NC State Extension Publications

Sanitation Practices

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  • Hand remove all weeds prior to seeding.
  • Remove any debris that may house insects.
  • Do not apply herbicide in greenhouse!


  • Thoroughly remove all soilless media and plant debris.
  • Steam at 176°F for 30 minutes.


  • Thoroughly rinse with 50% household bleach solution prior to each clipping and when transitioning between greenhouses.

Source Water Solution Analysis

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  • Analyze water to monitor pH, bicarbonates, and trace elements. Sample with a well-rinsed 16-oz drink bottle. Run water several minutes before sampling. Sampling can indicate when to reduce excessive bicarbonate concentration (Figure 1).
Uneven growth in seedlings from excessive bicarbonates

Figure 1. Excessive bicarbonate.

Germination Temperature

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  • Optimum daytime: 86°F
  • Optimum nighttime: 68°F
  • Fluctuate temperatures to break seed dormancy.
  • Reduce nighttime temperature to 55°-60°F after stand establishment (14-20 days after seeding).
  • Seed trays when five-day forecast predicts bright, sunny days.
  • Cold-injured seedlings (Figure 2) will recover with addition of warm air.
Photo showing the effects of cold injury on seedling leaves

Figure 2. Cold injury.

Fertilizer Management

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  • Use 2-1-2 or 3-1-3 ratio fertilizers (for example, 20-10-20 and 16-5-16).

Nitrogen (N)

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  • Add 100-150 ppm 7 to 10 days AFTER seeding. This will reduce soluble salts injury to seedlings and can reduce spiral roots (Figure 3A and Figure 3B).
  • Add an additional 100-125 ppm four weeks after seeding.
  • Maintain 125 ppm N if using an injector.
Photo of stunted seedlings due to spiral root and salt injury

Figure 3A. Spiral root and salt injury.

Photo of stunted seedlings due to spiral root and salt injury

Figure 3B. Spiral root and salt injury.

Phosphorus (P) and Potassium (K)

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  • 2-1-2 or 3-1-3 ratio fertilizers provide sufficient P and K when targeted N rate is achieved.
  • Excessive P can cause leggy or spindly transplants.
  • P deficiency (Figure 4) is rare when recommended fertilizers are used.
Photo of discolored leaves due to phosphorus deficiency

Figure 4. P deficiency.

Calcium (Ca)

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  • Sufficient Ca should be included in commercial media; if not, add 5 oz gypsum/100 gal of float water prior to seeding.
  • For deficiency (Figure 5), apply 3.51 oz of calcium nitrate/100 gal of water overhead or to float water. Either will provide 50 ppm Ca and 40 ppm N.
Photo of misshapen leaves due to calcium deficiency

Figure 5. Ca deficiency.

Magnesium (Mg) and Sulfur (S)

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  • Use standard fertilizers to supply sufficient Mg and S. For deficiency, add Epsom salts at a rate of 4 oz/100 gal of float water.

Boron (B)

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  • Use fertilizer with 0.02% B to ensure trace amount of B. Float bed concentration should be 1-2 ppm; less than 0.5 ppm is considered low. To correct, add no more than 0.2 oz Borax/100 gal of float water (this will supply 1.5 ppm B). Collect a water sample prior to seeding to determine B levels. Remember B is toxic to plants when concentration exceeds 2 ppm (Figure 6)! Add total source water B content to fertilizer B to determine total B concentration in float bed. Collect a diagnostic float water sample after fertilizer addition to analyze B concentration.
Photo showing the effects of boron toxicity

Figure 6. B toxicity.

Calculation for Gallons of Water in a Float Bed

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  • Length (ft) × width (ft) × depth (ft) × 7.48 gal/cubic foot = gal of water/bed.

Fertilizer Calculation

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  • Desired ppm of nutrient ÷ (% concentration in fertilizer × 0.75) = oz of fertilizer/100 gal.

Disease Control

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  • Sanitize and ventilate to control disease.
  • Dispose of old and diseased trays. Steam-sanitize trays (176°F for 30 min) between each use to prevent Pythium root rot and Rhizoctonia diseases.
  • Pythium Root Rot: Terramaster 4EC
    • Preventative rate = 1.0 fl. oz/100 gal of float water.
    • Curative rate = 1.4 fl. oz/100 gal of float water.
    • Mix Terramaster well throughout float bed. Root pruning is expected. Apply 3-8 weeks after seeding. Maximum use of Terramaster is 3.8 fl. oz/season.
  • Target Spot (Figure 7): Quadris F
    • 0.14 fl. oz/1,000 square feet. Use at least 5 gal of water/1,000 square feet. Coverage is critical! Apply only once prior to transplanting.
  • Black Root Rot/Tobacco Mosaic Virus/Collar Rot (Figure 8 and Figure 9):
    • No chemical control exists; discard infected trays and seedlings.
Photo showing brown discoloration on a seedling leaf

Figure 7. Target spot.

Photo showing lightened coloring of leaves due to black root rot

Figure 8. Black root rot.

Photo showing white fungus growing on seedling from collar rot

Figure 9. Collar rot.

Seedling Growth Management

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  • Clip properly to ensure plant uniformity, hardiness, and maximum usable seedlings.
  • Clip when plants are 2-2.5 inches above tray or 1.5 inches above bud.
  • Clipping 5 times improves viability by increasing stem diameter and reducing stem elongation. After fifth clipping, seedlings are only being “held” until transplanting.
  • Discard plant clippings at least 100 yards from greenhouse for sanitation.

Insect Control

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For aphid, flea beetle, or caterpillar management in greenhouse:

  • Orthene = 0.375 oz in 3 gal of water/1,000 square feet during seedling production.
  • Bt materials are another caterpillar control option.

For aphid, flea beetle, or thrips management in greenhouse within 5 days of transplanting or for field protection:

  • Imidacloprid (Admire Pro and others) = 0.6-0.8 fl. oz/1,000 plants (check formulation) for field protection.
  • Thiamethoxam (Platinum SC and others) = 0.8 fl. oz/1,000 plants (check formulation) for field protection.
  • Apply Admire Pro and Platinum overtop of transplants and rinse immediately to ensure media wash-in.
  • These materials can also provide control of soil insects such as wireworms at higher rates. Refer to the North Carolina Agricultural Chemicals Manual for soil insect control recommendations.

Estimated EC Meter Reading Examples

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100 ppm N

150 ppm N

Ultrasol 16-5-16

0.80 mS/cm

1.20 mS/cm

Ultrasol 20-10-20

0.65 mS/cm

0.98 mS/cm

* Correcting bicarbonates with acid will elevate EC reading.
* Many greenhouse fertilizer labels contain estimated EC readings.


Assistant Professor & Tobacco Extension Specialist
Crop & Soil Sciences
Research Assistant
Crop & Soil Sciences
Extension Agent, Agriculture - Field Crops
Professor and Extension Specialist (Berry, Tobacco and Specialty Crops)
Entomology & Plant Pathology
Assistant Professor & Extension Specialist
Entomology & Plant Pathology

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Publication date: Feb. 26, 2020

Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by NC State University or N.C. A&T State University nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your local N.C. Cooperative Extension county center.

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