Containers for Propagation and Seed Sowing

Containers for seed sowing, propagation and growing on

Pots are available in a wide range of sizes and shapes in plastic, clay, bituminized or treated paper, polystyrene or peat, in addition to which there are soil or peat blocks to consider. There has in recent years been many advances in containers and the trend has been towards space sowing in compartment or cell trays to avoid, where practical, the chore of pricking out when larger seeds are involved.


Plastic pots

Containers for Propagation and Seed Sowing

The swing to these in recent years has been dramatic, occasioned by hygiene, their very low manufacturing costs, transport, and their impermeability. When plastic pots made their debut they were viewed with considerable scepticism by older gardeners who failed to see how plants could grow in a non-sympathetic material incapable of admitting a sufficient supply of air to the roots. Clay with its porous nature seemed to offer all the advantages, yet practical experience has in the main shown otherwise. In a clay pot drying out occurs not only from the top but from the sides, whereas with a plastic pot the main area of evaporation is through the top surface.


Plant physiologists have been at a loss to say whether or not this in itself is a good thing, but less water is certainly lost by plastic pots and this must surely be considered an advantage, though admittedly over-watering can occur by failure to take into account the respective differences in ‘transpiration’ loss.

On the question of hygiene there can be no argument; plastic can be cleaned in warm water without scrubbing, and there is no risk of it absorbing disease as there is with clay. Plastic pots are also highly effective on capillary benches, coming in a wide range of sizes, round or square, light to handle and easy to store. Strips or blocks of small plastic pots are used commercially for a variety of purposes and are particularly useful for growing room activities where space is at a premium. The pot readily detaches by tearing, though some disturbance can occur when young plants require removal.


Paper pots

The role of paper pots is not so clear. While excellent as short-term containers for seeds or growing plants their cellulose content invariably results in a temporary nitrogen shortage until the cellulose decomposing fungi satisfy their own needs for nitrogen. There have been problems also with the saprophytic fungal agencies delaying the germination of seeds and the growth of plants. A major advantage of paper pots is that for seed sowing there is no disturbance on planting out or potting on, as the pot is planted intact, although the implications of temporary nitrogen shortage should not be overlooked. Paper pots are available in round or hexagonal shape, also in concertina-like strips from which pots can be taken as needed. Tubes of paper are very useful for sweet peas.

Red bituminized paper pots are available in large sizes and can be used for growing shrubs, tomatoes, chrysanthemums, and other long-term plants, lasting for about six months with normal handling.



Being inert, polythene offers no nutritional problems. While young plants can be lined out with their polythene pots intact, removal on planting is advisable in most cases, and must be effected with care to avoid damage to roots.


Peat pots

Their main advantage is to allow plants to be grown and planted out without check. They are made from a mixture of approximately 75% sterilized peat and 25% wood fibre, the proportions varying with different manufacturers. Small quantities of nitrogen, phosphorus and potash are usually contained in the peat/wood fibre, which is compressed to form a thin wall. Experience is needed to get the best out of peat pots, which retain moisture with tenacity, thus necessitating less frequent watering particularly if the peat pots are housed in boxes or trays on top of a layer of peat. Space should be allowed between the individual pots, otherwise they can deteriorate quickly. They must not be handled any more than is strictly necessary, otherwise after a certain stage they disintegrate easily. Nutritionally peat pots do not present great difficulties, although extra nitrogen may be required at a certain stage of decomposition, as indicated by paler green leaves on the plants.

Peat pots are more suitable for pricking off and growing on than for seed sowing, although for single large seeds or pelleted seed they are excellent, as the roots penetrate the walls and are poised ready for action when the peat pot is potted on or planted out. They are also available in strip form to save space.

Jiffy 7s are compressed peat pots contained in a thin mesh net made from a mixture of polyethylene and polypropylene; they have made a tremendous impact in all spheres of propagation throughout Europe and further afield in recent years. The pots are expanded by saturating with water on a clean surface, or by standing in a tray of water or placing in a tub; the overhead method of expansion is said to be better for seed sowing. Capillary or tub expansion allows more air in the Jiffy 7 and provides better conditions for the rooting of cuttings. Plants as they develop can be fed with a complete liquid fertilizer high in nitrogen and potash. Seeds can be sown or cuttings rooted individually in each Jiffy 7, which is then potted on or planted out, avoiding all check to growth. Compressed peat blocks without netting (e.g. Jiffy 9s) are now available.


Plastic blocks

Developed in the early 1970s, these consist of foam plastic cut into blocks which serve for seed sowing, rooting cuttings, or pricking off. They simply provide a compatible and moist medium for the production of growth, but experience in their use is limited. Rock wool cubes are also now available in different sizes.


Soil and peat blocks

Soil blocks have been used for a great many years with changing popularity. Either soil-containing compost or peat can be blocked with suitable machinery, manually or automatically operated, the latter being essential for blocks required by the large scale commercial grower. In the early days of soil blocks they were used as a substitute for pots, largely for pricking off seedlings, but in more recent years they have also been used for the rooting of cuttings and seed sowing. The automatic blockmaker not only makes the block but sows pelleted or split pill seeds automatically and is used mainly by large scale vegetable growers. Soil or peat blocks must be sufficiently compacted otherwise they can disintegrate, and it is essential to hold the blocks in trays or boxes.


Bedding plant pots, trays, seed boxes and propagators

While wooden seed boxes suitably treated against rapid deterioration are still available in standard sizes of 36.8 x 23.2 5cm (14 x 9 x 2in), these are tending to be ousted by plastic or trays. Fish boxes have long been the main plant container for commercial bedding plant production, but the trend is now towards polystyrene or plastic trays. Drainage occurs in wooden boxes through the spaces left between the bottom spars, although some gardeners make further holes. Ex-fish boxes tend to be smelly and salt contaminated; they are best kept outside for a weathering period.

Plastic seed trays are usually available in varying sizes with slight variations according to suppliers.

Containers for Propagation and Seed Sowing Polystyrene trays are generally of standard size (14 x 9 x 2in) although they are also available as break-up units (four units each constructed for 10-cell plants) so that customers can buy their bedding plants in small quantities without root disturbance. Commercial growers widely use four or five section polystyrene trays with 10 or 12 plants in each section, but it should be noted that these section trays are not generally available to the public. Gardeners with commercial aspirations can purchase the four or five section trays from wholesale sundry suppliers, or possibly by arrangement with a commercial grower. The trend towards cell trays is pronounced and it is ironical that in the U.K. Bedding plants are still produced and sold in communal trays, a practice which can find no favour whatsoever in any of the continental countries, USA or further afield. Plastic packs are also available in the same basic forms.

Apart from the break-up units, the role of all these boxes or trays is similar — seed sowing, rooting of cuttings and pricking off. Their respective merits culturally cause tremendous conflict of opinion; hygiene is a major advantage with plastic trays whereas the risk of disease carry-over in wooden boxes is very real, necessitating washing with a solution of 1:49 formaldehyde, or cresylic acid at 1:39, some weeks before use. Expanded polystyrene, due to its insulating properties, offers a warm home for the plant, but in some quarters the opinion is that plants do not thrive so readily in polystyrene as in plastic or wood; this is possibly because its porosity makes water application more tricky.

Plastic dome covers are designed to cover all sizes of plastic trays, forming very convenient mini-propagating cases, possibly an ideal arrangement for the gardener with limited propagating space in the greenhouse and needing to use a porch or even a light window in the home for additional space. Polythene bags make a very useful cover and conserve moisture effectively. Propagating cases play an increasing role for seed sowing.

27. March 2011 by Dave Pinkney
Categories: Greenhouse Gardening, Propagating | Tags: , , , | Comments Off on Containers for Propagation and Seed Sowing


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