Seeding and Transplanting

Seeding 

Always purchase high quality seed from a reputable source that is tested for germination rates, uniform emergences and viability. Request the germination percentages for the seed lots that you purchase. Seeds may also be coated, pelleted, primed, or treated with fungicides. Organic growers need to obtain certified organic seeds whenever possible. Plan your seedling schedule for your transplant season and keep good records. Most seeds have optimum temperatures for germination. Seeds can be sown in rows in open flats or in plug trays. Some seeds, such as lettuce, need light for germination and should not be covered. Larger seeds can be covered with vermiculite. Seeds can be sown by hand or with mechanical seeders such as vacuum seeders or needle seeders. After sowing the seeds, gently mist with tempered, warm water. 

Germination chambers are available from greenhouse distributors and provide uniformity of temperature and moisture.  Seeds can also be grown on greenhouse benches with bottom heat. As soon as seedlings develop their first true leaves, they can be transplanted. 

Never store your seeds in the greenhouse because their quality can quickly deteriorate due to the high temperatures and relative humidity in the greenhouse. 

Handling Growing Media

How soilless growing media is handled can greatly influence the air space and available water for plant roots. The major goal is to preserve the air space or porosity to ensure healthy root growth.

Add water to peat-based mixes before filling plug trays to help create more aeration. Satisfactory filling moisture is achieved if the slightest bit of visible water appears when media is squeezed between the fingers. Most growers work with a moisture content of 45-55% by weight. If mixing your own media, thoroughly mix components, but do not over-mix, which will cause particle size to decrease. Over-processed media quickly loses porosity, resulting in stunted transplants.

To prevent compaction, which encourages damping-off diseases and poor root growth, lightly fill containers, including plug trays, and brush the excess media off the top. Once filled, avoid nesting or stacking trays on top of one another. Stacking containers causes compacted media with reduced air space. This damage cannot be remedied after creating this compaction. Always stagger trays.

When dibbling seed trays, try to avoid compressing the mix; gently press to ensure a small indentation for seeds.

Production Schedules

Starting seeds too soon will result in overgrown transplants of poor quality. An excessively warm greenhouse will cause rapid, spindly growth. The following are guidelines for growing vegetable transplants. Note the number of weeks from seed to transplant. This will vary according to different growing conditions and should serve only as a guide. It is important for the seedlings to reach the proper level of maturity. Seedlings started too late may not transplant well, due to having a limited root system. Overgrown seedlings may have pot-bound roots that do not transplant well. 

Table 18: Germination and Growth of Vegetable Transplants

Germination Tips for Selected Crops

Warm temperatures and uniform moisture are needed to ensure successful germination and get plants off to an even start. Many germination chamber systems are commercially available including custom-built germination units. It is important to remove flats from the germination chamber as soon as radicles break through the seed coat to prevent seedling stretching.

Growers often use bottom heat or root zone heating to provide warm, even temperatures. Rubber tubing or mats with hot water are placed on the bench top under the plants. A weed mat barrier is placed on the top of the bench to help spread the heat with skirts on the side to help contain the heat. Experience and experimentation with your total seeding system is the key to uniformity and success.

Celery seeds germinate best at 70°F with continuous light. To prevent bolting, maintain greenhouse temperatures above 55°F.

Brassicas. To prevent premature seeding or bolting, avoid temperatures below 50°F. The cold temperatures cause the development of premature heads, or "buttoning", in cauliflower and broccoli. Any stress or check in growth results in a "wirestem" and plants will not establish well in the field, resulting in reduced yields and performance.

Eggplant seed can be directly sowed into 50-cell trays to shorten the time needed to produce transplants by approximately 1 week. Eggplants are susceptible to chilling injury and should not be grown below 40°F. Any stress or check in growth will result in tough woody stems and transplants that will have a tough time getting started later in the field.

Tomatoes. Exposure of tomato plants to temperature below 60°F will likely result in rough fruit (catfacing) on the first few clusters. Transplant young seedlings into 2-3" containers when they have 2 true leaves and grow on until planted in the field. For earliest production, some growers finish their transplants into 6" or larger containers.

Peppers. Germination is very slow at lower temperatures. Seedlings develop well at 65-80°F day and 60-70°F night temperatures. Seeds may be directly sown into 72-cell trays for early production. Peppers are prone to damping-off diseases especially if the media is compacted. Jalapeño pepper varieties may require much more time in the greenhouse than bell peppers to achieve adequately size plants.

Cucurbits do not transplant well, and are best sown in the final container. After germination, excess plants can be thinned. Cucurbit transplants should be field set with a maximum of 2 true leaves and before plants get leggy when exposed to high daytime temperatures.

Containers for Transplants

Transplants can be grown in all types and sizes of containers. Seeds can be sown in open seed flats or in single-cell (plug) trays. While many growers may sow seedlings in open flats, tender young roots of young seedlings may be damaged as the seedlings are pulled apart during the transplanting process. Open flats may also tend to hold onto water, so the young seedlings are more susceptible to damping-off. If you have had a problem with young seedlings drying out too quickly, choose plug trays with larger and deeper cells. Do not hold transplants in plug trays too long, as the young seedlings will stretch and dry out too quickly. 

Before sowing, decide whether germination and finishing will occur in the same container or whether seeds will be sown in one container followed by transplanting to a finishing container.

Plug Trays or Flats 

Germinating and growing small plugs requires close attention to detail and is probably best done by local, specialty propagators. Trays for transplants vary in size from 32 cells to 500 cells. The number of plants in a tray depends on the cell size needed for each plant. Large cell sizes such as 32, 50, or 72 are often used for cucurbit crops and early harvests. Plants are less stressed in larger cells if it is necessary to hold plants for several days before transplanting in the field. Mid-size cell sizes such as 72 and 128 are suitable for tomatoes, peppers, eggplant, and brassica crops. Small cells such as 128, 200, or 288 may be used for lettuce or onions. Consider your available labor, amount of greenhouse space, and the cost and benefit of growing in plug trays or flats in order to make a decision on which size container to use. Plug seedlings should be transplanted as soon as possible after they have reached finished size.

Soil Blocks 

With this method, wet media is pressed into forms to create separate blocks placed into flats. As the young seedlings grow, their roots reach the edges of the block and are air pruned due to the separation between the blocks. Young plants may be less prone to transplant shock in the field when soil blocks are used. Specialized equipment is needed to make the blocks. The growing mix needs to contain enough peat, compost, and sand or perlite so the material binds together and stays moist. 

Biodegradable Containers

Some growers will produce their vegetable transplants for sale to home gardeners in biodegradable pots such as coconut husk fiber (coir) pots, fiber or jiffy peat pots, composted manure pots (Cowpots), or 80% wood fiber-20% peat (Fertil) pots. Cowpots and Fertil pots are OMRI-listed.

Care of Purchased Plugs. Purchase transplants from a reputable local supplier to minimize the potential of importing severe disease and insect problems that are common in other regions of the country. Open and unpack the boxes immediately upon arrival and check the physical condition of the plants. Inspect plants for root and foliar diseases and for insects and mites. Report any damage or discrepancies immediately to your supplier (most companies want to hear within 24 hours). Photographs are also helpful.

Place plant trays on benches and water thoroughly with plain water (no fertilizer); be sure that plugs on the edges of the trays are thoroughly watered. Plugs can dry out quickly due to the small volume of growing medium; check the trays 2-3 times daily for watering. After the initial watering, apply a general-purpose fertilizer (such as 20-10-20) at 50-60 ppm of nitrogen at every other watering. Allow plants to acclimate to the greenhouse conditions for 24-48 hours before transplanting to larger containers.

Transplanting to a Finishing Container. Water the plug trays thoroughly 2-3 hours before transplanting; this aids in removing the plugs from the trays. Prepare your cell packs or pots by filling them with pre-moistened growing medium and pre-dibbling holes for the plugs. Lightly fill containers and brush off excess. To prevent compaction, do not pack down or stack filled flats.

Take special care during transplanting to handle plants gently and avoid planting too deeply. Stems of tender seedlings can be easily injured when workers grasp or "pinch" the stems too tightly. This often leads to stem cankers that can cause plants to wilt and die. Transplant to the same depth as the original plug. Some transplants may have elongated stems and it is tempting to bury the stem. Resist the temptation, except for more adaptable tomato plants.

See information under the specific crops for additional information on transplant production and planting.