

Plants in pots present a different problem. Too many potting soils are "sterile" which means there are no living organisms in the potting mix. Without microbes in the mix, the plant can't build a microbiome at all around its roots. Over time, some stray microbes will get into the soil, but the plant is completely at the mercy of what happens to be trapped in the pot with it. Over time, the problem can get worse instead of better, with the plant stuck in the same limited soil for years on end.

So can we add microbes to our soil, and how do we know which microbes to add? All the newest research says that diversity is the key. Studies where one specific beneficial microbe was added to the soil did some good, yes. But a huge mishmash of many different microbes, as in compost, did much more good. The plants that had many, many different microbes in their microbiome were the most disease resistant and the healthiest. Just the act of having new microbes to work with makes the microbiome stronger and more effective at fighting off diseases. We're not sure of the mechanism yet, but it seems to be that when plant roots are actively working at picking and choosing friendly microbes and driving away hostile microbes, they create a bigger, denser, stronger and more diverse microbiome that is able to fight off disease pathogens much more effectively than a passive microbiome without much work to do. So we need to give our plant roots a healthy workout with new doses of microbes to work with on a regular basis.

If you are a backyard composter, then you're in luck! Compost is perfect! It provides a wide array of microbes that is constantly changing, and helps the plant create a truly diverse, strong microbiome. Add your compost to the top inch or 2 of soil at the base of your plants 1-2 times per year. All the microbes in the compost will work their way into the soil around the roots and give the plants new material to build a stronger microbiome with.

Worm castings is an even easier and equally effective method for many of us who don't make our own compost. Worm castings provide a wide huge of high quality microbes to the plant, including many beneficial microbes that come from the gut of the worms themselves. All the organic material has been thoroughly digested by the worms, which makes all the microbes easily available to the plant roots. At HVH, worm castings are what we use in the potting soil we use and sell because we feel that it gives us the highest quality mix of diverse microbes in order to build the strongest possible microbiome in our plants. We recommend that 1-2 times per year you add about an inch of worm castings to the top of the soil below each plant. Gently scratch down into the surface soil to mix the worm castings into the soil a little bit, then water.

Hmph! My adorable kitten just added to the
microbiome in the plant pot he hopped out
of above. I know I should like this, but...
Of course, there is one other tried and true way of adding fresh diverse microbes to our hibiscus... The next time your beloved cat uses your hibiscus pot as its own personal kitty litter box, just take a deep breath and tell yourself that kitty poop is loaded with a smorgasbord of microbes for your hibiscus to enjoy. Or the next time your puppy digs in your garden beneath your hibiscus, salivates all over whatever little critter it decides to chew on out of the hole it made, then pees on the hole to mark its territory, consider this a lovely buffet of microbes for your hibiscus. In the end, worm castings might be easier!


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  2. Berendse, R.; Pierterse, C.; Bakker, P., 2012. "The rhizosphere microbiome and plant health." Trends in Plant Science, 7(8), 478–486. Published online May 7, 2012. doi: https://doi.org/10.1016/j.tplants.2012.04.001

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