In central Florida, it is commonplace to have hard freezes
and frosts every winter. If left uncovered, the leaves of Cycas
revoluta will usually get severely burned, or at least spotted.
In the spring, many people cut the burned leaves off. I have noticed
that usually, within a month, new leaves are produced. Even though
this is not an uncommon observation, I thought I would experiment
with the cutting off of leaves in greater detail.
Usually, on Cycas revoluta, I can force three to four flushes of leaves per year by applying a high nitrogen fertilizer every three months. In the landscape, I have noticed that some plants holding one hundred or more leaves will produce leaves only once a year no matter how much fertilizer I apply. After I cut the leaves off one of these 'stubborn' plants, it produced leaves within three weeks. I'm only guessing, but it appears that a plant with a certain amount of leaves is producing a certain amount of energy. With the reduction in the number of leaves, the plant compensates by producing more leaves. This would explain why a plant holding more leaves than usual may not need to produce leaves as often. It may also partially explain the effects of fire on some Encephalartos species in Africa.
The only negative aspect of this procedure is that the plant is temporarily stressed, and often the new leaves are dwarfed compared to normal leaves. Until new leaves are produced, the plant lives on the starch in the stem. After trying this procedure on one hundred species, ranging in size from seedlings to plants with five feet of clear trunk, I found that the smaller the stem size, the more the plant was stressed. Because of this, I killed a few single-leafed seedlings.
A local cycad grower that specializes in Cycas taitungensis told me a story about one of his plants and its growth during the winter of 1996. We had more days with below freezing temperatures that winter than any winter I have experienced. One unique attribute of Cycas taitungensis is that it naturally flushes new leaves in January. When this plant produced new leaves in January 1996, they were burned off by cold weather before they had a chance to harden off. Three weeks later, and, once more, new leaves were produced. By the time the winter was over, this plant had grown another twelve inches of trunk.
My next question was, What would happen if the majority of leaves were removed, but some were kept on? In my collection, I had eight Encephalartos ferox plants originating from the same seed source and potted in the same size container. The caudex sizes ranged from four inches to six inches in diameter. I chose the middle three to experiment with. If I chose the three largest ones, they would probably have grown faster anyway. If I chose the two smallest ones, they might not have reacted properly, for some reason they were weaker plants to start with. All the plants were holding around thirty leaves each. I cut off all but the top six leaves from the experimental plants. Within four weeks, all three plants produced new leaves, but none of the other five did. This time all the leaves were normal size. I waited a month after the new leaves had hardened and cut the lower leaves again. Within five weeks, new leaves were again produced. I repeated this procedure for thirteen months. The experimental plants produced five sets of leaves. The control plants produced two or three sets of leaves. To this day, the plants that were the medium size( the experimental plants) are now three inches larger in diameter than the plants that were previously the largest.
I have tried this experiment with around thirty species and found that this procedure is a good way to increase the growth rate of cycads while minimizing the stress to the plant. Plants that hold only a few leaves did not react as well as plants that hold many leaves. Some plants, such as Cycas taitungensis and Encephalartos ferox, will flush three to six times per year with proper fertilizer applications. Some plants, such as Dioon merolae, will only flush once a year at best. The former type species reacted well to this procedure, whereas the latter type species did not show much increase in growth.
Considering how well these experiments had worked, I wondered,
what would happen if I cut leaves off the offsets, but kept the
main plant intact? To begin with, I used a particularly vigorous
growing plant in my collection, a Zamia amblyphyllidia. In May
1996, it had four offsets. The largest was one-eighth inch in
diameter. The others had no obvious caudex size. At first, I cut
the leaf on just the largest offset to see what would happen.
To my amazement, two weeks later I observed a new leaf emerging
from the offset. Before that leaf had a chance to even harden,
I cut it-off, as well as all the leaves on the other offsets.
Within three weeks, all four offsets were producing leaves. Just
so I could observe this new growth easier, I left an inch of leaf
stem on each of the new leaves produced. I repeated this procedure
for four months, and a really amazing thing happened. Leaves started
coming out of the base of the main stem. By October, fourteen
'offsets' were on the plant. By May 1997, the largest of the original
offsets had produced leaves nine times and increased in size to
three-quarters of an inch.
I have tried this procedure on fifty species, with mixed results. Most all Zamia, Ceratozamia, and Bowenia species reacted well to this procedure. Cycas and Dioon had mixed results, and, for some reason, the Encephalartos species did not work very well. I have observed that the shorter the stem, the better this procedure works. Subterranean species worked the best out of all the plants. It appears that the farther the apex is to the base of the plant, the less the offsets are affected by this procedure.
As a nurseryman, I can perform experiments and observe the results. I do not, however, have a laboratory and cannot study these plants at the cellular level. It appears that there may be some type of hormone or auxin that is generated by the plant that initiates this new growth. This was very evident when the Zamia amblyphyllidia started producing offsets in mass quantity. If this hormone could be isolated,and given to a plant, there may be a way to increase growth without cutting off leaves.
Assuming there is a hormone that is produced when new leaves are emerging, I experimented with cutting the leaves from offsets only when the new leaves on the main stem were being produced. When I saw the first signs of new leaves on a Zamia portoricensis being produced, I cut the leaves on all the offsets. As soon as new leaves were being produced on these offsets, I would continue to cut them off again. By the time the leaves were hardened off on the main stem, all the offsets had produced four sets of leaves. I feel that this hormone is at it's peak during a leaf flush.Since this experiment worked so well, I continued to cut the leaves off the offsets of this plant every time the main plant naturally produced leaves. After a year, the offsets had produced up to 16 flushes of leaves. It has been 3/4 of a year since I ended this experiment, and the subject plant has grown to be much larger than the other plants from the same seed batch. There were two, almost indentical plants that each had two main stems and an offset. The plant on the right is the subject plant, and the plant on the left was grown naturally.
I have met many nursery owners that have made some amazing observations, but none of these observations end up in print. If nursery owners, with thousands of plants at their disposal, could coordinate their efforts with scientists and the equipment available to them, many of the questions that people have about cycads could be answered. With these questions answered, propagation and growth rates could be increased. If everyone interested in the welfare of cycads could work together, there would be no reason for any species to be endangered.