Between the published studies cited at the end, and my own side by side tests, some surprising discoveries have come out of the clone tray over this past year.
This first one is certain to get a hot debate rolling.
Surprise # 1
Removing Leaf Tips Reduces Rooting Success
Removing 30% of leaf tips from cuttings reduced rooting success rate and three leaves had higher root quality compared with two leaves without influencing rooting success rate.
Only a scientist could write a sentence like that and get away with it.
This very confusing statement can be broken down like this…
- Cuttings were harvested at 12 days after planting when approximately more than 50% of the cuttings showed visible roots at the bottom of the tray. Rooting success rate was measured on a binomial scale in which any visible adventitious root formation was considered rooted. The result: clipping the leaves reduced the percentage of clones that rooted in 12 days, from 71% to 53%.
- Among those that did root, those with 3 fully expanded leaves developed significantly larger roots than those with 2 fully expanded leaves, but both of these groups had the same rooting success rate. The observed quality improvement was likely caused by the additional carbohydrates and other factors that were stored in that extra leaf and made available as fuel for the growth of the developing roots.
Ok, instead of waiting for the onslaught, I’ll go ahead and answer the big disagreement that I know many of you clone-clippers already have on the tip of your tongue, and that is the argument that…
crowding a clone tray invites pathogen attack, so you HAVE to clip your leaves to create some airflow.
First of all, these studies are NOT about overcrowding problems in a clone tray. The studies ONLY measure the effect of clipping off 30% of the leaves. With that said, there are two things that come into play in the real world. One, this study very clearly shows that fully formed leaves speed up the rooting process – A LOT! The other is that fully formed leaves take up a lot of space in your clone tray. If you don’t do anything to address that level of crowding in your clone tray, you may indeed encounter some pathogen problems.
Nobody wants a clone room that takes up several acres, so the question is… is it possible to safely crowd the clone tray and still get the benefit of a couple days shorter turnaround?
If you take steps (which I believe are appropriate regardless of this issue) to sterilize your clones, your tools, your hands, the clone trays, and the dome, my own experience shows that you can safely pack them in there with no ill effects. To a human observer all those leaves look like a crowding issue. To a plant, those leaves are a big fat food source that promotes rooting MUCH faster.
I’m working on another article that goes into more detail about sterilizing your clones during the cutting process. Stay tuned for that one…
I should say that if you include really large fan leaves, they will very quickly block the light from reaching any clones below them. This is problematic because a cloning environment should have roughly 90-120 PPFD of light and anything below 50 PPFD will not support plant life. 90 is pretty close to 50, so when a bunch of big leaves crowd out the light, you could easily find your shorter cuttings dying out with black/brown tops.
Surprise # 2
Elongate Your Cuttings with Red Light
Ok this is no earth shattering surprise but more like a little “why didn’t I think of that” moment.
Of course it has long been known that cannabis plants stretch more when exposed to light with a red leaning spectrum. However, it never occurred to me that you could elongating your cuttings while they are still on the Mother simply by exposing her to a red spectrum for a couple weeks before the cut date. Then if you wish to put a Mother plant in “storage” mode, you can lower the intensity and lean the spectrum more to the blue side, or use T5’s which produce very short internodes. This concept could actually be useful if you have LED lights with some means of changing the spectrum, otherwise you would have to keep multiple lights hanging in the Mother room and switch between them.
If your farm is busy making clones every day, investing in a red spectrum light could give you a slight edge in stretching those cuttings out a little bit faster.
Surprise # 3
The Sharpness of your Cutting Tool is Irrelevant
To determine if the cutting tool influenced rooting success, branches were cut into clones (that were 15cm long) with either a disposable scalpel or with pruning shears. Sharp scalpels may cause less tissue damage than pruning shears, in addition to being easier to clean. In contrast, pruning shears can be quite a bit faster than a razor blade, and are much safer to use. I learned that lesson the hard way when someone called me and I tried to answer the phone without putting the razor blade down first…..
Thankfully, the results of this comparison showed absolutely no benefit to using a sharper tool.
Trimming up your clones with a good pair of sharp scissors is FAR faster and safer than with an open razor blade.
I personally did a test to see if I could just clip clones off with my fingernail…. they all grew roots.
Surprise # 4
Wounding the Base Speeds Rooting By 160%
“Wounding” by removing epidermal tissue from the bottom 5cm of a clone’s stem by scraping a scalpel parallel to the stem surface, caused clones to pop roots 1.5 days faster, and with a 160% increase in rooting success (see the data).
It is important to note, that while wounding was found to increase the rate of rooting, (the speed) the study did not measure the ability to root, which means that if the test groups had all been left alone indefinitely, they might have all had a 100% success rate. To measure the speed difference, the test was stopped at a certain point when most of them were rooted, but before they all rooted. At that point they were dissected and measured.
It is theorized that wounding largely acts to break the very formidable barrier formed by the cuticle and make pericycle cells (which initiate lateral root formation) more accessible to external applications of rooting hormone.
My own tests with wounding…
I find that scraping the surface of the stem does work as mentioned here, but it also increases the chance of pathogen invasion compared to other types of wounding. After performing several sets of side-by-side tests and observing the root growth from the stem under a microscope, I’ve found that scraping creates a section with no growth in the center of the scraped area, and growth around the border of the scrape. If the scraping action is prolific enough to create overlapping wounds that cover the majority of the stem, I see growth appear only at the top of scraped area and not within or below the scraped zone. This indicates that the scraping technique itself is actually inhibiting root growth, but at the same time exposing the pericycle around the edges of the scrape. Based on this observation, the pericycle exposure is the important part. Root growth appearing only at the top of the scrape, indicates that the nutrient flow which fuels root growth comes down from the leaves. Root growth does not ever appear in the middle of the scraped off areas.
Scraping is very susceptible to human error, so that no two technicians will ever scrape to the same degree or depth, and even the same technician would have a hard time replicating a scraping action with any degree of accuracy. If slightly overdone, the scraping action removes the phloem and exposes the woody xylem of the clone. If the clone is allowed to run over 10 days, this large open wound will frequently become infected with a bacterial pathogen. This elevated rot probability seems to be associated with exposing a large open area to the growing medium without the protection normally provided by the cuticle.
In an effort to find a wounding method that provides the least exposure to pathogens, while still exposing the pericycle to the rooting medium, I’ve tried all sorts of wounding styles…
Circular and spiral cuts around the stalk produced roots in a ring above the wounded area and nothing below that point. It seems clear that the flow of plant fluids is stopped or inhibited by any wounding style that circles the stalk.
The same effect was caused by an abundance of shallow cuts all around the stem. Roots only formed above the cut area.
Split Stalks performed the worst of all methods attempted. For unknown reasons, if your wound divides the stalk completely through the middle to create two separate pieces, that cutting will not grow roots.
The magic combo is 3 lengthwise cuts spaced evenly around the stem and only deep enough to break through the thin phloem layer. With this pattern, there is ample undamaged area in the phloem to allow the unrestricted flow of nutrition coming down from the leaves. The thin opening made by a longitudinal razor blade cut is enough of a break in the cuticle to allow entry of the rooting gel, while not so big as to provide a large open surface wound for bacterial invasion. Prolific root growth bursts directly out of the full length of the cuts and rarely ever from the uncut cuticle.
Caution: Wounded areas must be set fully into the media. Wounds that are exposed above the media after the clone is set, become doorways for fungus invasion.
Surprise # 5
You Don’t Need A Node At The Base
My side-by-side testing shows that stalks with 3 lengthwise wounds grow roots far faster and more prolific than stalks with a node and no wounds.
Previous rumors of nodes sprouting roots faster than bare stems are more likely due to the fact that the node was the only place where the clone was wounded to remove the branch and leaf at that junction. If your only wound is at the node, that cut is the ONLY exposed pericycle tissue available, so the rooting hormones soak into the cut at that point.
I believe the root growth at that location has been mistakenly attributed to the presence of the node and not the presence of the wound.
I tested this theory by cutting clones with 2” of straight stem, and no node in the section that was inserted into the media. I compared this with stalks having a node just above the end of the base. There was no difference in root growth on a node-free stem, compared to a stem with a node. However when compared to cuttings with 3 lengthwise cut wounds and no node, the cut wounds grew far more prolific roots than the node alone.
Surprise # 6
Exposing The Cut End Is Bad News
Ok so this is not really a surprise. It is pretty widely accepted that the tubes in the xylem and phloem should not be allowed to dry out from exposing the freshly cut end. However, it was also widely accepted that you should clip off your leaf tips, so I was half expecting to find this to be somewhat of a myth as well.
It’s not!
I made three groups of cuttings and exposed the freshly cut ends for… 5 minutes, 30 minutes and 90 minutes.
After 10 days in Peat Pellets I dissected all clones.
The 5 minute exposure group had 2 out of 6 that produced roots.
The 30 minute and 90 minute groups produced ZERO roots.
This one is no myth.
Sources:
(1) Phenotypic plasticity influences the success of clonal propagation in industrial pharmaceutical Cannabis sativa.
Campbell LG, Naraine SGU, Dusfresne J (2019) PLoS ONE 14(3): e0213434. https://doi.org/10.1371/journal.pone.0213434
(2) Vegetative propagation of cannabis by stem cuttings: effects of leaf number, cutting position, rooting hormone, and leaf tip removal. Deron Caplan, Jonathan Stemeroff, Mike Dixon, and Youbin Zheng School of Environmental Sciences, University of Guelph, Journal of Plant Sci. 98: 1126–1132 (2018) dx.doi.org/10.1139/cjps-2018-0038