Container sowing should always be used for seeds that are very fine, expensive, slow to germinate or just to get a jump on the season.
The advantage to sowing in containers is it gives you greater control and flexibility. The containers can be placed in a warm position allowing you to start seeds earlier in spring. It is possible to control watering and fertilising more accurately. Pests can more easily be monitored; you are far less likely to lose a whole batch of seedlings to a hungry snail.
The disadvantage is the time involved in 'pricking out' the seedlings and the need for greater attention to watering. It is also more expensive to buy the trays and seed raising mix. For gardeners with limited space it is possible to grow many vegetables to maturity in containers. Larger containers are useful for this such as recycled styrofoam vegetable boxes or Super Tubes.
Types Of Containers
• Seedling Trays or Punnets Seedling trays and punnets are shallow so the seed raising mix stays warm. Shallow seed trays also a have a better surface/depth ratio to improve aeration. Seed can be sown directly into seedling trays or the seedling tray used as a tray to hold punnets, jiffy pots, jiffy starters or 48 cell growing trays. 48 cell growing trays are made of a soft plastic that allows you to squeeze the entire seedling out without damage to the root ball. Seedling trays are designed to fit bottom heat propagators and Mini Propagators. Sowing seed directly into a seedling tray gives you a good surface to work from in order to 'prick out' or just thin your seedlings. Small tree seeds should first be sown in a seedling tray and later transplanted at the 4-6-leaf stage, into individual tree tubes, before planting out into their final position. This is because pots are too deep and stay too cold and wet for good germination. Tree seeds can take 3-6 months to germinate, depending on factors like soil temperature so they should be labelled with the name and date of sowing and left alone in a seed tray. One day when you have forgotten about them there may be a tray with hundreds of seedlings!
• Plantable Pots These include Jiffy Pots, Jiffy Plant Starters and Potmaker Pots. These allow you to sow individual seeds in controlled conditions, without the need for pricking out. A big advantage is they reduce transplanting shock as the whole container is planted. The Jiffy Plant Starters are particularly useful for starting tomatoes, capsicums and eggplants. The Jiffy Pots are great for starting larger seeds such as zucchini, corn, melon and cucumber. A Potmaker is used to make your own small pots out of newspaper. Jiffy Pots and handmade Potmaker pots need to be filled with a seed raising mix, while the Jiffy Plant Starters are a container and mix combined.
How To Sow Seed in Containers
• Seed Raising Mix
Choose a good quality seed raising mix, a small expense that will pay dividends. Look for brands that meet or exceed the Australian Standard. Seed raising mixes are designed to allow the right aeration for the germinating seed. Seeds need oxygen for respiration or they suffocate and die, which is why seed raising mixes usually have quite a bit of sand/vermiculite/perlite to try and improve aeration. You can make your own but it is better to become an experienced seed raiser before attempting this. Never just use garden soil in a container as it shrinks and makes watering very difficult and lacks aeration. Usually potting mix is too coarse to be used to raise seeds.
• Preparing the Containers
First fill the seedling tray or punnet right to the surface and firm down, to 1 cm below the rim. A handy tool for firming down can be made from plywood with a handle glued to the back, to fit the punnet or seedling tray. Don't press down too hard; you need to keep air in the mix. Then water the punnet or tray gently until water runs out the bottom. Sow seeds evenly, try to leave space around all seeds. Fine seed can be mixed with dry, clean sand to help spread it evenly. Paying attention to spreading the seed out will save time later and produces a sturdier seedling. It avoids wasting seed. If you are using a Mini Propagator keep the ventilator closed until germination, this allows moisture to condense and be recycled, keeping the soil evenly moist. Heat is retained which speeds germination. Place seed trays in a warm, sheltered position with good light but not direct sun. Remember to label your trays or punnets to avoid later confusion.
How to Raise Seeds in Containers
• Pricking Out
Seedlings can develop disease, such as damping-off or to grow too tall and spindly, if they are left too close together. Pricking out is done at the 4-6-leaf stage, usually 2-4 weeks after germination. Select the sturdiest seedlings, discard any that are badly formed, damaged in the pricking out process or with insufficient root development. Seedlings are lifted carefully by their leaves (never the stems), using a dibber to gently loosen the roots and transplanted into punnets, Potmaker pots, recycled yoghurt cups, individual Jiffy Pots or tree tubes. Certain vegetables like to be re-planted deeper than they originally germinated. These include cabbage, broccoli, cauliflower and tomato. To do this remove the first set of ‘juvenile‘ seed leaves and plant up to the first set of true leaves. Doing this can increase the root area of a plant and hasten fruit development. In general though it is best to replant to the same depth. Once pricked out water gently, and if using a Mini Propagator, open the ventilator to encourage air circulation. At this stage seedlings need continuous moisture and nutrients, which can be provided by half-strength liquid manure, Seed-Start or Natrakelp, applied every few days. Remove the propagator lid during the day once the seedlings are growing vigorously and replace it at night. Once roots reach the bottom of the growing container, usually after 2-4 weeks, they will be ready to transplant into their final position.
• Hardening Off
All seedlings need to be toughened up to full sun exposure before transplanting. This is done over a few days by moving the trays or punnets into steadily more exposed sun conditions. Keep a close eye on them so that they don't dry out.
Water gently, with a fine spray, to avoid washing seeds away. Avoid over-watering, as this will cause 'damping off'. Damping off pathogens can attack seeds before they germinate or after seedlings have emerged. Remember that seeds need to breathe in order to grow and there is no room for air if all the available spaces in the soil are constantly filled with water. Seeds vary in their vulnerability to damping off. Water when the surface feels dry, once a day may be too much. Once the green leaves appear it is better to combine watering with weak liquid feeding as seed raising mixes contain very few nutrients.
Article by Frances Michaels
Green Harvest Organic Gardening Supplies