Plant growth and enegy reserves

Joe1980(5)April 13, 2011

Ok, I have a few questions regarding plants and their most "robust" growth periods, and stored energy.

In my readings here, in particular, posts by Al, I've been reading about repotting times based on the time of most robust growth. I guess what I'm getting at here is that I'm not really sure when that is. In my case, a lot of my plants grow well all year, and don't seem to have a down time. The exception would be my Pachira, which clearly slows down in winter, although as the older leaves fall off, new ones grow, but slower. Some plants, like my chinese evergreens, seem to grow MORE in winter, probably due to deeper sun penetration into the house.

All of my plants are situated in my dining area, which has a large south-west facing window, as well as the large sliding glass doors, which face south-east. Needless to say, I get a LOT of sun in this room, which results in great performance. If certain plants grow well all year, is it safe to say that anytime is ok for repotting and pruning??

Also, the whole "stored energy" thing confuses me a bit sometimes. I guess I always assumed that when a plant starts putting up new growth, that it must have plenty of energy available. This question comes based off of my post about the ficus alii, and the work I just did on it. Because it has new growth going on, I figured it was ok to root prune and such. I'd like to know a little more about how a plant builds up energy reserves, and how/when it is stored and/or used. I understand fully well how photosynthesis works to create energy, and always assumed that if the plant has leaves, it has the ability to produce energy, assuming you have healthy roots.

Thanks in advance to all who indulge in this one, although I do recognize that this is a bit scientific. Al, I know you have a lot of available knowledge in this department, so I look forward to learning some stuff, and I also realize you are busy, so I'll be patient. I'm also interested in everyone else's knowledge as well, or add-in questions for that matter.


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tapla (mid-MI z5b-6a)

Most of the plants we grow are long-day plants. That term is sort of a misnomer though, because it is actually the dark-period and phytochrome ratios that determine how plants respond to light stimuli. Basically, most houseplants will be at their lowest energy levels after the stresses of winter and before long days stimulate serious growth (after the vernal equinox). In most cases, unless there was a lot of leaf loss over the winter, most plants, and particularly trees, wouldn't be back-budding in the spring unless the branch apical meristem was damaged or removed. Spring is the time for branch EXTENSION and new foliage. Plants will gleefully use nearly the last of their reserves to invest in longer branches and a new flush of growth.

Most plants have a tendency to exhibit the most visible growth before Father's Day (summer solstice), as they put most of their current photosynthate production into extending branches/stems and growing leaves. As the period of darkness becomes longer than the light period, the plant recognizes the shift in the ratio between 2 forms of phytochrome and shifts its growing habit from EXPENDING energy to STORING it. So for the second half of Jun/Jul/Aug, and into Sep in the South, plants are mostly storing energy, unless they are stimulated to using some of it by the chemical messengers associated with pruning/root-pruning.

Plants that have been growing robustly due to long days and bright light will always recover from any serious work because of their high energy levels. We see this effect at work in the fact that cuttings taken from weak plants often languish or fail, while cuttings taken from healthy plants with lots of stored energy strike in a fraction of the time.

I grow under lights & my plants always come out of the winter in great shape. I've tried repotting Ficus and other long-day plants in the spring, and though I wouldn't be too worried about losing them because they ARE in good vitality, I know that the plants I repot in late Jun/early Jul recover in a fraction of the time. Most of the people I try to help are talking about plants in desperate straits, so it's a judgment call as to whether or not a plant can tolerate the additional stress of a repot when it's already likely circling the drain. There's nothing like having the plant in hand and a lot of experience repotting to help make the best decision; but lacking that, I tend to be on the conservative side with other folk's plants, usually suggesting that unless the repotting is required immediately to save the plant, that the grower tries to nurse it back to a better state of vitality so it can more easily tolerate additional indignities.


    Bookmark   April 14, 2011 at 4:22PM
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So realistically should I be lopping branches in winter?? Due to the low humidity in winter, I've always cut back my pachira around the 1st of the year, when a lot of leaves have dropped and it looks unruley. By this point, including this year, it is growing rampant, although this year there is significantly more back budding then in previous years. I've always just kind of done things when I felt necessary, although that usually equates to spring, because the yard isn't ready for me to go around messing with stuff, so I hit up my plants. My thought was always similar to pruning the trees in my yard; that you do it in spring before the rampant growth begins. This of course does not apply to conifers, although I do seek and destroy the sawfly larvae before they defoliate my scotch & austrian pines.

Because we grow tropical trees indoors, where the stresses of the winter dry season don't exist, wouldn't the trees beable to produce energy, even with the shorter photoperiod? I always figured that trees store up energy because they have to defoliate for winter, thus are not able to photosynthesize and create energy, and then use this energy to put up new growth and leaves come spring time. But, my assumption was that because our indoor trees don't have to defoliate (in most cases) that the stored energy isn't quite as important due to the fact that they can still photosynthesize throughout winter.


    Bookmark   April 14, 2011 at 5:57PM
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You are right about plants that defoliate & enter winter dormancy - they are sending energy into storage to make leaves come spring.

Considering tropical "long-day" plants to be "short-night" plants: their chemical signals switch from "saving" to "spending" when their phytochrome & phototropin light receptors respond to shorter nights & longer days.

The quality & intensity of the light matters; I think indoor light from human housemates is disruptive enough that we see some LDP foliage come on in winter (& why poinsettias, SDP's, benefit from time in dim rooms/closets to induce flowering.), but it's not enough to flip the chemical switch from "save energy!" to "spend it".

Al, you've said when we root-prune in summer the high energy levels make it easier for the plant to recover, but does the sudden need for storage area induce root growth too? And, of course, the sudden need for enough roots to supply those hard-working leaves with sufficient water...

Why the response late summer top pruning? Is the energy expenditure on new leaves outweighed by the potential for them to make lots of food? Even if the leaves might not mature for a while?

Further aside: my Monsteras put on quite a bit of foliage this winter - because they're understory plants for much of their lives, are they less sensitive to photoperiod length?

    Bookmark   April 15, 2011 at 2:05AM
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tapla (mid-MI z5b-6a)

Joe - I wouldn't be removing any substantial amounts of foliage in winter. My advice here tends to run heavily toward "It's ok to pinch in winter, but wait until the plant is growing robustly to do anything radical to roots or shoots." Temperate plants that are dormant or quiescent are another matter. They go into spring with most of the energy reserves they had when the went dormant intact, so spring IS the best and most logical time to work on these plants, though most can also be worked on almost any time if their roots aren't to be subject to freezing.

In most cases, houseplants use more energy than they produce during the late fall and winter months. Even the plants I grow under lights are no where near as robust in the spring as they are from Mem Day to mid-Sep. What plants will tolerate isn't always the ultimate question. Plants that are additionally stressed or worked when they are already weak, exhibit a much longer recovery period, leaving them much more susceptible to insect predation and diseases, so choosing the point in their growth cycle when energy reserves and the ability to make food are at their highest makes sense.

GB - plants 'understand' through chemical messengers when they need to devote energy to roots or shoots. They go through a balancing process whereby they will shed roots after a top pruning just as readily as they might shed foliage and shoots after a hard root pruning, but roots always come first. For houseplants, I generally reduce roots and repot in the month prior to the most robust growth. I might do some light pruning or top reductions, but by and large I leave the top mostly intact so it can make the food required to quickly regenerate roots. When I see the plant has returned to normal growth, I know the roots are in good shape & the plant will have goods amounts of stored energy. I can then cut the top back hard, relying on stored energy PLUS the fact that the plant's photosynthesizing ability is at peak levels to replace above-ground parts.

The way I manage my plants is different than the way most hobby growers manage theirs. I'm not passing judgment, only saying that I prioritize a little differently than most by adopting a much more holistic approach. Where most growers look at rapid top growth as the benchmark by which they judge success, I tend to look first at health/vitality of the entire tree but with an emphasis on root health, then eye appeal that includes not only how the plant looks now, but also how what I do today will affect the plant's appearance in years to come; only then do I consider growth, and then only as one of the indicators of how the plant is doing o/a. If a plant is stagnant, not growing, it is dying. That is to say, it is using more energy than it is producing, which can only end in one result, unless the trend can be reversed.

Leaves are energy sinks until they are somewhere around 75% mature; only then do they become net producers of energy instead of net users. If I have a tree going to a summer show, and it is growing well, I'll often defoliate the entire tree around Father's Day. Because the tree has been expending almost all of its reserves on making new leaves and expending branches, it won't have the energy reserves it will a month later, say at July 4th. The result is it will back-bud profusely after the defoliation, AND produce a flush of leaves much smaller than those removed. The reason is twofold: the back-budding can double the number of growing points, and the tree's lowered energy reserves reduce both the size of leaves and the length of internodes on new foliage & branching, making for a much denser plant.

All plants are encoded with their own set of genetic responses to light relations, but even plants growing on or near the equator exhibit varying responses to even subtle change in photo exposure. I'm not sure if we can say an understory plant would respond differently to variations in light exposure than say, a plant that preferred full sun because I can't help but believe there are other understory plants growing (in situ) in near proximity to your plant's relatives that ARE significantly affected by photo-period.

Switching back to the main topic now, it's important to consider that the advice I often give is offered knowing that general indoor conditions are usually pretty poor, and most plants are weak in the spring. If you are experienced enough to know your plant has been growing in good light and actually has ample amounts of energy in reserve that should enable it to tolerate the way you want to work it, I'd be the last to make a large effort to convince you otherwise. I've just worked with lots of other peoples plants and have done thousands of my own repots and have found that tropicals/houseplants respond much better/faster and with a much reduced rate of complications if they are repotted or worked hard just before the most robust portion of their growth cycle.


    Bookmark   April 16, 2011 at 2:32PM
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Very informative, Al, thank you.

From my perspective, growing mainly indoors, and not being much more than a novice, I've learned to view the roots as the best and most important indicator of my plants' health. If their roots are healthy and growing well, then I know the plant above the soil surface will grow well, other factors considered.

If I only viewed what was visible above the soil surface as the main indicator, I'd be too often disappointed, and often wrong. Plants can look just peachy above soil, while below the surface roots are rotting or dying, and the plant is trying desperately to produce an offshoot or bloom in order to have its genetic information carried forward.

I know that's not the main point of the topic, but as I've gathered information and learned, I've noticed that several of my plants behaved in exactly this fashion... their root systems were totally decimated, but the plants looked fine and in fact were putting on growth, trying to bloom. In essence, they were making a last ditch attempt to leave progeny, expending all their remaining energy to this end.

So, I guess my point would be... the roots are definitely of priority in growing anything, and it's to our advantage to ensure continued health under the soil first.

    Bookmark   April 17, 2011 at 11:39AM
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tapla (mid-MI z5b-6a)

I've posted this collection of assessments by Dr Carl Whitcomb, PhD, about his take on the role of root health as it relates to o/a plant health. It's worth considering if you haven't seen it:

Copy/pasted from another post:

Dr Carl Whitcomb, PhD, wrote what is probably the bible on growing plants in containers. Some "Whitcomb-isms":

"If the root system ain't happy, ain't no part of the plant happy"

"Roots control the tree, the stems and branches just think [not my emphasis] they are in charge."

"The more roots to share the load, the faster the dirty work gets done"

"Roots provide the fuel for the plant engines we call leaves"

"Each root tip casts a vote to decide what the top will be allowed to do"

"Top growth gets all the glory, but the roots do all the dirty work"

He also notes that "Stress can ALWAYS be measured in the root system before symptoms appear in the top [of the plant]".


    Bookmark   April 17, 2011 at 2:18PM
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Very "Whit-ty", indeed, Al! ;-)

    Bookmark   April 18, 2011 at 8:10AM
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Is back budding just a descriptive term for the formation and growth of axillary buds?

    Bookmark   April 19, 2011 at 7:04PM
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Yes, for the activation & growth of axillary buds.
(I know I'm not Al, but...)

Another question about movement of a plant's energy pool - is the best time to air-root when the plant has just turned roots to sinks (my thought being that a top suddenly without the "right" sink will create it), or does it matter, as long as the plant's energy is high?

Thanks for all your helpful info Al!


PS - curiosity killing the cat here, Al - do all indeterminate shoots of apically dominant plants have an auxin gradient, or just the top? Or, does it depend on the plant?


    Bookmark   April 19, 2011 at 8:04PM
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tapla (mid-MI z5b-6a)

Back-budding includes but isn't limited to the formation of buds in leaf axils. I've cut very large plants back hard and removed all branching and had more than 100 adventitious buds pop on what little remained of the plant.

Auxin growth regulators are not synthesized in all plant cells, even though every cell always has the ability to do so. Auxin transport in the plant is polar, primarily from apical meristems downward to root meristems but auxin is also produced in leaves - primarily in emerging l;eaves, but to a far lesser degree in older leaves and even root tissues.

All shoots of apically dominant plants exhibit the effects of a diffusion gradient of the hormone auxin, if that's what your question was; how great or minor that effect is does vary by species ... even by cultivar/variety.

The best time to get roots to form (fastest) on most plants (air layers) is late in the summer, when energy levels are highest & phloem tissues are transporting the heaviest carbohydrate load. Whether or not that is the best timing in view of other cultural considerations (for instance on a zone 4-8 outdoor plant) is another matter. If I had an indoor tree or houseplant plant that would lend itself to air-layering, I would be thinking, "Phatten it up until late summer - do the air-layer then - separate & pot up in the subsequent late spring or early summer", unless the roots were so aggressive it was obvious the plant really wanted to be separated sooner.


    Bookmark   April 19, 2011 at 11:21PM
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