Environmental Engineering Reference
In-Depth Information
Phytoremediation sites can be planted with seedlings.
Seedlings in their first year are little more than a shoot
with some roots. They are often available free of charge
from local forestry departments, where they are grown for
reforestation purposes. Seedlings usually are established in a
funnel-shaped container of soil, and immediate planting is
not necessary as long as water and light are provided.
Seedlings typically are planted by using manual methods,
which include a dibble bar or shovel.
Hardwood cuttings supplied by most third parties typi-
cally are not available throughout the entire calendar year
and, therefore, their availability will control when a
phytoremediation planting occurs. Such cuttings typically
are made in the fall when the tree is dormant, usually after
the leaves have fallen and the nights are longer. These
cuttings are stored in coolers until the next season and
typically are available from January to July. They are
shipped coming out of dormancy and soon begin to accli-
mate to warmer air temperatures; this in combination with
rising ground temperature breaks the cuttings' induced dor-
mancy. Thus, the ease of plant acquisition often will deter-
mine the timing of planting. For example, planting in the fall
may require the purchase of older unsold stock that has
remained in a cooler for more than 6 m. Similar
circumstances can be corrected if plants have been placed
in specially designed grow bags while in storage.
Cuttings also can be made from trees that are on site or
have been planted previously. Cuttings acquired this way
can be used to replace those previously planted trees that
have been lost for a variety of reasons. The cuttings have to
be done correctly, however, to ensure survival of both the
donor tree and the cutting. Also, because these cuttings may
be from cuttings themselves, there is an increased risk of
potentially devastating effects if the plants are susceptible to
pests or diseases.
Cuttings of softwood, or the new, green, unsuberized
growth, are best when cut early in the morning when water
supplies are highest prior to transpiration. These stems are
actively growing. Cuttings should be no more than 12 in.
(30 cm) long, and placed in plastic bags in the dark to
prevent drying. Leaves should be removed in most cases,
or the largest one at least cut in half. Growth hormone can be
added to the cut.
Cuttings made from hardwood (seasoned old growth that
is dormant) can be taken after the leaves have fallen. These
can be planted as soon after they are collected and planted
leaving only the top bud above the ground. If some time will
elapse before the cuttings can be installed, the cuttings can
be placed into a plastic bag wrapped with a wet towel and
this placed in another plastic bag to decrease water loss. The
bundle can then be placed in a cooler or refrigerator. Prior to
planting, the cutting can be placed in a root hormone pow-
der, such as indolbutyric acid (IBA). Cuttings made during
dormancy should occur only after a period of colder
temperatures.
After a cutting is taken, the plant hormone auxin travels
from the cutting tip to the site of the cut, and begins to
stimulate root growth in the separated cutting. This transfer
of auxin must be an adaptive response to tree damage,
perhaps by ice or wind, such that these natural cuttings
would have a chance to grow on their own. At the site of
the cutting where auxin travels, root cells begin to grow and
7.2.2.2 Vegetative stock
Because of the time needed to establish plants from
seedlings, other techniques can be used, such as plant
cuttings. A cutting from a plant, be it leaf, stem, root, etc.,
can form an entirely new plant without having to go through
sexual reproduction, and is a form of asexual propagation.
Asexual reproduction, or vegetative reproduction, does not
involve the fusion of separate sex cells from separate plants.
Rather, it is the propagation of a new plant from a piece of
the existing adult plant. The produced plant is not the off-
spring of the parent but instead is a younger identical version
of the adult plant. For example,
Romulus, once in a trial of his strength, cast hither from the
Aventine Hill a spear, the shaft of which was made of
cornelwood [cornelian cherry]; the head of the spear sank
deep into the ground, and no one had the strength to pull it up,
though many tried, but the earth, which was fertile, cherished
the wooden shaft, and sent up shoots from it, and produced a
cornel trunk of good size.
Plutarch (1914; translation)
There are downsides to generational advances with
cuttings, because plants become more susceptible to disease,
as the genetic mixing and dilution of sexual reproduction
does not occur. As an alternative, the cloning of trees with
desirable traits can stop future genetic dilution and instead
capture the necessary genetic makeup to assist with environ-
mental restoration.
A cutting can be taken from different parts of the plant
during different times of the growing season. If a stem is
used to produce a cutting, it can be from the growing tip,
called a softwood cutting, or from the part of a stem that
contains bark, called a semi ripe to hardwood cutting. Soft
(herbaceous), firm (semi lignified), or hard (woody) is the
way that Garner (2003) described making cuttings. In most
cases cuttings made from the growing tips are best, as these
contain the highest concentrations of natural growth
hormones that will stimulate shoot and root growth, as
discussed in Chap. 3. However, internode cuttings can be
taken successfully even if many are made from the same
cutting. This is because even though the tip contains the
highest levels of auxin, the nodes contain higher levels
than the internodes.
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