Environmental Engineering Reference
In-Depth Information
applied at this stage if no precipitation occurs. Adventitious
roots develop below ground from stem nodes or root initials.
Once the leaves are established, auxin present in the new
leaves and apical meristem is transported to roots to encour-
age root cell growth and elongation.
If the drainage of the site is poor and precipitation is high,
such that the potential exists for oxygen to be excluded from
the subsurface, then the drainage needs to be improved
before the plants are placed in the ground. To increase
drainage around the roots, a large, deep hole can be dug
and coarse-grained media can be added prior to soil backfill.
Other techniques that can be used as a conduit for oxygen to
reach the root zone include the installation near the roots of
pipe material that contains holes, or well-screen material,
such that air can enter the pipe and diffuse into the soils
around the roots (Ferro et al. 2001; Quinn et al. 2001; Tsao
2003). The rate of oxygen diffusion will depend on the soil
tortuosity and other factors, such as sediment, chemical, and
biological oxygen demands, which may limit the actual
amount of oxygen supplied to the roots. Also, these holes
may become clogged over time with roots. In order to
maintain air flow and prevent stagnation in the pipes, a
U-shaped pipe with an influent and effluent stovepipe
above ground can be used. At any rate, the installation of
such pipes may help to bring air in contact with the subsur-
face to support root respiration. There are a few liabilities
using such pipes with respect to contaminant fate, which are
discussed in Chap. 15.
Alternatively, a trencher can be used to create a long
trench where the plants can be added with backfill. This
method, however, greatly disturbs the soil, and can result
in increased permeability and, therefore, increased infiltra-
tion and recharge.
have extensive root and shoot biomass, assuming that the
plants have been removed from the nursery using the correct
methods. Dormant cuttings or whips can be planted anytime
from immediately after harvest to before bud break the
following spring. Fall planting is favored in the warmer
Southern United States where winters are milder than in
the Northeast. Bare-root plants should be planted right
after leaf drop if fall planting is to be performed.
Conversely, spring planting also has been successful in
some areas. Planting in the spring after the last frost, how-
ever, tends to induce shoot growth relative to root growth.
This may cause excessive wilting because the roots cannot
provide the top growth with sufficient water to meet increas-
ing transpiration demands. The negative effects of this con-
dition can be remedied, however, by pruning. For bare-root
plants that must be planted in the spring, they should be
installed before they break dormancy.
Regardless of planting in the fall or spring, an important
but often overlooked criterion is the soil temperature. Gen-
erally, planting can be accomplished successfully when soil
temperatures reach a stable 50
F. Warm air temperatures
may indicate planting, but if the soil temperature is too
low, seeds will not germinate even if water is available,
and root growth from cuttings will be slowed. It is advanta-
geous for tree growers to sell poplars from January through
July, a good time for such dormant cuttings to be installed in
most parts of the United States or northern hemisphere. The
soil temperature also is an important variable in determining
the type of plant for a particular area to achieve biomass at a
phytoremediation site. An evergreen tree adapted to cold air
and soil temperatures if planted in a more temperate area
will die of dehydration, because warmer air temperatures
result in increased transpiration while the cool soil
temperatures decrease water absorption by the root hairs.
Acquisition of plants also deserves attention. As any
florist knows, the length of time that a cut flower
remains sellable is determined by the conditions in which
it is stored. Cut flowers must be stored in cold, dark, and
humid storage containers and wrapped in plastic with very
little solid media, such as soil. The treatment of cuttings for
phytoremediation purposes follows the same practice.
Cuttings need to warm up slowly from being stored at cooler
temperatures. This can be done by removing them from
storage and placing them in a bucket of water in indirect
sunlight, as stated previously. Cuttings or bare-root cuttings
should be placed in bags to maintain high humidity levels or
in buckets of water until planted so they do not dry out. As
much as 1-2 weeks of warming time should precede planting
the cuttings. Soil moisture should be high enough to reduce
water stress in newly planted cuttings but not so high as to
induce anoxic conditions, which leads to death.
In some areas, the use of birches generally is not
recommended. They can be prone to disease caused by the
7.2.2.3 Time of Plant Installation
Timing of plant installation is as much an art as it is a
science. In short, plants can be added to soil at most sites
any time during the year; even in frozen soils. A random
planting approach without regard to the relation between
plants and season, temperature, light, and moisture will not
result in the most successful planting, however. On the other
hand, there is no single perfect time to plant in a particular
area. In many cases, the perfect time is based on individual
experience and preference or plant availability rather than
plant ecological considerations.
In general, for most areas and most types of plantings,
it is advised to plant in the fall, on a cool and overcast
day, because constraints on plant-available water are
decreased and below-ground soil temperatures resist sea-
sonal fluctuations in air temperature. Warm soil encourages
roots to continue growth, even while shoots are dormant.
Planting during the fall is best for larger plants that already
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