The University of Georgia College of Agricultural & Environmental Sciences
Cooperative Extension Service

Transplant Production in Greenhouses

W. David Smith and Michael D. Boyette, North Carolina State University
J. Michael Moore and Paul E. Sumner, University of Georgia

The greenhouse method produces excellent-quality transplants with uniform stem lengths in a very predictable time period. Successful transplant production in a greenhouse requires few people, but it does demand intensive management with great attention to details. Little problems can become big problems very quickly in the greenhouse. Transplant production in greenhouses is less dependent on the weather than with plant beds. However, the weather does affect production in the greenhouse. For example, in 1998, cool cloudy conditions delayed germination and contributed to spiral roots, particularly with non primed NC 71. In 1997, unseasonably warm temperatures in February and March increased the rate of plant growth and caused plants to reach transplant size earlier than normal. Then, cold conditions in mid-April caused a delay in transplanting. As a result, transplants remained in greenhouses for an extended time, which resulted in numerous problems with Pythium root rot and collar rot.

Greenhouses are also very susceptible to structural damage from wind and snow. Snow collapsed numerous greenhouses last spring and the two hurricanes last summer and fall caused extensive damage in North Carolina.

WATER ANALYSIS AND SOURCES

Water quality is an important component of successful tobacco transplant production in greenhouses. Production problems with seedlings have been observed with excessive bicarbonate in float water in several counties, primarily in Eastern North Carolina. Excessive boron levels have occasionally been found in water samples from the east, and boron deficiencies have been observed on seedlings in float systems from the piedmont. Sodium is occasionally high, which can cause salt problems for overhead-irrigated systems, although recent research has shown tobacco seedlings in float systems to be very tolerant to sodium. Surface water should be avoided because of potential disease problems.

The three most important water quality parameters for most growers are: pH, soluble salts (conductivity), and alkalinity or total carbonates. A low pH (5.0 to 6.0) indicates an acidic condition and a fertilizer that will raise the pH should be used. A higher pH (greater than 7.5) will generally indicate high alkalinity levels (high carbonates). Acidifying fertilizers that lower the pH and neutralize alkalinity should be used. Remedies fo haigh alkalinty will depend on the severity. Alkalinity levels of 100-200 ppm would correspond to total carbonate levels of 2-4 meq and would not be expected to cause substantial growth problems. Corrective action in this case would only involve the use of an aciifying fertilizer such as the Perters Excel 15-5-15. Alkalinity levels of 200+ and carbonate levels of 4+ call for corrective action such as the addition of calculated amounts of acid.

Analysis: The University of Georgia Agricultural Services Laboratories provides water analysis for pH and Basic Cations (P, K, Ca, Mg, Mn, Fe, Al, B, Cu, Zn, Na, Cr, Cd, Ni, and Mo) at a cost of $10.00 per sample (Soil, Plant and Water Laboratory, 2400 College Station Road, Athens, GA 30602, Phone: (706) 542-5350. Analysis for Alkalinity (the total bicarbonate content) of the water is performed for $12.00 per sample (Feed and Environmental Water Laboratory, 2300 College Station Road, Athens, GA 30602, Phone: (706) 542-7690. Results for alkalinity are reported in parts per million (ppm ).

A 16-ounce sample should be collected from each potential water source and for each analysis requested. A clean, nonreturnable drink bottle with a screw-on cap is an excellent sample bottle. Rinse the bottle (but do not use soap) several times before collecting the sample, and allow the water to run several minutes before collecting the sample. Fill the bottle completely so that no air remains. Forms and assistance are available from county Extension offices. Sample reports should be requested to be sent to the local County Extension Agent and to the Extension Agronomist - Tobacco. Recommendations related to the nutritional suitability of the water for transplant production will be discussed and forwarded by the local agent.

Wells are the most desirable water sources. Municipal sources that have been treated and filtered also are satisfactory. Pond or river water usually is suitable nutritionally. However, black shank has been observed on seedlings in float-systems filled with pond water in Kentucky. The potential for water contamination with soil-borne pathogens also exists for tobacco in North Carolina. Herbicides that injure tobacco also could be carried in soil runoff into ponds. Therefore, most (if not all), surface water sources should be avoided.

FLOAT SYSTEM MANAGEMENT

Selection of the Growing Medium: In general, tobacco media are peat-based with various combinations of vermiculite and perlite. Particle size distribution and nutrient charge are important factors in the suitability of a medium for tobacco transplant production. Particle size in a soilless medium is similar to the texture of a soil; and is determined by the relative amounts and size of the components ( peat, vermiculite, and perlite in traditional tobacco media) in the mix. The particle size distribution of a medium determines many characteristics that are important in plant growth such as: aeration, water holding capacity, drainage, and capillarity (wicking). Research has shown that a wide range of particle sizes are suitable. Very coarse-textured media such as those with 50percent or more perlite, promote dry cells and those with 1000 percent peat are less satisfactory than media that contain perlite and/or vermiculite. Experience has also shown that bark-based media are not as good as peat-based media. Within the range of particle sizes in media that have been used in tobacco production, quality control factors such as moisture content, uniformity, fertilizer charge, and the presence of sticks, stems, clods, and weed seeds are the most important factors in medium selection.

Management and monitoring of fertilizer salts in the growing medium Research conducted in 1994 and 1995 showed that successful transplant production can be obtained with noncharged (lime and gypsum only, no additional fertilizer added) media when all of the nutrients were applied through the waterbed. This research also showed that fertilizer salts accumulated in the medium from the upper half-inch of the tray. High temperatures, low humidity, and excessive air movement promote water evaporation from the surface of the growing medium, which results in the accumulation of fertilizer salts in the upper portion of the cell. Salts can accumulate to levels high enough to injure seedlings, even when recommended fertilization programs are followed. Fertilizer salts in the upper half-inch are directly related to the total amount of fertilizer (fertilizer in the waterbed plus that in the medium) applied. Therefore, media with no fertilizer or with only a minimal amount are preferable to highly charged media.

Electrical conductivity is a commonly used indicator of fertilizer salts levels in media and water. Pocket-sized conductivity meters are available for a reasonable price from many farm supply dealerships. When properly calibrated, these meters are very helpful in a salt-monitoring program for float water and growing media.

Salts should be monitored in the growing medium on a 24 to 48 hour basis from seedling emergence until the plant roots grow into the waterbed. Collect a sample of the medium from the upper ½ inch of the cell from several trays, then add twice as much distilled water as growing medium on a volume basis ( a 2:1 water-to- growing-medium dilution). Shake or stir the sample and wait two to three minutes before measuring the conductivity. Normal levels will range from 500 to 1000 microseimens (0.5 to 1 millimhos). Readings of 1000 to 1500 microseimens (1 to 1.5 millimhos) are moderately high and readings above 1500 microseimens are very high. Water should be applied from overhead to leach and dilute salts when: 1) Conductivity readings are above 1000 microseimens and plants show a pale color or have stopped growing; or 2) Conductivity readings are 1500 microseimens or above.

Tray Filling and Seeding: Uniform emergence and growth are necessary to provide a high percentage of usable transplants. Begin seeding 50 to 55 days before the anticipated transplanting date using only high-quality, pelleted seeds. Care should be taken during seeding to ensure that one seed is placed in each cell. Misting trays from overtop after floating has not been shown to speed the rate of seedling emergence. However, the use of a premoistened medium decreases the amount of medium that falls through the holes in the bottom of the tray and increases the speed of emergence as compared to a dry medium. Over-wet media do not flow from the hopper box as uniformly as dry media. Check trays carefully to see that they are filled uniformly.

Dry cells are a common problem in float systems, particularly with new trays because they float higher than old trays and because it is difficult to keep the medium from falling through the hole in the bottom of the tray. Since these new trays float high, it is very important to pack the medium all the way to the bottom of the cell. Dry cells can be reduced by: (1) using a moist medium; (2) uniformly filling the trays; (3) screening media that have excessive sticks and clods before filling trays, and; (4) wetting new trays prior to filling.

Spiral roots (aerial roots) are normally a minor problem in flue-cured transplant production. An incidence of 2-3% spiral rooting is normal. However, in 1998 the incidence was as much as 20 percent per tray in many growers' greenhouses and in seed studies conducted by Extension personnel (Table 1). Past research with burley tobacco at the University of Kentucky has shown a strong relationship between the increased incidence of spiral roots and low oxygen levels in the medium. Excessive compaction reduces the size of pore spaces in the medium. Water then displaces the oxygen in the smaller pores after the trays are floated.

Compaction and low oxygen levels are significant contributors to spiral roots. But, other factors also seem to be involved. For example, a greater incidence has been observed when the weather was cool and cloudy during germination in combination with some brands of media (such as Carolina Gold in 1998), certain varieties, and when the seeds of some varieties (such as K 326) where primed. At this point it is not known if these environmental and seed-related factors contribute to spiral roots by reducing seedling vigor, affecting the timing of germination in relation to changing oxygen levels in the medium, or if cool, cloudy conditions cause a wetter (and lower oxygen level) medium.

Although there are still many questions related to the cause of spiral roots, the risk can be reduced by: (1) Packing trays just tight enough for proper capillary water movement (wicking); and (2) when possible, avoiding cool, cloudy conditions during germination by seeding according to the five-day weather forecast.

Primed Seeds: Priming is a seed treatment process that promotes the beginning of the germination process in the laboratory. After the early stages of germination occur from exposure to warm temperature, darkness, water, and then light, the seeds are dried. The effect of this treatment is that all of the seeds are at the same stage of germination when purchased by the grower, and the emergence after seeding in the greenhouse is quick and uniform. Priming is particularly beneficial for varieties with a high light requirement for germination.

In 1997, the variety NC 71 was released with a very limited seed supply. The performance of these seeds was outstanding, with 95% emergence in five days a common observation. Growers were impressed with the performance of these seeds and because they were primed, there was high demand for primed seeds of other varieties such as K 326 and K 346 in 1998.

Germination tests by the seed producer of NC 71 in 1998 showed good emergence of nonprimed seeds, and the decision was made not to prime NC 71 seeds, except by special order. Priming is an additional expense to the seed company and primed seeds have a limited storage life. Therefore, priming was only done on a custom basis for most varieties from Gold Leaf Seed in 1998. Seeds from all Speight varieties were primed in 1998.

Some growers had poor stands in 1998. In some situations priming improved emergence, but in others, primed seeds seemed to contribute to uneven emergence and spiral roots. Nonprimed NC 71 often resulted in slow, uneven emergence over a 14-day period. Also, the seedling emergence from primed K 326 and K 346 seeds was slow and more uneven than from nonprimed seeds.

Preliminary research in 1998 with a limited number of varieties showed varying differences in response to priming. In these studies, seedling emergence was improved, decreased, or unaffected, depending on the variety. For example, priming increased the rate of emergence of NC 71, NC 72, and K 394. Later emergence was observed from primed seeds of K 326 and K 346. The emergence of K 730, K 149, and RG 17 was unaffected by priming.

In a separate study, the germination of primed seeds was earlier at 72^oF than at 62^oF degrees. Therefore, it appears that the use of primed seeds does not reduce the need for warm temperatures for rapid germination. Based on the limited experiences of 1998, priming is probably more of an advantage in low-light situations (cloudy days, and when seeds are covered by the growing medium) than in low-temperature situations.

Temperature: A temperature of 70 to 75^o F should be maintained at the tray level for the first seven to 10 days or until maximum emergence is obtained. After maximum seedling emergence, the temperature can be reduced to 55 to 60^oF degrees at night. Daytime temperatures of 80 to 85^o F are adequate. Heat injury (browning of leaves or seedling death) has been observed when air temperatures inside the structure exceed 100^o F.

Tray Selection: A wide variety of trays with a range of cell volume and number are available (Table 2). All trays produce suitable transplants. Most growers prefer the large size of transplants from the 200-cell tray but like the lower cost per acre associated the smaller transplants produced with the 338- or 392-cell trays. It takes fewer 338- or 392-cell trays and a smaller greenhouse per transplanted acre than those required for the 200-cell tray. The 242-, 253-, and 288-cell trays are a nice compromise. Research suggests that stem length is similar among transplants produced with the three trays and intermediate when compared to the stem length of transplants from the 200- and 338-cell trays. Research conducted by Dr. David Reed in Virginia showed no difference in field survival and yield due to cell size.

Fertilization: Growers with fertilizer injection systems have been successful with a constant application rate of 125 ppm nitrogen from 20-10-20, 16-4-16, 15-5-15 or similar ratio fertilizers. For noninjected systems, fertilizer can be added to the water in two steps. Research has shown that excellent transplants can be obtained from an initial application of fertilizer to supply 100 to 150 ppm nitrogen at or within seven days after seeding plus a second application of fertilizer to supply 100 ppm nitrogen four weeks later. A complete fertilizer, such as 20-10-20, or

similar ratios, should be used for the first application. The same fertilizer can be used for the second application. Ammonium nitrate is also acceptable. Higher application rates can cause tender, succulent seedlings that are more susceptible to diseases. Also, high application rates promote fertilizer salts injury to seedlings (see discussion under Selection of Growing Medium ). If high fertilizer salts levels are detected during the first four weeks after seeding (>1000 microseimens in the medium from the upper half-inch of the cell), water should be applied uniformly from over-top to reduce fertilizer salts levels.

Nitrogen form: Fertilizers commonly provide nitrogen from various combinations of nitrate, ammonium, and urea sources. Tobacco seedlings can use nitrogen in the nitrate and ammonium form, but urea must be converted to ammonium before the nitrogen is available for use by the plant. Research conducted in 1994 and 1996 showed reduced seedling growth when more than half of the total nitrogen in a fertilizer was provided from urea, as compared to growth obtained from a fertilizer with all of the nitrogen supplied as nitrate and ammonium (Table 3).

Similar results have been observed in Kentucky. Research in Kentucky by Dr. Bob Pierce suggests that observed reductions in plant growth may be a result of nitrite toxicity. Nitrite is an intermediate nitrogen form that occurs in the process of ammonium conversion to nitrate. Nitrite can accumulate to levels high enough to cause plant injury when high levels of ammonium are present.

Exclusive use of nitrate nitrogen has been observed to raise the pH of the medium, which causes plant-growth problems similar to those caused by bicarbonates. Therefore, it is important to study the fertilizer label carefully to determine the nitrogen form as well as the concentration of nitrogen and micronutrients. The best choice is a fertilizer that contains a balance of the nitrogen in the ammonium and nitrate forms.

Phosphorus: Research at Clemson University has shown the need to limit phosphorus concentrations to 35 to 50 ppm in the waterbed. Excessive phosphorus application causes spindly transplants, and more phosphorus is left remaining in the waterbed for disposal at the completion of transplant production. Therefore, 20-10-20 and 20-9-20 are better choices than a 20-20-20 fertilizer, which is no longer recommended because of excessive phosphorus application and because it often contains a high proportion of nitrogen as urea. Other fertilizers such as 15-5-15, 16-4-16, and 16-5-16 are also acceptable. However, over application of acidic fertilizers in low-alkalinity water can reduce the solution pH to less than 4.0 which damages roots (provided that plant roots are into the waterbed).

Fertilizer effects on waterbed pH. The pH of well water in North Carolina ranges from pH 4.5 to 8.5. Several fertilizers such as, 16-4-16, 15-5-15, and 21-5-20, are acidic and were designed for use in high pH (high bicarbonate water). However, in water sources without bicarbonates, or those with a low pH, acidic fertilizers can reduce the pH to levels low enough to damage roots.

Studies conducted in 1997 in water with a low pH (4.8) showed that the application of 16-4-16 and 15-5-15 at seeding reduced water pH, as expected, since they were developed for alkaline waters (Figure 1). The water pH rose to above pH 4.0 in two weeks and was even higher by four weeks. Root damage was not observed (Table 4). The second application at four weeks also reduced the pH but not enough to affect roots. In 1998, waterbed pH was reduced to below pH 4.0 at seeding and again at four weeks. Root growth was reduced with 16-4-16 and 15-5-15 (at the high rate) compared to 20-10-20 (Table 5). These data indicate that the use of acidic fertilizers such as 15-5-15 and 16-4-16 in low pH water can reduce the water pH to below 4.0 and result in temporary root damage, particularly when applied at higher than recommended rates. The drop in water pH is temporary, and root growth recovers when the pH rises to 4.0 or higher. Effects on stem length and plant weight have not been observed.

Plant size and weight were similar among 16-4-16, 15-5-15, 16-5-16, and 20-10-20 treatments when applied at 150 ppm N at seeding followed by 100 ppm N four weeks later (Table 4). However, the 50 ppm N treatment every two weeks (for a total of five applications) resulted in small seedlings. Therefore it appears that current recommendations of 125 ppm N with injectors and the 150 ppm N + 100 ppm N at four weeks are satisfactory.

Sulfur deficiency is occasionally observed in float systems if the medium was not supplemented with magnesium sulfate (epsom salts) or calcium sulfate (gypsum). The major media marketed for tobacco should contain sulfur. Also, some fertilizers such as 16-5-16 contain sulfur. If the sulfur content in a medium is questionable, the fertilizer used does not contain sulfur, or a sulfur deficiency is observed, epsom salts should be added to the waterbed at a rate of 4 ounces per 100 gallons of water.

Boron deficiency (bud distortion and death) has been observed in several float systems. In most cases, the water did not contain any boron, and the fertilizer also did not contain boron. The best solution to this situation is to choose a fertilizer such as a 20-10-20 with a guaranteed micronutrient charge if the water analysis indicates 0 boron. If a fertilizer with boron is unavailable, the addition of no more than 0.25 ounces of Borax per 150 gallons of float water is sufficient to prevent a deficiency.

Figure 1. Effect of fertilizer application on solution pH in low alkalinity water, Lenoir County 1997. Note: 4 wks/before = solution pH immediately prior to second fertilizer application to supply 100 ppm N. 4 wks after = solution pH immediately after second addition of fertilizer to supply 100 ppm N. Unfertilized water pH = 4.8

Clipping: Proper clipping is an important tool in increasing the number of usable transplants, transplant hardiness, stem-length uniformity, and stem diameter. A properly clipped plant is essential for carousel transplanters because uniform stem lengths are important for the machine to transplant seedlings at the proper depth, and excessive foliage disturbs the timing mechanism. Clipping can also be used to delay transplanting when field conditions are unfavorable. Past research has shown that maximum usability is obtained with three to five clippings. However, many growers clip 15 to 20 times. Excessive numbers of clippings indicate that the greenhouse was seeded too early. Early seeding increases heating costs and also increases the potential for collar rot.

Although proper clipping is a significant benefit, research has shown that improper clipping can adversely affect stem length, increase stem rots, and slow plant growth in the field.

Research conducted by Walter Gutierrez showed that collar rot infection increased in the presence of clipping residue on tobacco stems and leaves. Therefore, removing as much residue as possible is important in reducing the incidence of this disease. The use of high-suction rotary mower and the proper use and collection of residue with reel mowers is important. Also, removal of excessive foliage that results from waiting too long between clippings increases the amount of clipping residue that falls onto leaves and stems.

Research conducted by David Reed in Virginia showed that the severity of clipping affects stem length at the time of transplanting. For example, severe clipping (0.5 inch above the bud) decreased stem length but did not increase stem diameter as compared to normal clipping (1.5 inches above the bud). Therefore, there is no advantage in severe clipping. Severe clipping early in the season was particularly detrimental, resulting in very short transplants that grew slowly in the field. Additional work in North Carolina showed that severe clipping, down to the bud, immediately before transplanting reduced early season growth and delayed flowering.

Current recommendations are to begin clipping at three- to five-day intervals when total plant height is 2 to 2.5 inches above the tray and to set the blade height at 1 to 1.5 inches above the bud. This procedure provides the best balance of uniformity, stem length, and disease management.

Disease Control: Damping-off or target spot caused by Rhizoctonia, collar rot caused by Sclerotinia, and black leg caused by Erwinia are the most common diseases seen in float systems. Sanitation is extremely important in disease prevention. Adequate greenhouse ventilation and horizontal air flow are a must for successful production. Over-top applications of water or fertilizer should be avoided to keep leaf surfaces dry and to reduce the spread of pathogens by splashing from diseased to nondiseased trays. Nitrogen rates above 150 ppm promote rapid seedling growth, which results in succulent seedlings that are more susceptible to disease.

CALCULATION OF WATER VOLUME

The number of gallons of water in a foat bed may be calculated by:

length (ft.) X width (ft.) x depth (in.) X 7.48 gal./cu. ft.
12

Example:
50 ft. X 16 ft. X 4 in. x 7.48   = 1994 gal.
12

CALCULATING PARTS PER MILLION

Because nutrient recommendations in the float system are given on a concentration basis, it is necessary to calculate these concentrations as parts per million (ppm). While this is very different from the traditional pounds per acre or pounds per plantbed, it really is not very difficult. The following formula is a useful way to calculate the amount of fertilizer necessary for a given concentration in the waterbed.

Fertilizer added per 100 gal = Concentration

% x 0.75

Where: Fertilizer added per 100 gallons = amount of fertilizer to add to each

100 gallons of water in the waterbed;

Concentration = Desired concentration in parts per million; and

% = Concentration of the nutrient in the fertilizer.

An example: A grower wishes to obtain 100 parts per million nitrogen from 20-10-20. This product is 20 percent nitrogen. Therefore: 100 = 6.6 ounces of 20-10-20 per 100 gallons of water.

20 x 0.75

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