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Botany Made Easy

Getting Down to the Roots


What is it that makes these hibiscus so healthy?
A big part of the answer is under the ground.
Let's get down to the roots!
All of us spend a lot of time looking at our hibiscus flowers, and many of us are careful about inspecting the leaves and branches. But how many of us pay much attention to the roots? The sad fact is, when a hibiscus plant dies, most of the time it is because of root problems, and most root problems are either indirectly or directly caused by improper watering. So if we want to keep our hibiscus healthy and blooming, it is very important to have at least some knowledge of how roots work.

It helps to think of a plant as a giant soda straw. The sun effectively sucks on the straw by evaporating water out of the leaves - what we call transpiration. On a cold or cloudy day, the sun draws less water out of the leaves, so less water is sucked up through the straw.

The part of roots that we are most familiar with are the big, crisp, tough branch-like roots. These are filled with a spongy center of dead cells, called the xylem that functions like the hole in a straw, where water moves up through the plant as the sun sucks it out the leaves. The xylem is tough and turns woody to help support the plant and to provide a rigid straw that does not collapse with strong sucking action. Because the xylem is made of dead cells, it does not use up any of the water and nutrition that passes through it. It very efficiently functions as a non-living tube that transports all its goodies up to the above-ground parts of the plant. It is a one-way tube, that moves water only upward, and it only works when the suction of the sun is pulling the water up. The xylem has no way to push water up or to help in the transport process. It is just a straw, non-living, non-active.


Cross Section of a Hibiscus Root
Surrounding the xylem are living cells, called "phloem" that are capable of actively pushing nutrients both up and down the plant. These cells push and pull sugars and proteins from the leaves down to the root cells and to all other parts of the plant to nourish them. Roots have a lot of hard growing work to do, pushing downward into hard and rocky soil, growing through every kind of obstacle, creating new cells just as much as the top part of the plant does. The phloem brings the growing root cells the food and energy for all this hard work.

But the most important part of the root is still to be seen. The very bottom of the straw where the water and nutrients come into the plant is not these big parts of the roots that we see when we dig up a plant. The bottom holes in the straw are all the little tiny, thin, fine, delicate root hairs that we barely notice on the roots. Root hairs have very special cells that stretch out like little tiny absorbent wicks to soak up water and nutrients from the soil. They are very active cells, more active than all the other root cells, to actively, quickly, and efficiently move water and nutrients into the main part of the roots. When we transplant plants, we tend to be careful with the tough, visible parts of the roots, but we often do not notice the soft, tiny root hairs, which are actually the most important parts of the root. If we crush or destroy these tiny root hairs in our transplanting, the plant will experience much more "transplant shock." If we are very gentle and careful not to crush them or knock them off, and spread them gently into soft, fluffly soil, the plant will survive the transplant with much less suffering.


Healthy roots need air & oxygen
in fluffy soil that drains well.

Roots Need Oxygen!

We know that roots need water, of course. But the less obvious thing that roots cannot live without is air - oxygen to be exact. Yes, just like us, they need to be able to breathe oxygen down there in that dark, damp soil. The phloem sends them sugars and proteins, but they need oxygen to turn this food into energy for growing and working. We forget how hard roots have to work, how much energy it takes, and how much oxygen it takes to burn food for energy. The green parts of plants can make their own oxygen through photosynthesis. But the roots have no green chlorophyll and no light. They can't photosynthesize, and they can't make their own oxygen. The only way they can get oxygen is to get it out of the soil.

Helping Roots Get Oxygen

Prepare the Soil: So how do we help our hibiscus roots get the oxygen they need? Before we plant, we fluff up the soil, dig it up, break it up, and put things like coco coir into it to make it light and fluffy and full of air pockets.

Check for Drainage: Next we check our hole or pot to make sure it drains properly by filling it with water and watching to see that the water drains out quickly. If water sits in our hole or pot for more than 24 hours, the plant will be starved of oxygen, and "drown" just like we would. When a plant drowns, it is a very sudden death. One day the plant will be healthy and green, the next day it will be completely dead. Nothing will bring it back. As for all living things, drowning is fast and fatal for plants.

Use Proper Watering Techniques: From this point on, the way we water determines how much air stays in the soil. We all know that too little water will dry the plant out and kill it from thirst, so we are very careful not to underwater. But the most common cause of plant deaths, is overwatering by flooding the soil too quickly. Flooding the soil with water that doesn't drain away quickly, drives all air pockets out of the soil. In order to understand this, we need to learn a little bit more.

Why do Plants Wilt?

Wilting is simple. It just means that the sun is sucking water out of the leaves faster than the roots are able to fill the straw and send water up. Wilting means that water is not able to pass through the roots and up to the plant for some reason - there's no water in the straw. Many things can cause this problem though, and it's not always obvious what the cause is.

Drought and Flooding: The most obvious reason for a wilted plant is drought. You forgot to water it, the soil got too dry, and there's no water for the roots to absorb. We all do this at times, and of course, there's an easy fix! We just need to water the plant. BUT.... at this point we need to be very, very careful. A plant that has dried out to the point of wilting has dried its roots out as well as its top, green growth. When roots dry out, the tiny root hairs die very quickly and easily. Remember! Those little root hairs are the end of the straw that suck up the water. If many of them have died, the plant will not be able to suck up very much water! So after you have let a plant wilt from drying out to much, water very gingerly, and do not let the plant stay in standing water at all. Make sure all excess water runs out quickly. It is very easy to flood the remaining root hairs and drown the plant at this point. This is one of the biggest mistakes that all of us make at some time in our lives. We feel so guilty for letting our poor baby dry out to the point of wilting that we give it a HUGE drink of water that it never recovers from.


Early stages of root disease:
Plant is starting to wilt, leaves look yellowish,
Plant doesn't look quite right.
Overwatering: Overwatering is like very slowly drowning a plant, bit by bit, day by day. When you overwater a hibiscus, the soil stays very wet, loses all its fluffiness, and begins to close up all the little air spaces that roots use to get oxygen. Over time, the soil gets more and more compacted until there are almost no air spaces left at all, and the plant finally dies of drowning. But in the early stages of overwatering, your plant will just look a teeny bit wilted and stressed. The leaves will be a bit off-color, and you'll notice that something is not quite right. If you catch your mistake early, poke some holes in the soil and fluff it up a little bit to get some air into it, then reduce your watering or improve your drainage, you can save your plant. You will need to be very patient as the plant regrows the roots that have been damaged due to a lack of oxygen, but with time and patience, the plant can recover.

Root Diseases: Root diseases are a common cause of plant wilting and death. Sick roots stop working properly, which means they can't take in water, and the plant dries out and dies, no matter how wet the soil is. A very common cause of root disease is fungi that can occur naturally in garden soil. Some fungi make a hole in the plant's outer skin and enter through that hole. Other fungi enter the plant through the breathing holes in the skin, called stomata. Then they fill up the spaces that water is supposed to move through, and steal the plant's nutrition. Or they exude toxins that kill the cells that water is supposed to move through, and then move in and eat those cells. Either way, the fungi fill up the "straw" and block the flow of water up to the plant. Our poor hibiscus plant slowly dies of a lack of water.

Fungal infections are nearly impossible to cure; it is much better to use good prevention techniques. A healthy hibiscus in healthy soil can usually fend off fungal attacks. But a hibiscus that sits in soggy soil becomes weakened and loses its natural defenses. The roots are already damaged by the soggy conditions, and the fungus is able to invade them easily. Strong and healthy roots are much harder for fungi to enter.


The black tip on the center root is the start of a Fusarium infection
The roots on both sides are completely black & fully infected.
Courtesy of The Korean Society of Mycology
In order to prevent poor soil conditions that weaken roots, try to keep the crown, the point where the roots meet the trunk, of your hibiscus as dry as possible. This is especially important in very wet, cool, or humid climates or growing conditions. Plant your hibiscus so that the crown is slightly above the ground, and try not to water directly onto the crown. Try to get plenty of air and sunlight down onto the crown too so that it dries out during the daylight hours. But most important, make sure your soil drains well and quickly. Roots that sit in water for long hours every day are sitting ducks for fungal infections.

If you live in an area that gets a lot of rainfall and you can't avoid wet soil for your plants, you may want to plant your hibiscus in raised beds or in pots that you raise up off the ground onto a deck or pallet. Finding a way to provide optimum drainage is key to preventing root diseases and to keeping your hibiscus roots healthy and vigorous.

 


  1. The Korean Society of Mycology, 2014. "Phytophthora Root Rot and its Control on Established Woody Ornamentals." Mycobiology. Mar 2014; 42(1): 66-72. Published online Mar 31, 2014. doi: 10.5941/MYCO.2014.42.1.66

  2. Benson, D.M.; Jones, R.K. 2000. "Phytophthora Root Rot and its Control on Established Woody Ornamentals ." North Carolina State University, Plant Pathology Extension, http://www.ces.ncsu.edu/depts/pp/notes/oldnotes/odin13/od13.htm

  3. Malloch, D., 2013. "Fungal Parasites of Plants." New Brunswick Museum, 'The Mycology Web Pages,' http://website.nbm-mnb.ca/mycologywebpages/NaturalHistoryOfFungi/PlantParasites.html

  4. Smith, K. L. "Root Problems on Plants in the Garden and Landscape." Ohio State University Extension,HYG-3061-96, http://edis.ifas.ufl.edu/ep362.

  5. Thompson, L.K., 2003. "Plant Roots" Furman University. http://facweb.furman.edu/~lthompson/bgy34/plantanatomy/plant_root.htm

  6. "Transport In Plants." Extreme Papers: GSCE Biology. http://www.xtremepapers.com/revision/gcse/biology/transport_in_plants.php