Plug transplants for processing tomatoes

Introduction 

Plug transplant establishment is one of the key factors in producing high yielding tomato crops. Rapid early growth and complete plant survival is influenced by the practices used by the greenhouse grower, and by the transplant management practices used by the field tomato grower.

How to Recognize a Good Quality Transplant

The first step in tomato stand establishment is obtaining high-quality transplants. Growers should look for seedlings with straight, thick stems, 12 to 17 cm tall with good uniformity. Leaves should be well developed: flat, not cupped or puckered, and green. A slight amount of purpling at the base of the stem and on the underside of leaves is a sign of carbohydrate development which will improve survival. Extensive purple colour on the top surface of leaves is a sign of phosphorus deficiency which will delay early growth. Roots should be white, thick, and should fill the plug from top to bottom. Roots which are brown in colour and don't extend to the bottom of the plug are a sign that the plants have been grown under moisture stress which can delay rooting in the field. A well-grown transplant will have adequate food reserves to ensure rapid establishment under a wide variety of field conditions.

Transplant Trays - Plug Size and Depth

Cell depth of 288 plugs has been a controversial topic. Deep cells have not shown an improvement in plant survival and yield over shallow cells in research trials. However, if deep cells are well managed, the plug will be heavier, and may be easier to plant than shallow cells.

Deep cell trays are usually less durable and may need to be replaced sooner.

The use of smaller plugs, such as 406 trays, would allow more production per unit of greenhouse space and thus reduce transplant cost. However, in several years of trials 406's had reduced plant stand and yield compared to 288's in early May plantings.

When compared under more favorable field conditions in late-season plantings, the stand and yield of 406's was as good as 288's (see Table 1).

If 406 trays are to be used successfully, greenhouse production practices will have to be modified to produce a plant of acceptable size.

Table 1. The Effect of Tray Size on Transplant Height, Plant Stand and Yields.

Trials conducted at the Harrow Research Station, 1991-93.  

Growing Media

There are various growing medias available for the production of tomato transplants in plug trays. The commercially available soilless mixtures generally contain peat moss, perlite, vermiculite, a nutrient charge and a wetting agent. More recently some other materials such as rock-wool have been used. These mixes vary in their moisture retention, drainage characteristics, fertilizer content and particle size.

Soilless mixtures are recommended since they are sterile and there are fewer disease problems They are light weight, easier to handle, convenient for storage and handling, fairly uniform from year to year and they are noncrusting. Good-quality transplants can be grown in a range of soilless mixtures.

Low levels of fertilizers containing minor elements are preferred but not essential, as nutrients can be applied with each watering if necessary. It is advisable to check the pH and salt concentration (EC) of the soilless mixture before seeding. The pH should be in the range from 5.0 to 6.5.

Salt levels vary depending on the amount of fertilizer in the mix. An acceptable EC range for seedlings is 1.0 to 2.0 MMHO/cm.

Drainage and water retention are important to maintain the soilless mixture at a uniform moisture level. Therefore select a mix with medium particle size to ensure proper aeration, drainage and ease of filling the trays.

It is important to know the nutrient concentrations in the growing media. Nutrient concentrations vary greatly between different types of media (see Table 2). It is important to adjust the greenhouse watering and fertilizer program according to the water-holding capacity and the nutrient charge of the soilless mixture.

Table 2. Nutrient Concentrations in Growing Media.

Media vary in nutrient concentration, so fertility and watering practices must be adjusted. 

Seeding and Germination

High-quality seed is required for plug transplant production in order to produce acceptable plant counts and uniform transplants. Tomato seeds should have uniform, rapid germination and good seedling vigor. The germination should be greater than 90%. Seed companies can provide information on the optimum temperatures to germinate specific lots of seed.

Many different types of commercial seeders are available for seeding plug trays. When selecting a seeder a grower should consider the ease of converting between different cell sizes, the flexibility of the seeder to handle raw or pelleted seed and seeding capacity in trays per hour.

Germination of seed is a critical step in plug transplant production. Seed should be covered with fine vermiculite to ensure that the seed does not dry out. After seeding trays, apply warm water (approx. 27-28°C). Seeds require aeration to germinate so it is important to avoid saturating the growing media with water.

Tomatoes should be germinated in a germination chamber. This is usually an insulated room in which temperature and relative humidity can be maintainedat a precise level. Air circulation is important to ensure uniform temperature and humidity throughout the chamber.

The time required for germination varies between cultivars and seed lots. Growers should check the trays regularly and as soon as the seedlings break through the surface, the trays should be moved out of the germination chamber. Never allow the seedlings to elongate by leaving them in the germination chamber too long. Trays should then be placed on racks in the greenhouse.

Trays should be positioned at least 20-25 cm off the ground and should be level and placed tightly together.

Greenhouse Environment

Good ventilation and temperature management in the greenhouse is a key to growing transplants. Tomato transplants can be grown successfully in a variety of different greenhouse structures, but when using small volume plugs, control of the greenhouse environment is more critical.

Low temperatures can control growth, but if taken to an extreme, can damage tomato seedlings. Chilling injury can occur when tomato transplants are exposed to temperatures above freezing but below 10°C for an extended period. Chilling causes stunting of growth and can have a long lasting effect on field establishment. Tomato plug growers should maintain a minimum temperature of 10°C, and use ventilation to maintain air movement in the greenhouse.

DIF Method is a method of managing greenhouse temperatures to control plant height. Seedling elongation is most rapid early in the morning, so if early morning temperature is reduced, elongation is also reduced. A negative DIF means that the day temperature is lower than the night time temperature, which results in reduced plant height.

The steps in using DIF to control tomato seedling height are:

• Open the vents at sunrise and cool the greenhouse to a minimum of 10°C. Do not allow temperatures to fall below 10°C because of the possibility of chilling injury.
• Maintain cool temperature for as long as possible during the day.
• In most cases, a morning temperature which is slightly lower than the night temperature (slight negative DIF) will produce good quality plants.

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Figure 1.Uniform, high-quality processing tomato transplants in 288 plug trays in a well-ventilated quonset greenhouse.

Greenhouse Nutrition and Watering

There are numerous factors affecting the growth of tomato plug transplants. Two factors which have a dramatic effect on growth are the watering and fertilization programs used in the greenhouse.

• Water Quality. Prior to the growing season it is advisable to have a detailed water analysis completed. The greenhouse fertilizer program should be adjusted accordingly or an alternative water supply should be obtained.
• Watering. The amount of water and frequency of watering will vary depending on cell type, growing media, greenhouse ventilation and weather conditions. It is important to water thoroughly, to moisten the entire plug, which will promote root growth to the bottom of the plug.

If the plug is not watered thoroughly, root growth will be confined to the top of the plug. Allow the plug to dry down before watering, but do not let the plant wilt severely, as this will damage roots. Plug transplants should be watered thoroughly in the morning, but should not be watered late in the afternoon. If the plants remain wet overnight, disease problems are increased. If an overhead watering boom is used, it is advisable to remove and rearrange the nozzles occasionally to avoid "streaking" which results from variations in output from different nozzles.

• Nitrogen. Nitrogen concentration in the greenhouse fertilizer program has a greater affect on the growth of transplants in the greenhouse than the other two major nutrients. Increasing the level of nitrogen results in taller transplants with thicker stem diameters and heavier plant weights. Applying too much nitrogen in the greenhouse results in soft, poor quality transplants. Generally, between 50 and 100 ppm nitrogen is adequate if the fertilizer is applied on a daily basis. When the transplants are fertilized once or twice per week, concentrations of 200 to 350 ppm have resulted in good-quality transplants.
• Phosphorus. Phosphorus has a limited affect on the growth of plug tomato transplants when compared to nitrogen. Increasing the phosphorus concentration from 0 to 10 ppm (0 to 25 ppm P205) results in a moderate increase in transplant height, stem diameter, and shoot fresh and dry weight. Generally, phosphorus concentrations above 10 ppm (25 ppm P205) do not result in a significant increase in plant height. If phosphorus is restricted to the point at which the plants show extreme phosphorus deficiency, field performance will be reduced.
• Potassium. Potassium has the least affect on the growth of plug tomato transplants of the three major nutrients. Increasing the potassium concentration from 0 to 200 ppm (0 to 200 ppm K2O) results in a small increase in tomato transplant height, stem diameter, shoot fresh weight and shoot dry weight. Equally or more important than the actual concentration of nutrients in the fertilizer program is the amount of fertilizer solution applied per tray and the frequency of fertilizer application in the greenhouse.
• Fertilizer Solution Volume. The volume of fertilizer solution applied has a dramatic affect on the growth of the tomato transplants. Increasing the volume of fertilizer solution applied per tray results in an increase in height, stem diameter and plant weight.

Applying too much fertilizer solution results in soft, tall, poor-quality transplants. Growers are advised to determine accurately how much fertilizer solution is being applied per tray. Generally, volumes greater than 500 mL/tray are excessive and will result in soft transplants if applied on a daily basis.

• Frequency of Feeding. Preliminary research results indicate that transplant quality can be improved by applying higher concentrations of fertilizer less frequently (referred to as pulse feeding). Pulse-fed plants tend to have thicker stem diameters and a heavier transplant weight while being approximately the same in height.

Summary

• Water thoroughly to ensure the entire plug is moistened.
• Do not let tomato plug plants wilt excessively.
• Begin fertilizing the seedlings shortly after they emerge.
• Apply 50 - 100 ppm of Nitrogen and a minimum of 5 -10 ppm of Phosphorous if feeding on a daily basis.
• Less frequent applications of higher fertilizer concentrations may improve transplant quality.
• Adjust fertilizer rates depending on the nutrient concentration in the growing media, environmental conditions and growth habit of the cultivar grown.

Disease Prevention

Very few fungicides are registered for use on processing tomato transplants in greenhouse. Therefore, the primary means of controlling disease is by sanitation and by managing the greenhouse environment to suppress disease development.

Sanitation

• Control weeds inside and outside the greenhouse which may harbour disease organisms.
• If plug trays are to be reused from one year to the next, they should be washed and sanitized using "D.C.D." disinfectant.
• Other greenhouse equipment which comes in contact with the transplants may also be sanitized with D.C.D. between seasons.

Damping-off Control

• Ventilation, to promote air movement around the plants is the best method of preventing most damping-off and foliar fungal diseases.
• If signs of damping-off diseases are noticed in the greenhouse, transplants can be treated with Captan or Ferbam fungicides (refer to OMAF Publication 371, Growing Greenhouse Vegetables, for rates and application information).

Watering

• Plugs should be watered thoroughly in the morning and "spot watered" in mid-afternoon if necessary. Do not water plug plants in the evening.
• Foliage should be dry overnight to reduce the development of foliar fungal disease.

Cultural Controls

• Maintain a slow, steady growth rate for the transplants. Plants which are severely held-back are more susceptible to many types of disease.

Chemical Controls

• If tomato plug transplants are held for longer than normal periods, they may require additional disease protection. In this situation apply DYRENE 50 WP at 0.04 g/tray. An alternative method of applying fungicides is with smoke fumigators.
• Exotherm Termil contains the fungicide chlorothalonil, and is registered for the control of stem rots and leaf spots caused by Botrytis.
• Chemical controls for bacterial diseases may be fairly effective if applications are started at the transplant stage. Apply one of the copper bactericides at .04 g/tray if bacterial diseases are a suspected problem. Caution: Do not apply foliar fungicides under high temperature conditions (>30°C) as this may injure foliage. Use only sufficient water to wet the foliage, to prevent fungicide from dripping into the growing media and possibly damaging roots.

Finishing and Hardening Transplants

Hardening-off transplants is important, especially if they are to be planted under stressful, early season conditions. Tomato transplants may be hardened-off by reducing temperature in the greenhouse through ventilation. Reduced watering will also provide some hardening effect. Do not let plants wilt excessively. Do not harden-off transplants by reducing fertilizer application, as this often results in stunted plants which do not establish well in the field.

In some cases, it is difficult to harden plants in a greenhouse enough to withstand early season planting conditions. Research has shown that if plug plants are slightly soft, an additional period of conditioning can improve field performance.

Pre-Plant Conditioning

The pre-plant conditioning period involves holding the plants outside for 5 to 7 days prior to planting. This process allows the plant to become acclimated to outside conditions while still in the tray. Plants which are hardened-off in this manner often have improved field performance as compared to those planted directly from the greenhouse. This method of hardening transplants requires extra labour and close monitoring by the grower. Growers who are not prepared to put in this additional management should not attempt it.

Transporting and Handling Plugs

• Plug plants should only be transported in a properly designed trailer. The trailer should be tarped tightly while transporting plants. Use the trailer to transport the plants to the field, and protect them from drying winds.
• Ensure that the plants are thoroughly watered before leaving the greenhouse grower's yard to prevent drying out in transit.
• It is important to prevent the plugs from drying out. Check the plants regularly and water when the leaves start to wilt. Don't let the plant stem wilt down before watering.

Storage of Plug Transplants

If field planting is delayed by rain, plug transplants can be held for an extended period of time with very little reduction in plant stand or vigour. Research indicates that plug plants can be stored for up to two weeks, but the method of storage has a great influence on field performance (see Table 3).

If tomato transplants have to be held more than one day they should either be returned to the greenhouse, or stored outside in a protected area, and watered as required. Do not store plants in an enclosed trailer or barn for more than one or two days, as this will result in very soft, elongated plants.

If growers are storing plants outside the racks of plants should be placed in an area which receives direct sunlight but which is sheltered from the wind. The racks should be elevated, to prevent root growth through the bottom of the plugs. Use irrigation pipes, cement or wood blocks, or any other method to elevate the racks 4 to 6 inches off the ground.

If plug plants are stored outside and frost is forecast, the racks of plants should be moved inside a covered trailer or into a barn until the risk of frost is over.

Check the plants through the day, making sure they are watered regularly, never allow the plugs to dry out. The transplants should be watered thoroughly in the morning, then dry areas can be spot-watered in mid-afternoon. Do not water plants heavily late in the day as this can promote disease.

If planting is delayed longer than 5 to 7 days and they begin to show nutrient deficiency symptoms (yellowing or purple discoloration of leaves), they should be fertilized. Use a soluble fertilizer such as 20-8-20 or 20-10-20. This can be applied in water, using a 16:1 siphon mixer on a watering hose. Follow the manufacturer's directions for rates and mixing procedures.

Table 3. The Influence of Storage Methods on Plant Survival and Yield of Tomato.

Plug Transplants Held for 7 to 14 days.
Trials conducted at the Harrow Research Station, 1988-90.

Dipping, Planting Depth, Planter Water

Trays should be dipped in a shallow tank of water for a minimum of 2 minutes, to moisten the plug media before transplanting (See Figure 2). This will help the plant drop through the transplanter for better placement and will prevent it from drying out in the soil. Starter fertilizer should not be used in the dip tank as it may burn roots.

Transplanter water should always be used. Apply as much water as possible without causing the plug to float in the furrow. A pressurized watering system is recommended to increase volume and uniformity of transplant water. If this type of system is used, a fertilizer injector can be added to improve precision of starter fertilizer application (see Figure 3).

When planting, check to ensure the plants are being set properly and the planting furrow is closing completely. Transplants should be set deep enough that the plug media is in moisture and is below the zone of herbicide incorporation.

After planting, knock the empty plugs out of the tray while they are still moist, stack the trays and promptly return the trays and racks to the greenhouse grower. Don't allow the empty plug trays to come in contact with any herbicides or herbicide drift.

Starter Fertilizer

The use of starter fertilizer is a key factor in the field establishment of transplanted tomatoes. Starter fertilizer is recommended under the following conditions:

• soil temperatures less than 18°C
• soil phosphorus concentrations less than 15 ppm

Starter fertilizer may not always be beneficial, and may reduce plant survival under the following conditions:

• coarse, sandy soils
• high soil temperatures
• very high soil fertility levels (high salts levels)
• dry soil conditions

The determination of whether or not to use starter fertilizer must be made on a field by field, and day to day basis.

Wind Abatement

Early spring conditions are often not favourable for transplant establishment. Wind-whipping and sandblasting can reduce plant survival. Some type of wind abatement system (usually rye strips spaced every 1 to 3 tomato beds) is recommended, especially on fields which are prone to wind erosion.

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Figure 2.Dipping tomato transplants to moisten the plug media prior to field transplanting.

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Figure 3.Injection system on a tomato transplanter for uniform application of starter fertilizer in planting water.

The assistance of Dr. R. Pitblado, University of Guelph, Ridgetown College in preparing the Disease Prevention section, is gratefully acknowledged.

Related Links

• Publication 835, Crop Protection Guide for Greenhouse Vegetables
• Publication 836, Growing Greenhouse Vegetables in Ontario