One of the factors that has revolutionized the
world of hostas in the past few decades is the introduction of
tissue culture (TC) for the propagation of hostas. There seems to be
a bit of mystery associated with TC but, in reality, it is just a
"high tech" method for making new plants from cuttings, a "low tech"
propagation technique that has been around for a very long time.
We have all taken a cutting from a geranium
plant and put it in moist potting soil or perlite to form another
new plant. Tissue culture involves taking a very small piece of a
plant and turning it into hundreds or even thousands of new copies
in a relatively short timeframe.
The key difference between TC and taking
regular cuttings is that this space age technique must be done under
totally sterile conditions. It is called an in vitro process
which means that it takes place in glass. This makes it very labor
and equipment intensive so it is a costly procedure. Therefore, TC
is generally used to multiply plants that are very difficult to
propagate with other more low-tech methods. Or, it may be used to
very rapidly reproduce high value plants where large numbers are in
demand...such as hostas.
Although some of the specifics will vary from
TC lab to TC lab, the general process will consist of four stages:
Initiation, Multiplication, Rooting and Growing On.
The first three stages are completed in vitro
under carefully managed, sterile conditions. During these steps, the
TC lab must be able to manipulate the temperature, light, chemical
and other environmental conditions to fairly precise levels.
The fourth step moves out of the test tube and
away from the laboratory. It consists of growing the plants in
regular greenhouse conditions the same as any other seedlings or
A. Initiation of the TC Process -
As the name implies, this first stage is where the whole TC process
gets started. In the case of hostas, a small bud is usually removed
from the crown of the plant i.e. the explant, to be propagated. This
bud is delicately "peeled" back to a very small point and is cleaned
and disinfected thoroughly.
Throughout the entire TC process, the constant
challenge is to eliminate any bacteria or fungi which might
contaminate things and ruining the crop. To achieve the proper
conditions, all cutting is done using sterilized equipment and
usually in front of a laminar flow hood to prevent recontamination
by fungal spores floating around in the air.
Prior to making the cutting, a mixture of plant
nutrients, plant hormones and a jelly-like material called agar is
prepared under sterile conditions and poured into the bottom of a
test tube. The exact composition of the agar mixture may vary from
TC lab to TC lab. The miniscule piece of the explant is carefully
placed in the bottom of the tube nestled on top of the agar mixture.
The test tubes are then placed on racks in a
sterile, growing chamber with artificial light and strict
temperature and humidity control. People entering this room will be
required to sterilize the soles of their shoes, use medical gloves
and, perhaps, wear a sterile gown to prevent any contamination.
There is nothing more frustrating for a TC lab than peering into a
glass tube or jar and finding grey mold or bacterial slime instead
of nice green plant tissue.
At this point, the plantlets have no roots so
they depend on the sugars in the agar for energy rather than
B. Multiplication of the Plant Tissue
- As the plant tissue in the test tubes grows and buds multiply, it
needs more room. So, periodically, it is taken out of the test tube,
cut into smaller pieces and distributed into even more tubes. This
may be done every 4 to 6 weeks and, again, requires strictly sterile
Each time the plantlets are divided and
replaced in the tube, represents the equivalent of about one year's
growth in the garden. That means that, if one bud on a division in
the garden would put on 4 buds during an entire summer, it would do
the same in the tube in about 4 weeks time. A good estimate is that
each bud will produce as many new buds in one year as it would in 10
years in the garden. So, you can see how these plants multiply so
rapidly under TC.
Generally speaking, two plant hormones are used
during TC. One is called a cytokinin which encourages bud formation
and the other is an auxin that encourages root growth. During the
multiplication stage, the level of cytokinin in the agar is
relatively higher than the auxin because the TC lab is encouraging
buds to multiply and is not yet interested in forming roots.
C. Rooting the New Plants - After
several rounds of multiplication have been completed, the plants
will be switched over to an agar mixture that has a higher level of
auxin relative to the cytokinin. That will trigger the formation of
roots on the small plantlets.
D. Growing the Plants in the Greenhouse
- Once the plantlets have formed adequate root masses, they will be
divided once more and each one will be planting in a container of
potting media. Then, they will be transferred to a standard
greenhouse for growing on to a saleable size at the retail or
wholesale nursery. This is called the ex vitro (not in glass)
phase of the process.
Why go to the trouble and expense of tissue
culture? Of course for many plants that multiply easily from
cuttings or come true from seeds, the cost of TC would be
prohibitive. So, it is generally used for the following reasons:
_ 1. Clone Production - Since the
resulting plants are actually exact copies of the original explant,
TC is a way to maintain the precise characteristics of the original
specimen. This is vital in reproducing named cultivars of hostas
where it is important that each one be exactly the same as the
_ 2. Difficult to Propagate Plants
- TC may be a last resort option for some plants that do not easily
propagate through standard cuttings, seeds, layering, grafting or
other methods. Of course, the resulting plants will need to be of
high value to justify the cost.
_ 3. Fast Production - Plants
reproduced through TC can be rapidly multiplied and quickly made
available for sale. This is especially useful for plants with high
demand which can still maintain a decent price level. From a small
piece of a plant, a good lab can turn out hundreds to thousands of
clones in a few short months.
Mr. PGC Comment: In the earlier days of
hostas, new cultivar introductions almost always carried a large
price tag when they were first introduced. This was because that
first plant was the only one of its kind in the world. If it were a
slow growing type, it might take 4 or 5 years to make 10 to 15
divisions. For a wonderful new plant, people would often be willing
to pay hundreds of dollars to have one of those early, rare
divisions in their garden. In this scenario, it would take many,
many years before the cultivar would be generally available at a
more affordable price for the gardening public. With TC, that same
plant would produce many new plants for sale within a year or two.
This larger supply usually results in a more reasonable price tag
_ 4. Pest Free Plants - Because it
is done under highly controlled, sterile conditions, TC may often be
used to produce certified, disease free plants. By using specific
techniques, virus infected plants may often be propagated with the
resulting clones being virus free. In addition, other problems such
as foliar nematodes may be eliminated during the TC process.
_ 5. Sports - Hostas tend to be a
plant that frequently mutates or "sports" on their own compared to
other plant genera. Because various levels of growth hormones are
routinely used in TC and since huge numbers of plants are produced
in a short period of time, this tendency is enhanced.
Mr. PGC Comment: In a population of say,
ten thousand plants of a particular hosta cultivar, there may occur
dozens of sports. Imagine how many gardens you would have to visit
to observe this phenomenon in the home landscape environment. With
TC, however, you might have ten thousand plants of the same cultivar
produced by the same lab in one year. During the quality control
inspections of the plants, you are bound to find...guess
what...dozens of sports. Of course, only a small number of these
variations will be worthy of naming but TC has become a major source
of new hosta cultivars in recent years.
So, why not use TC to propagate all hostas?
_ 1. Expense - TC requires a high
level of investment in equipment and highly trained staff.
Therefore, it is not economically viable for every nursery or for
cultivars with a limited demand. It is usually more cost effective
for small nurseries or growers to propagate a plant by some of the
older, low-tech methods which do a good job at a much lower cost.
However, in recent years, more independent TC
labs have come into existence who only propagate the plants and sell
small "plug" size specimens to wholesale and retail nurseries. As a
result, even many of the smaller retail outlets are able to afford
to use the benefits of tissue culture.
_ 2. Sports - While this can be an
advantage with certain species such as hostas, it can also be a
negative when too many, useless sports result and must be trashed.
This factor is another cost consideration since plants must be
inspected closely at all steps to assure quality and to be sure that
only "true to name" plants are sold to the consumer.
_ 3. Sanitation - The specter of
fungal infections ruining an entire environmentally controlled room
full of test tubes hangs heavily over the head of every TC lab.
Extensive efforts must be made to take care in every step of the
process to prevent contamination.
The big question that has circulated around the
hosta world since the beginning of TC is, "Are tissue cultured
plants as good as those divided from a plant growing in the ground?"
The answer is a resounding, YES! Just like taking a knife and
cutting off a part of a hosta crown to make a division, TC plants
are exactly the same as the mother plant.
I think some of the confusion comes from a
couple of factors. First, in their effort to make a quicker return
on their investment, some nurseries sell tiny, little TC plants in
two inch pots. Of course, it may take an extra year or two for these
to grow to the same size as a plant that originated from a single
division taken directly from a mother plant.
For example, if both plants came from an H.
'Sum and Substance' mother plant, they should both eventually grow
to the same mature size given similar growing conditions. It may
take the TC plant a little longer because it started as a smaller
plant. If the TC plant is grown to a good size at the retail nursery
before you purchase it, there should be no difference between it and
a similar size hand-made division.
Another point of confusion may come from the
fact that there are a lot of sports produced during TC. People seem
to think that since the TC lab has used plant hormones to manipulate
that process, there must be something "unnatural" about all of the
plants produced in this manner.
To avoid this confusion, remember a couple of facts. As mentioned
previously, hostas naturally produce a lot of sports. TC produces
many, many plants in a short period of time which just accelerates
this natural process. The "non-sports" should be perfectly fine and
just exactly like the mother plant cultivar. The second factor is
that the plant hormones i.e. cytokinins and auxins, used in TC are
the same as those naturally produced by plants anyway.