THE MANY MERITS OF MULCHING AND A DEEP DIVE INTO WHY ROOTS REQUIRE RESPIRATION
(A.k.a. Properly mulching plants, why roots need to breathe, and learn about lenticels)
A properly applied, breathable yet insulative, organic mulch, like pine straw, can help to gain a gardener as much or more than half of a cold hardiness zone. A proper mulch will help to trap stored heat from the soil and hold it around the crown of the plant as well as serving to shield tender buds beneath it from repeated frosts and freezes. A mulch helps to moderate soil, root, and crown temperatures from extremes not only in winter but also throughout the growing season. A late winter or early spring warm spell can heat the exposed soil's surface prematurely possibly triggering early spring growth that can literally be nipped in the bud. With a mulch in place the soil temperature is kept slightly lower and initial spring growth is delayed until temperatures have stabilized and are more conducive to growth. A mulch can be critically important for young starter plants and cold tender species that need this extra blanket of protection during their first year or more of establishment.
By ‘properly applied’ it’s implied that it must be at an appropriate depth for the type of mulch being used as well as taking into consideration the size of the plant or plants that it is to be used around. For pine straw, once it has settled, and most mulches, LSU AgCenter recommends that it should be about 2-4 inches deep to be effective. Use the shallower end for smaller transplants and lower growing plants as well as for plants that may be sensitive to higher soil moisture levels (Lavender and Agastache come to mind). The mulch can be slightly deeper for larger trees and shrubs but still is generally not recommended to be over about 4 inches deep. Mulch volcanoes are a common gardening and landscaping mistake. This is where there is a mound of mulch that is much deeper at the base of the plant and then it quickly slopes outward to nothing. Mulch should be evenly applied across the entire root zone. Optimally, it should extend from the base of the plant out to the edge of the plant's dripline. The dripline is an imaginary circle with a radius that extends to the outermost edges of the plant's branches. The fine young feeder roots typically extend to the outermost edges of this circle and this is where the most competition for resources such as water and nutrients occurs. Apply mulch to the proper depth across the entire root zone for maximum effectiveness against weeds, for optimum moisture retention and temperature moderation, and for creating healthier soil conditions to help your plants grow to their full potential.
Mulches around desirable plants should be ‘breathable’. Oxygen absorption by roots is critical for cellular respiration and thus plant growth. This is why the Bald Cypress and Mangroves produce aerial roots above the natural water line. These aerial roots, or pneumatophores, allow for this air exchange to occur even when these plants are growing in what are anaerobic or oxygen deprived conditions. These plants are the exceptions and the great majority of terrestrial plants do not have the ability to survive in anaerobic conditions. In my humble opinion, this is one of the most important lessons that a gardener can learn and apply to be successful in growing plants: “Leaves need light AND roots require respiration!”. A proper mulch is breathable in that it will still allow for this natural air exchange to occur with surface roots as well as allowing moisture and fresh air to penetrate into the root zone. As moisture passes downward into and through the soil, fresh air is drawn in behind it and exhaust gases are pushed out helping to maintain these healthy aerobic or oxygen rich conditions. As a result, properly applied, breathable, organically mulched plants are healthier, more vigorous, and are more resistant to attack by pests and disease.
What do you mean by plant roots need oxygen?!? We all learned in school that plant leaves use carbon dioxide, water, and light energy in photosynthesis (light energy + 6CO2 + 6H2O → C6H12O6+ 6O2) to create sugars with life giving oxygen as a byproduct. Ah, but this is only half of the equation young grasshopper. In order to release and utilize the sun’s energy that is stored as sugars for cellular processes, like cell division and thus plant growth, the cell’s mitochondria use respiration (C6H12O6 + 6O2 → 6CO2 + 6H2O + energy). As you can see in the chemical equation, respiration requires exposing these sugars to oxygen and this reaction then releases the stored chemical bond energy along with carbon dioxide and water as byproducts. Without oxygen for respiration root hairs die; water, oxygen and nutrient uptake is no longer able take place; cell division and photosynthesis are unable to continue due to a lack of water; exhaust gases build up and eventually cell death occurs. This is the same reasoning behind aerating your lawn if you have soil compaction, it provides a means to get more oxygen to the root system and thus improve plant health and vigor. Remember: “Roots Require Respiration!”
The root system of a plant uses the stored sugars which flow downward in a plant from the leaves via the phloem in the stems for plant growth on a cellular level (see diagram at left). The phloem is composed of tiny bamboo-like tubular cells stacked atop one another that are just below the bark in woody plants with gate-like sieve plates at each end of the cells to control flow. If you scratch the bark of a woody plant stem and you see green beneath it, like in the image at right, this is the secondary vascular cambium that is responsible for lateral growth or stem and root thickening. The cambium is green because it too contains chlorophyll and it too captures UV light and produces sugars even beneath the bark of a woody plant. This is where much of the photosynthesis occurs in deciduous plants during winter and we can see by the thinner darker tree rings that growth is still occurring during this ‘dormant’ period (see Photosynthesis in Aspen Bark by L. C. Pearson and D.B. Lawrence – American Journal of Botany in references below to learn more).
Scratching a plant stem is one way that we can check to see if a deciduous plant, or that stem at least, is alive. If there is no green layer, then there is no growth and so that part of the plant may be dead. Water and dissolved nutrients travel upward from the roots via the xylem bathing one side of the few cells thick layer of the vascular cambium in moisture and dissolved nutrients, while it is bathed in sugars on the other via the phloem. The xylem is basically composed of dead phloem cells that have lost their gates creating tubular pathways for water to flow upwards with minimal resistance like a straw. Respiration and thus the presence and availability of not only moisture and sugars but also the presence of oxygen is critically important for plants to be able to continue to grow and thrive. As one of my most memorable and esteemed professors, the late Dr. Carlos Smith, explained during our Master Gardener classes “Once a plant stops growing (or respiring) it dies, period.” Plants can live for a short time without light (they do it for about half of each day already) but will not survive long without necessary moisture, sugars, oxygen and the resulting cellular respiration. Even ‘dormant’ plants and bulbs are all still growing, and the process of respiration is still occurring but just at a very slow rate. This is why horticulturalists continually refer to the fact that “roots need oxygen” and thus a proper mulch must be breathable to be beneficial.
Let’s dig a little deeper into the root of the problem, pun intended. The reduction or complete elimination of air exchange that occurs within the root system of a plant, both in surface roots and the fine hair-like aerobic feeder roots, is the most basic cause behind some of the most common mistakes that new gardeners make. Planting plants too deeply, over-watering, over-uppotting small plants into too large of a container and keeping water in a plant's saucer continuously can all lead to the death of these oxygen requiring surface roots and feeder roots. We often aptly call this condition ‘root rot’ because this is literally what happens when these roots are suffocated and, like most any other living organism that is starved of air, they die. To replace the dead and dying root hairs, the root tips must grow but it may be unable to do so because of a lack of oxygen for cellular respiration. Too wet from over-watering, using saucers as basins instead of as catch pans, and planting too small of a plant into too large of a container can all lead to soil that remains super-saturated for too long of a period and is oxygen deprived. The excess moisture fills the natural air pockets in the soil pushing the oxygen out of the root zone. This can also potentially even lead to the collapse of these natural air pockets creating additional soil compaction issues. These anaerobic or oxygen deprived conditions literally cause the suffocation and death of the hair-like feeder roots that perform so much of the air exchange and moisture absorption that occurs in this part of the plant. At that point, no matter how much you water, the plant will often remain wilted because it has lost these fine roots that do the heavy lifting when it comes to moisture and nutrient uptake from the soil.
Let’s step back and look at the photosynthesis equation again, noting particularly the oxygen that it gives off: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2. As photosynthetic plant cells create sugar molecules, they are also giving off oxygen. The excess oxygen, not utilized by that cell’s mitochondria or other processes, diffuses through the cell walls into air spaces between the plant’s cells and may diffuse out into the atmosphere via the stomata and lenticels (learn more about Lenticels in the sidebar at right) or may be transported to other parts of the plant. The further removed from the oxygen producing photosynthetic cells, lenticels, and stomata you move, the lower the oxygen concentration becomes as it is used up by non-oxygen producing plant cells. So in the most basic sense, the further from the foliage and lenticels you move, the more the oxygen is depleted and the more it is needed. What plant part is furthest from the leaf? The root tip. Once the moisture absorbing root hair cells die those roots are no longer able to pull moisture and oxygen from the soil. Now think about what plant part is farthest from the water absorbing root hairs at the root tip: the leaf, and more specifically the leaf tips and edges, as well as the primary cambium that is responsible for stem elongation or growth. The further you get from the root tip the more important moisture becomes and so we start to see signs of cell death at the leaf edges due to drought like conditions. These plant parts no longer have enough available moisture to continue respiration and photosynthesis and thus cell death occurs. Browning at the leaf edges, leaf tips, and the growing points where stem elongation is occurring is the first sign of plants that are staying too wet even though at first that seems counterintuitive. We must remember that roots need oxygen, even in water loving species, and without oxygen plants become ‘moisture stressed’, diseases are able to invade and attack, the root system is destroyed and ultimately, we lose precious plants. Remember, as my father (John) has always tried to stress “Take care of the roots because the roots take care of the top!”. Roots require oxygen for respiration, and it is just as important as photosynthesis in the foliage and stems, if not more so.
Yes, there are plants that have the ability to grow in water. One of the most common and memorable instances that offer proof of this concept is the beautiful glossy bright yellow and green heart-shaped foliage of a Pothos Ivy wrapping around an entire office space or basking in the light of a kitchen window with only a vase or jar of water to grow in. So how do you explain to this proud plant parent why the Closet Plant or Spathiphyllum in the corner is turning dark brown to black due to too much water? A Closet Plant, a.k.a. Spathiphyllum, or other indoor plant, is normally planted in potting soil and as the organic matter and dead roots begin to rot this creates extremely toxic conditions for the root system further degrading growing conditions. In fact, a Closet Plant can be grown in just water (hydroponically), without soil, if it is oxygenated, like in a fish tank. So, yes, some plants can survive indefinitely under healthy anaerobic conditions. Anaerobic cellular respiration is not only a more complex process, but it is less efficient at utilizing the chemical bond energy available in the sugars. Cell growth in the root system is then slowed down and thus overall plant growth is slower. Most terrestrial plants are not able to survive for long under anaerobic conditions and even those that can still need healthy root zone conditions for suitable growth and vigor.
Newspaper, cardboard, sheets of plastic, and fresh green grass clippings (that are hopefully free of weed seeds) are all examples of mulches that can mat together and/or create a barrier preventing natural air exchange and thus the loss of respiration in the root zone. These types of mulches can be used to good effect to smother undesirable weeds and grasses in future garden areas without the use of herbicides. My father-in-law, the noted botanist, author, and retired professor Dr. Charles Allen, uses this method on a regular basis to good effect. These types of non-breathable mulches are generally not suitable for use around desirable plants because they can create an impenetrable layer preventing this all-important air exchange and even eliminating many of the desirable microbes and mycorrhizal fungi. Now we are back to anaerobic conditions with roots starved of oxygen. Of course, over time the biodegradable mulches in this group will break down and the microbes and mycorrhizal fungi will eventually re-colonize the area once the anaerobic conditions have dissipated. The area can then be utilized to successfully establish new plantings.
Properly applied, breathable, organic mulches have many more advantages than just aesthetic value (beautification), moisture retention, the promotion and improvement of respiration in the root zone, weed control and prevention, and temperature moderation. The organic mulch that is in contact with the soil is broken down further and further via the natural decay processes of soil microbes and symbiotic mycorrhizal fungi. Did you know that over 90% of plants are known to develop symbiotic relationships with mycorrhizal fungi! Some plants simply are unable to survive without mycorrhizae, like the magnificent Longleaf Pine and Blueberries, and most other plant species benefit greatly from this symbiotic relationship. As the microbes work, including the beneficial fungi, they break the organic matter down into its most basic components and some of this is released back into the soil or is shared with the roots via the beneficial fungi in a form that plants can utilize. The actions of the microbes and mycorrhizae make the soil more porous and these air pockets and passages created by the actions of these microbes in the soil are filled by oxygen rich air and moisture. The soil becomes friable, or loose and crumbly, and less compacted making it easier for roots to penetrate the soil where they can acquire new nutrients as well as making it so the plant can grow larger, stronger and more vigorously. The plants are healthier and less stressed and so it is easier for them to fight off and resist attack by pests and diseases. Plants can flower and fruit more readily, produce and ripen larger crops, and often even at a younger age. Organic mulches also create an environment that promotes populations of beneficial nematode predators. Mulches that do not float away even help prevent and reduce erosion. There are so many advantages to a properly applied, breathable, organic mulch that it should become a requisite part of every gardener’s planting process and landscape regimen.
You can check to see what the horticulturists in your region recommend for mulches by using the search function at extension.org or find your local county extension offices via the USDA's NIFA Land-Grant University Website Directory.
Hidden Partners: Mycorrhizal Fungi and Plants by NYBG Herbarium Intern Matthew Pace on the New York Botanical Garden website at: https://sciweb.nybg.org/science2/hcol/mycorrhizae.asp.html
Mulch Aids Plant, Soil Health by Dr. Allen Owings, (now retired) LSU AgCenter Horticulturist on the LSU AgCenter website at: https://www.lsuagcenter.com/portals/communications/news/news_archive/2015/september/headline_news/mulch-aids-plant-soil-health
Reviewing The Use of Landscape Mulch by Richard Bogren, John Young, Dan Gill, and Allen Owings – LSU AgCenter News You Can Use:
Photosynthesis in Aspen Bark by L. C. Pearson and D. B. Lawrence in the American Journal of Botany - Vol. 45, No. 5 (May, 1958), pp. 383-387: https://www.cedarcreek.umn.edu/biblio/fulltext/Amer%20Jou%20Botany%20Vol%2045%201958%20Pearson.pdf
Adapted from: Almost Eden’s 2020 Fall Newsletter
Textual version of the link: https://almostedenplants.com/shopping/growing_guides/Almost Eden's 2020 Fall Newsletter.pdf
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Thank You & Good Growing,
John, Bonnie, & Jeff McMillian
& the Crew at Almost Eden
1240 Smith Rd
Merryville, LA 70653