Plug Transplants for Processing Tomatoes: Production, Handling and Stand Establishment
Table of Contents
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.
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.
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.
Media vary in nutrient concentration, so fertility and watering practices must be adjusted.
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.
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:
Figure 1. Uniform, high-quality processing tomato transplants in 288 plug trays in a well-ventilated quonset greenhouse.
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.
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.
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.
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.
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.
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.
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.
Transplants Held for 7 to 14 days.
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.
The use of starter fertilizer is a key factor in the field establishment of transplanted tomatoes. Starter fertilizer is recommended under the following conditions:
Starter fertilizer may not always be beneficial, and may reduce plant survival under the following conditions:
The determination of whether or not to use starter fertilizer must be made on a field by field, and day to day basis.
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.
Figure 2. Dipping tomato transplants to moisten the plug media prior to field transplanting.
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.
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