Culture of Tobacco Seedlings in
Todd Cells
Table of Contents
- The Todd System
- Space Requirements
- Time of Seeding and Heat Requirements
- Sterilization and Disease Conrol
- Media
- Loading of Trays, Seeding and Seed
- Watering
- Fertilization
- Advantages of Todd Cells
- Disadvantages of Todd Cells
- Comparative Costs of Seedling Production and of Transplanting
Flue-cured tobacco seedlings normally are grown in beds of muck within
glass-, fibreglass-, or polyethylene-covered greenhouses. In recent years
there has been increased interest in containers for seedling culture.
Most such systems have produced seedlings superior in many respects to
seedbed seedlings, but capital and labor costs have prohibited their use.
The Todd system is a container method less prohibitive with respect to
labor requirements than previously-examined container methods. Capital
costs are higher for Todd- than for seedbed- culture, but labor costs
are lower for the Todd system.
Although there has been considerable research and grower experience with
Todd culture, some management practices require further study and it is
suggested that no one should convert to the system without gaining experience
with a few trays. Management techniques differ from those used for seedbeds,
and success with Todd culture is even more dependent upon good management.
The Todd System
Numerous open-ended, tapered air spaces (cells) are provided within a
67 by 34 cm framework of styrofoam (tray). Various cell sizes are available,
but only those with an upper opening 2.5 cm square, a cell volume of 28
cc and a depth of 7.6 cm can be considered suitable for tobacco
culture. With this cell size there are 200 cells per tray. Todd trays
are re-usable for as many as 10 to 20 times.
The trays are filled with a peat-based medium containing large amounts
of vermiculite and perlite. Dolomitic limestone is added to lower acidity,
wetting agent is added to improve the wettability of peat, and small amounts
of major and secondary fertilizer nutrients are also included. Muck is
an unsatisfactory medium as it tends to stick to the styrofoam and remain
in the cells on pulling of plants, thereby causing loss of part of the
root system. Peat-filled trays also are much lighter and easier to handle.
Tobacco seed is too small to permit placement, without complex and expensive
equipment, of a single seed in each cell. Therefore, seed for Todd culture
is pelleted. The use of pellets permits seeding by manual or inexpensive
mechanical methods. After filling and seeding, a light application of
medium may be applied to cover the seed. The trays then are thoroughly
watered and supported on aluminum rails about 1m above grade in the greenhouse.
At this elevated position roots growing through the small hole at the
base of the cell are air-pruned.
Uniformity of water application is extremely important as there is no
lateral movement of water among cells. It is necessary, therefore, to
use travelling boom systems. Since cells at the exposed edges of trays
by the central walkway and the outer wall are subject to rapid drying,
provision should be
made, in the interest of uniformity of growth, to apply extra water at
these points by increasing size or density of nozzles.
Seedlings in Todd cells are fertilized with soluble fertilizers metered
from a siphon bucket into the watering system. The growth medium contains
enough fertilizer nutrients for 3 weeks, that is during germination and
early growth. Thereafter, amount and frequency of fertilizer addition
increases with plant size.
Fertilizer applications normally are discontinued just before transplanting
to curtail growth and help harden the plants. During this period there
may be some reduction in water application as well. Compared to seedbed
plants, however, those in Todd cells require little change in management
before transplanting.
Plants are pulled from trays in the field for transplanting with conventional
or with cup transplanters. The latter type has potential for reducing
labor requirements for transplanting, but testing has been insufficient
yet to evaluate its effectiveness and reliability compared to conventional
types.
Space Requirements
A standard greenhouse 30.5 m long with two beds each 3.66 m wide should
produce enough plants for 10 to 12 ha. The potential population of Todd
seedlings in this greenhouse, based upon 100% stand and use of all available
space, would be enough for 12 ha. In practice, stand always is less than
100% and some space must be left at the ends of the greenhouse for positioning
of the watering system after use. With an effective length of about 29
m there would be enough plants at 100% stand for 11.33 ha. If the germination
and survival percentage was only 85%, however, there would be enough plants
only for 9.5 ha. The stand with Todd culture must, therefore, be at -
or close to - 100% to obtain about the same number of plants obtainable
from seedbeds.
Time of Seeding and Heat Requirements
Todd trays can be filled and seeded ahead of the seedling production
period, then watered all at one time to initiate germination. Alternately,
trays can be watered at time of filling and seeding in order to stagger
start of germination over several days.
The germination period preferably should start 4 to 5 days earlier than
is usual for seedbeds. This is partly because dry seed is used in Todd
cells compared to partially-germinated seed in seedbed. Also, early growth
is slower in Todd cells because of poorer heat absorption by peat than
muck and more rapid cooling of trays than seedbeds at night. Utilization
of some supplementary heat during the first few weeks largely eleminates
differences in growth rate. Normally, germination should begin about April
1 with Todd cells in order to obtain transplantable seedlings by May 24.
Supplementary heat is necessary to prevent damage to young seedlings
from low temperatures. There may be little or no need for heat to prevent
damage in some years, but growth in all years will be improved by sufficient
heat to maintain 50C or more. It is suggested that use of heat be strictly
supplemental with no attempt being made to maintain ideal temperatures
for growth.
A standard household fumace of about 160,000 kilojoule capacity will
readily maintain 50C in a 280 m" greenhouse. Warm air is best distributed
to the far end of the greenhouse by means of a polyethylene tube located
under the trays. The inlet is at the furnace with no ducting necessary.
To assure that products of combustion do not contaminate the greenhouse
air and damage plants the fumace must be kept in good operating condition
and be equipped with a stack.
Sterilization and Disease Control
Commercial media normally do not need sterilization, but trays should
be sterilized. This can be done by dipping in a formalin solution (1 part
formaldehyde to 25 parts water), or by spraying trays, on rails in the
greenhouse, with formalin. In either case the trays must be well aerated
before use. When using formalin it is advisable to wear a mask. If home-made
mixes are used, ingredients such as commercial peat, perlite, vermiculite
and fertilizer ingredients require no sterilization, but muck or compost
should be steam-sterilized before use.
Standard control practices for diseases such as blue mold and damping
off in seedbeds suffice for Todd culture. Algae growth may often be a
problem in Todd cells. Satisfactory
control measures have not yet been developed, but the use of supplemental
heat during cool and cloudy weather discourages algae growth. Top-dressing
of trays with vermiculite, a very early start of germination, and too
much compression of media are practices that can lead to excessive algae.
In some cases the concentration may be such as to form a crust on the
media surface that prevents water entry into the cell.
Media
A good quality peat-based medium is essential and various commercial
mixes are available. Such mixes contain shredded peat, horticultural vermiculite,
perlite, limestone, superphosphate, nitrate fertilizer, minor elements,
a wetting agent, and, frequently, slow release fertilizers. Commercial
mixes have been tested for a number of years and found to be generally
acceptable. Compositions of these media are unknown, but the most desirable
media appear to have added compost
Figure 1.
Mechanical filler and seeder for Todd cell trays post.
Although it is most convenient to use a commercial mix, satisfactory
mixes can be made by proper mixing of various ingredients in correct proportions.
The following recipe, which provides about 5400 liters or enough to fill
900 trays, has been successfully used for several seasons.
2295 liters peat
1630 liters vermiculite (horticultural grade)
656 liters perlite
810 liters muck
13 kg finely-ground dolomitic limestone
5 kg 0-20-0 superphosphate
2.6 kg 15.5-0-0 calcium nitrate
0.33 kg trace element package
1 liter wetting agent - added to vermiculite
before mixing
The muck addition is insufficient to cause an appreciable gain in weight
of trays or problems in pulling plants such as occur with pure muck.
Loading of Trays, Seeding and Seed
Trays may be loaded by hand or from a hopper equipped with agitation
to maintain flow of medium. In hand loading, tamp the loaded tray to settle
the medium in the cells, re-load, and scrape off the excess. A seed pellet
then is placed in each cell. After seeding the tray can be placed, if
desired, in a shallow box with a hinged lid containing round-head bolts
(one per cell). On closing the lid the bolt heads provide slight compaction
and press the seed into closer contact with the medium. Excessive compaction
of the growth medium at this time should be avoided.
Loading of trays and placement of pellets by hand, particularly the latter,
is laborious. In fact, the amount of labor required for a greenhouse would
be prohibitive. Labor for seeding can be reduced with a double or triple
plate manual seeder. The double plate type is, in particular, relatively
inexpensive and easily made. With this seeder two people can fill and
seed 150 to 250 trays per day.
Figure 2.
Overhead, travelling, double boom watering system for Todd cell trays
Mechanized equipment developed to fill and seed trays in 1981 (Figure
1) first moved the tray on a metal link belt under a hopper of growth
medium, while another metal link belt at the bottom of the hopper moved
medium out of the hopper on to the tray below. The tray then passed a
metal wiper that levelled the medium. After some hand levelling the tray
continued to the seeder, which consisted of a wooden seed container and
a vacuum seed lifter. The seed container had holes (200) of 1.9 cm diameter
and depth and centres to match those of cells in a Todd tray. The seed
lifter was made of upper and lower plates separated by a 0.635 cm air
cavity. A fitting for a vacuum cleaner and a means of releasing vacuum
was fixed to the upper plate. The lower plate had 200 attached plastic
tips (disposable pipette tips) spaced to coincide with centres of the
seed holder cavities and of Todd cells. To seed a Todd tray, vacuum was
applied, the seed container was lifted to bring tips into contact with
the pellets briefly and then lowered, the lifter, with one pellet per
tip, was moved over a media-filled tray, and vacuum was released to deposit
seed. With this equipment, which was designed such that the unit could
be constructed in a farm workshop, about 60 trays could be filled and
seeded per hour.
Pelleted seed can be obtained from seed growers, dealers handling Todd
culture supplies, or directly from the pelleting company. In order to
obtain stands of 90% or more, seed for pelleting must be well-cleaned
and have a count of 12,400 or less seeds per g and a germination percentage
of no less than 90%.
Watering
An overhead, travelling boom system providing uniform application is
essential. Such a system preferably should apply a fine spray during the
first few weeks. After the stand is well-established, the nozzle size
can be increased to provide a coarser spray and reduce watering time.
A double boom system used in 1981 (Figure 2) gave very good results. With
this system 8002 nozzles were used for about three weeks then replaced
with 8003 and 8004 nozzles. More water was applied at the boom ends by
increasing nozzle density in order to compensate for the rapid drying
rate of exposed cells at these locations.
The exposed position of Todd trays results in more rapid drying of the
growth medium than is the case in seedbeds. In both methods the media
surface must be kept continually moist during germination and early growth
to prevent drying of seed or small seedlings. This is particularly important
with Todd cells in order to obtain as good a stand as possible and thereby
reduce labor of transplanting small seedlings into empty cells. Waterings
of Todd Trays during this period should be both light and frequent. On
sunny days 4 to 5 light waterings may be necessary. Later on, when seedlings
are well-established, waterings should be less frequent and sufficient
in amount that some water passes though the cells. Except for the initial
period, management practices for seedbeds and Todd cells are similar,
and the media surface should be relatively dry during the night-time hours
in each case to reduce the chance for disease occurrence. Nevertheless,
it usually is necessary, particularly on warm, sunny days, to water Todd
cells more often than seedbeds. A light watering would consist of 100
to 250 liters - and a regular watering of 500 to 800 liters - over 900
trays.
Fertilization
Contrary to seedbeds, where dry fertilizer worked into beds before transplanting
suffices to produce seedlings to transplant size, Todd seedlings, except
for the first few weeks, require frequent additions of soluble fertilizer.
Sufficient nutrients are present in the media to satisfy the small requirements
of very young seedlings. Use soluble fertilizer such as 10-15-20, 15-16-17
or 20-20-20 in sufficient amount that each plant receives 0.016 to 0.022
g nitrogen in about 15 applications during the greenhouses season. For
900 trays the total required amounts during the season for each fertilizer
would be as follows:
28.8 to 39.6 kg of 10-15-20
19.2 to 26.4 kg of 15-16-17
14.4 to 19.8 kg of 20-20-20
It is best to apply the fertilizer through the watering system using a
siphon bucket adjustable in rate of discharge into the water line. The
amount of fertilizer per application will be only a fraction of the above
amounts depending upon the number of applications during the season. If
fertilizer is applied at each watering the amount should be very low.
However, the above amounts, which are based on fertilizer needs with 15
applications, would rquire some increase in total amount if fertilizer
was applied at each watering. Preferably, apply the fertilizer in 15 applications
starting three weeks after seeding and as follows: 2 applications per
week for 2 weeks, 3 applications for 1 week, then 4 applications per week
for the final 2 weeks. The required amount of fertilizer at each application
would be 1/15 of the above total amounts.
The foregoing suggestions for fertilization of Todd cells are for a complete
greenhouse equipped with a traveling boom watering system and a means
of applying fertilizer with the water. For only a few trays and without
proper watering and fertilizer metering equipment it is necessary to apply
fertilizer by a hand-spray method. Examples of requirements per application
for 100 flats are as follows:
213 to 293 g of 10-15-20
142 to 195 g of 15-16-17
107 to 147 g of 20-20-20
These amounts should be mixed in sufScient water to give good coverage.
For 100 flats 40 liters of water is satisfactory. Instead of preparing
a separate spray for each application, the 600 liters required for the
entire season can be made up in containers ahead of time. The percentage
of the total nitrogen in soluble fertilizers that is in the nitrate form
does not appear to be a critical factor in Todd cell fertilization. The
10-15-20, 15-16-17 and 20-20-20 fertilizers used for this purpose have
70, 50 and 28% of the total nitrogen, respectively, in the nitrate form.
The amount of fertilizer applied may be more critical. Twice as much 10-15-20
as 20-20-20 must be applied to obtain an
equivalent amount of nitrogen, and slightly poorer results with 10-15-20
than with 15-16-17 and 20-20-20 have been associated with a somewhat higher
soluble salt content.
Advantages of Todd Cells
Each seedling has the same space and, if the watering system maintains
uniformity in water and fertilizer applications among cells, seedling
growth is very uniform. Seedling size also is more readily controlled
in Todd cells than in seedbeds simply by eliminating soluble fertilizer.
Like seedbed seedlings Todd seedlings are bareroot. There is, however,
an enormous difference in amount of root retained on pulling. Few roots
are lost from Todd seedlings and the seedlings are better able to withstand
the field environment and commence growth quickly. A large proportion
of the roots of seedbed plants are lost on pulling even if the beds have
been previously loosened by forking. Todd seedlings also have thicker
stems as each seedling has more space. If hardiness is measured by a plant's
ability to survive the field environment and grow quickly, then Todd seedlings
are much hardier than seedbed seedlings. The latter are more variable
in this respect depending upon greenhouse management. Survival rate always
is lower and days to flower longer --- for seedbed --- than for Todd-seedlings.
Initially slow establishment with seedbed seedlings delays days to flower
and may have a significant effect on maturity and quality as
well. Todd culture is a potentially important technique in a short Ontario
growing season that is characterized by difficulty in achieving acceptable
maturity and quality and by
the ever present danger of cold weather loss or damage to late-harvested
crops.
The elimination of pulling labor is an attractive aspect of Todd culture.
The method also is adaptable to further mechanization of transplanting,
although significant developments have yet to be made. The rapid establishment
of Todd seedlings in the field makes their use desirable as replants in
a crop obtained from seedbeds. Growers producing a few trays only should
consider such use and, thereby, reduce maturity differences between replants
and the main planting.
Disadvantages of Todd Cells
Many years are required, on the basis of labor savings, to recover the
capital investment required for Todd culture. In actual fact the reduction
in labor is less than elimination of pulling labor would indicate, as
hand labor is necessary to attain 100% stands and Todd cells require closer
attention than seedbeds for watering. If stands are above 90%, the grower
can readily do the replanting essential to reach 100% stand without using
extra labor as it is necessary during a period of minimal demand for labor.
In any event the grower must be present to look after the greenhouse.
A supply of plants for transplanting into empty cells should be provided
in small beds or trays. These plants should be relatively small when transplanted
(2 to 4 weeks after germination). Such small plants can be dug and transplanted
quickly into the peat-based media of the cells. Simply press the roots
or roots and adhering media slightly into the spongy media and keep moist
until the young plant is re-established. As is the case with seedbeds
extra plants should be grown to achieve 100% stands in the field and for
complete replanting if initial plantings are destroyed by frost or other
causes. Survival of Todd seedlings is such that relatively few
field replants are required, but, if the entire crop is in Todd cells,
growers should strive to produce 50% more than required for the initial
field planting of the crop.
Comparative Costs of Seedling Production and of
Transplanting
In considering such costs no allowance was made for potential benefits
of Todd culture relative to maturity and quality. These benefits are less
tangible than labor savings,but could very well represent an economic
advantage in most years. In this respect the following analysis of seedling
production and transplanting costs may understate the case for Todd culture.
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Costs of seedling production and transplanting were evolved for Todd cell
culture and for seedbed culture
without- and seedbed culture with- a mechanical plant puller in a standard-sized
greenhouse and for transplanting of 10.5 ha. In each case the costs were
grouped into capital, annual material and annual labor categories. The
purpose of the analysis was to determine differences in cost of production
and permit calculation of time required to recover the extra investment
in Todd culture. The capital investment for Todd culture was $4100 and
$1500 more than that for seedbed culture without and with a mechanical
puller, respectively. Annual material costs were $70 higher for Todd culture
than for both seedbed methods, and annual labor costs were $661 and $518
lower for Todd culture than for the
respective seedbed methods.
If a new greenhouse was required, the length of time required to recover
the extra investment in Todd culture was 6.9 years without - and 3.3 years
with - a mechanical puller for seedbed culture. Conversion of an existing
greenhouse to Todd cell culture required a pay-back period of 7.7 years.
