Culture of Tobacco Seedlings in Todd Cells

Agdex#:	181
Publication Date:	04/82
Order#:	82-038
Last Reviewed:	04/82
History:
Written by:	E. K. Walker - Agricultural Canada Research Station; L. B. Reynolds - Agricultural Canada Research Station; D. A. Stier - Delhi Engineering Research Group

Table of Contents

1. The Todd System
2. Space Requirements
3. Time of Seeding and Heat Requirements
4. Sterilization and Disease Conrol
5. Media
6. Loading of Trays, Seeding and Seed
7. Watering
8. Fertilization
9. Advantages of Todd Cells
10. Disadvantages of Todd Cells
11. 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.

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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.

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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.

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Sterilization and Disease Conrol

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.

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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.

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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%.

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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.

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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.

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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.

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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.

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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.

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