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.
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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.
