Seedling production using cell trays
Note Number: AG0304
Published: July 1995
Updated: March 2009
Seedlings may be produced in seedling punnets, flats or cell trays.
Cell trays are used by commercial growers to produce seedlings for planting out. The seedlings are easily removed from the tray for transplanting and the growth check to transplants from cell trays is minimal when planted in the field compared to the use of other types of transplants. Seedlings from cell trays may be used in manual or automatic transplanters.
Soil-less mixes are usually used for commercial seedling production. Commercial sterilised soil-less mixtures are available with added fertiliser and this simplifies the production process.
Advantages and disadvantages of using transplants
Use of transplants has a number of advantages compared to direct seeding, although some vegetables are much more suited to growing as transplants than others. For example, beans, sweet corn, carrots and peas are amongst those that are not usually grown as transplants.
- Shorter growing season and more efficient use of land.
- Improved crop uniformity.
- More accurate prediction of harvest date.
- Facilitates the use of a wider range of herbicides.
- Extends the growing season.
- More efficient use of expensive hybrid seed.
- Transplant shock which delays growth but is not as severe on cell raised seedlings compared to bare rooted seedlings.
- High cost of seedlings which adds significantly to production cost.
- Extra labour to establish crop.
- Changes crop growth pattern.
The capital cost needed to begin growing seedlings as cell transplants makes it more expensive than in seedbeds. The use of cell-grown plants have many advantages over seed bed or seed tray plants; a fact demonstrated by their widespread use.
Some flower and vegetable growers have set up their own cell production units. Growers must give the same careful attention to planting and design of the seedling growing area, just as if it were a commercial nursery.
Commercial nurseries constantly assess their production process to ensure they are only marketing quality seedlings. They seek to produce uniform, healthy plants in quantities that will meet their customers' demands.
Good nursery hygiene is essential if disease free seedlings are to be produced for transplanting in the field. Planning is essential to maintain nursery hygiene and to ensure a high standard of plant health. A seedling nursery must be clearly separated from any other growing system or area and there must be strict control over entry to the nursery to limit the possibility of introducing pests and diseases.
In other words, a nursery must be treated as a quarantine area, with restrictions on entry and movement inside the boundary. This applies to people and to materials or equipment.
The design of the nursery should include defined areas of operation. Briefly these areas can be defined as:
- stores - general goods, chemicals and growing media. etc. as delivered.
- preparation - media mixing area including a section for sterilisation of media and trays; sterile material must be protected.'
- seeding house - includes a special seed store, seed treatment section, seeding and germination rooms.'
- pricking out - this is an extension of the germination room.,
- growing on - defined areas for different types and ages of plants.'
- order preparation - a direct but isolated extension of the growing areas.'
- sales/dispatch - the only area open to the public.,
- recycling holding area for reject, surplus material or returned trays for recycling and composting., Until sterilised recycled materials need to be kept separate and isolated from the nursery .
- waste disposal - isolated, secure, well separated from seedling houses and downwind prior to collection or treatment for disposal.
It is essential to design a flow-through system to assist with hygiene controls. For example, a steam sterilisation kiln should have an entry and an exit door to a storage-holding area. So there is a clear separation of functions.
The provision of concrete aprons, floors and pathways designed for rapid drainage is more than just a sign of a successful nursery; it is essential for a healthy business.
Glass or plastic greenhouses or frames give the best protection to seedlings. Supplementary heating may be needed to promote healthy growth of warm season plants. Inflation of the air gap between a twin-skinned plastic house will help to improve heat retention inside the house.
On hot and sunny days shading may be required along with proper ventilation and air circulation. Ceiling fans will assist in further air movement and in less condensation.
Wind breaks will help reduce damage from a strong wind within the nursery area and help reduce the entry of disease-laden dust. They should be sited to protect the houses from the prevailing winds
Shade houses are used to harden greenhouse raised plants to conditions outside. They offer some protection from the extremes of temperature and from winds.
An alternative method to harden plants is to place them in an open area surrounded by a wind break such as shade cloth.
Hygiene of growing mixes is vital and it is important to ensure that all growing mixtures have been sterilised, especially soil based media.
Soil has traditionally been accepted as part of seedling growing mixture. To most it would seem that the use of soil keeps seedling growing simple and straightforward.
This is not necessarily true. Consider these points:
- Quality standards will vary within and between loads of delivered soil. This applies to standards such as fineness or coarseness of the soil; the depth at which it was mined; and the amount of undesirable material coming with the soil such as diseases, weeds, trash and clay.
- Because the soil can vary, so can the desired standards of a growing mixture such as density, where balance is important for uniform strong plants. These differences may not be easily seen, but could be a reason for indifferent plant growth.
- Stock-piled soil can support unwanted plant growth (weeds) which can increase the amount of disease inoculum in the soil or the spread through a nursery.
- Soils may contain unknown plant toxins, extremes in soil pH variations, nutrient content or unknown chemical residues.
The alternatives to soil, either in part or in total are not without problems. The worth of these materials and how they can be used must be understood.
Some soil-less materials:
- Pine bark: Two key disadvantages with pine bark are the effect of its toxins on plant growth and in its hydrophobic (or difficult to wet) nature. There have also been problems at times with purity of sample, and in uniformity of particle size following milling. For seedling mixes bark should be milled to a maximum particle size of 5 mm.Most of the toxin content can be leached by ageing in open air stacks.' however the bark must be wet first. Newly stripped bark is harder to wet following drying than stockpiled, weathered bark.Pine bark can be composted with the addition of small amounts for slow-release fertilisers, to help in the process. This process can give some control over certain diseases which may contaminate the bark. In open stacks', the heat generated throughout the heap will not be even and therefore in the cool areas, the temperature will not rise to the thermal death point of many diseases.
- Hardwood sawdust: There are similar problems with sawdust to those of pine bark, i.e. some timbers do have quite high toxin levels. Treatment is the same.Sawdust composted in the recommended manner can be combined with pine bark and with peat moss. It is desirable to keep the percentage by volume of sawdust to around 40 percent or less.
- Soft wood sawdust: The most common softwood sawdust available is from Pinus radiata, which contains less toxins than some of the Australian hardwoods.
- Sand: The value of adding sand, an inert mineral, to any seedling mix is to obtain a balance between the other components and to give some bulk to the final product. A common belief is that sand helps with drainage' in fact the reverse can occur. For example, if added to peat moss it may fill in many of the narrow spaces between peat particles, thereby reducing loss of moisture. Also re-wetting of a dried-out mix is improved if there is a small amount of sand present. However overmixing of peat and sand in a rotary mixer can lead to pulverising of the peat, thus changing its structural qualities. On the other hand large amounts of sand added to a seedling mix is likely to produce rather dense mixtures. Likewise there needs to be a balance between the extremes of fine and coarse sands. Sand 2 mm or larger is classed as coarse material.
- Perlite: As an inert light material, perlite is used to reduce bulk density . It does not retain moisture or hold any plant nutrient. There is no real value of having perlite as part of the growing media.
- Vermiculite: Horticultural vermiculite has long been valued by the nurse industry. It has most value in short-term mixes, such as those needed for seedlings. As a material with a high base exchange capacity, it is able to reduce the loss of nutrients through leaching. It also contains small amounts of magnesium and potassium.
- Polystyrene pellets or beads: This material is used sometimes, if for no other reason than to add bulk- It has no nutrient value or water holding capacity. Polystyrene is difficult to handle outside when there is a wind. although the results are better if mixed dry with a little brown coal, before moisture is added.
- Brown coal: This material is known also as Lignum Peat or coal fines. The ratio of brown coal in a soil-less mix to other components needs to be low, about 20 percent by volume or even less. Higher volumes with adequate nutrition may cause plants to respond with lush growth and will be difficult to harden. Brown coal has a high water-holding capacity, with a high level of unavailable water. Its water loss is in fact high too, and it has been shown that plants transpire less water growing in brown coal, than is the case with peat moss.
- Peat moss: The use of peat moss for horticulture is increasing world wide. In Australia there are a number of peat deposits, but not many are suited for seedling mixtures. A common local peat used in Victoria comes from Tasmania, but because of its trash content (old plant material) its quality does not compare with many imported peats. The fine sedge-peats are generally not suited to seedling mixtures due to their dense nature. Most peats are relatively stable against rapid decay. They are valued for their high water-holding capacity; the sphagnum peats having a better balance between water and air, than most sedge peats. Even though peats have this capacity to hold water, their loss of moisture is high through transpiration and evaporation. Because of this it is claimed that plants growing under optimum conditions would grow faster in this material.
There is no such thing as one perfect mix for all types of plants. With so many mixes to choose from it can be confusing.
Commercial mixes: Most nurseries buy pre-mixed commercial mixes and bulk lots of a prepared mix can be obtained with or without fertiliser. The standard for ingredients should never change unless by agreement.
Bulk commercial mixes simplify the process and fertiliser can be included, eliminating the need for adding fertiliser, lime etc to obtain a balanced mix.
The most universal is a 50:50 peat/vermiculite mix. Some nurseries use it successfully as a standard mix for a range of plant species.
When changes are made from a standard mix, never change more than one ingredient for each test. For example, if you change the 50:50 peat/vermiculite to a 60:40, do not add another ingredient until this change has been tested. This also applies to added fertilisers. In this way change is controlled and results understood.
Plant nutrition for soil-less mixes: During the final mixing, lime, gypsum, fine grade superphosphate, a trace element mix and small amounts of nitrogen and potassium are added. A fine grade slow-release fertiliser may be used instead of adding N.P.K. separately.
Only enough nutrients should be added to the mix to obtain steady growth for the first 7 to 14 days. Further growth is maintained through foliar feeding with a soluble complete fertiliser every 7 to 10 days, until the seedlings are near maturity. Do not use fertiliser at higher than recommended rates.
Water plants with care because over-watering can easily leach nutrients out of soil-less mixes. Careful leaching of nutrients can be used as a growth control aid. Slowing seedling growth by this method is preferred to trimming the leaves of plants or with-holding irrigation before transplanting.
Media pH: Lime has been normally added to correct a pH that is strongly acid. In fact the greatest benefit from lime and gypsum is the calcium content. Calcium, like phosphorus and other nutrients', is easily leached from soil-less mixes.
Seed health and quality
Healthy plants are encouraged by a balanced nutrition program giving steady growth. Excessive wetness of the leaf may promote disease. Poor ventilation or air circulation will add further to the problem.
A healthy seed is a must. In some circumstances it may be advisable to treat the seed for diseases by using a seed coating treatment if this has not already been done by the seed supplier.
When using a seed dust always follow the instructions on the label and remove surplus dust from the seed and from the seed container or germination may be affected.
For some vegetables a hot water treatment should be used to control seed borne diseases, but care must be taken as germination percentage can be reduced.
Only use seed from reliable sources. Do not accept grower seed or seed that is not sealed in a seed company container.
Infected seed can introduce disease into the nursery. The results can be expensive and the disease difficult to control. Disease can lead to reduced quantities for orders or unnecessary production to make up for losses and hence tie-up expensive nursery space.
Quality begins with healthy seed; in seed vigour, and in high germination levels. Sometimes there are problems with seed, therefore it is essential that all sowings and seedling orders delivered are identified in a trace back system that includes date supplied, seed lot number, the date treated and sown, and the name of the seed company.
When direct seeding only seed of the highest germination count should be used. For example, if the stated minimum germination is 85 per cent, then a minimum of 15 per cent more trays and greenhouse space will be required to meet an order.
Air seeders are almost the only method used today in the production of cell-grown plants, because of their cost effectiveness. Such units vary from small hand operated single tray units to high volume, automated, flow-through systems. The high volume systems include auto tray-fill and topping of the sown tray.
Plug seeding: This is a method where seed is sown into small plugs for germination. At an early stage the plug is slipped into the normal cell-tray to complete growth to a mature seedling. The advantage of this system is the huge saving in greenhouse space.
Use only quality water low in salt. Disease organisms may survive in water, so the source of the water must be considered carefully. Water should not be recycled due to the risk of infecting plants with diseases.
The type of irrigation to achieve the most even growth is a travelling boom irrigator. It can also be used as an efficient management tool. The initial cost for a boom irrigator can be high if only looked at as an irrigation system. A boom irrigator becomes a management tool when it waters, feeds, applies most chemicals and is used to slow down plant growth. The whole process can be programmed through a computer taking into account different groups of plants.
Irrigate only to the point of drainage, unless it is desired to slow growth down by leaching of some nutrients. Do not allow soil-less mixes to dry out to a point where plants wilt. Plant growth is affected and because over-watering is needed to re-wet the media in each cell, nutrients are leached away.
Pests and diseases
Hygiene as outlined under Nursery design has a vital role in the control of pests and diseases. Use steam or chemical sterilisation of the growing media, structures, tools and trays. Effective ventilation and air movement is also a sound disease prevention method.
Individual crop Agriculture Notes should be consulted to understand pests and diseases that could affect the growth of healthy seedlings. A guide to control measures will also be found in these Agriculture Notes.
Care must be taken with the use of pesticides within enclosed areas. Note also that in a greenhouse plants can be more sensitive to chemicals than in the open field.
Follow the directions on the label. If in doubt consult the chemical company named on the label.
There are two choices in marketing the plants:
- Trays of plants are sent out as orders. or
- Plants are pulled and bulk delivered in containers.
Extreme care is required where used trays or containers are returned to the nursery due to the risk of disease. Returned trays and containers should be kept in a separate area. Do not place these items along side or above trays containing plants or clean trays and containers. All equipment must first be sterilised before re-entering the main nursery complex.
Pulled plants may be stored in the cool room for two to three days, but should be planted as soon as possible. A high rejection rate of plants before delivery indicates a production problem which must be addressed and cannot be tolerated.
Costs are high in a modern seedling nursery. But then the returns are even better from a healthy business. To maintain long-term profitability there must be sound planning, operation and investment. This will only be achieved when the buyers are satisfied.
For the vegetable grower, the loss of farm income must not result from inferior quality in seedlings delivered to the farm
This Agnote was developed by Rob Dimsey, July 1995.
It was reviewed by:
Rob Dimsey, October 2007.
Rob Dimsey,Farm Services Victoria. March 2009.
Published and Authorised by:
Department of Environment and Primary Industries
1 Spring Street
This publication is copyright. No part may be reproduced by any process except in accordance with the provisions of the Copyright Act 1968.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication