How to plant seeds in punnets


Robin Gale-Baker, from Sustainable Macleod, discusses how to plant seeds in punnets. This is one of a series of articles she has written about growing techniques (see right hand sidebar). She has also written a number of articles about growing various vegetables, herbs and fruit trees.

In September, it’s time to plant seeds in punnets for the spring veggie and flower garden. A seed is essentially a hard outer covering with an embryo inside and your job is to allow that embryo to develop in prime conditions. This article provides some tips for optimum results.

Assuming that you will be using secondhand punnets and pots, your first job is to clean them. Use a brush to remove loose dirt, rinse them and soak for a minimum of 30 minutes in 9 parts water to one part vinegar to remove any pathogens.

Next select an Australian Standards seed raising mix (and later a potting mix). Never use garden soil in punnets as it is too heavy and compacts. Also, buy from a nursery rather than a hardware store or supermarket even though it will costs a little more: seeds can be expensive so don’t skimp on good quality mix.

Seed raising mix is much finer than potting mix and, when firmed down, prevents small seeds falling through the mix and disappearing out the drainage holes. It also doesn’t contain much in the way of nutrients which is a good thing as you do not want your seedlings growing too fast and crowding as this leads to ‘damping off’ whereby your seedlings collapse due to pathogen growth.

Fill your punnets with seed raising mix to 1cm below the rim. Using a second filled, watered punnet, place it inside the first punnet and use it as a weight to firm down the mix so it forms a flat seed bed and then water it well. For all seed types except for cucurbits (see next paragraph) and lettuce (see the following paragraph), sprinkle your seed onto this moist surface, cover with 1cm of soil (or whatever is suggested on the seed packet) and water again gently. This results in the seed being in contact with moisture, being secure in its bed and prevents loss of seed. For fine seed which is close to the surface, use a spray bottle daily to spray the surface so that it remains moist. If your soil surface dries out, this is likely to result in your seed drying out too, killing the embryo.

Cucurbit seeds, such as pumpkin, cucumber, zucchini and melon, are large seeds with a flattish surface. If laid in the punnet horizontally, water can pool on the upper surface of these seeds, causing rotting. Instead, plant these vertically into the seed bed with the pointy end upward, and cover with the required amount of mix. Generally the rule is to cover the seed to a depth of up to 4 times its thickness but this does not work with seeds planted vertically and I recommend 2 cm cover in these cases.

Lettuce seeds germinate in light so they should not be covered with seed mix. Rather, prepare a firm, moist seed bed, sprinkle the seed onto this and press the seed down so that it connects with the moist bed. This means that, when it germinates and roots begin to emerge, those roots will be ‘in touch’ with the soil and able to penetrate it rather than laying along its surface.

When the seeds have germinated and the seedlings are at the 4 leaf stage (2 pairs of leaves), they are ready to transplant into seed trays (wash and sterilise trays too). Use a good quality potting mix which will contain a balance of nutrients, drain freely and be more open allowing roots to develop unimpeded. If your seedlings were a yellow or purplish colour (indicating a lack of nutrients), they will soon turn green as they take up nutrients. When you transplant, take out a clump at a time and gently pull the roots apart. Hold the seedlings by the leaves not the stem. If you damage the leaves, they will re-grow but if you damage the stem, the seedling will die. Also check that you haven’t bent the stem when separating as these won’t survive either.

Most veggie seeds can be started in punnets but root vegetables are best planted direct into soil cultivated to a fine tilth. Some other seeds can be planted direct but I advise against it in the case of zucchini, cucumber, pumpkin and melons as these seedlings are often eaten when young and tender by mice or rats or scratched out by birds. Better to grow them in pots until the plants are strong and well developed and then transplant.

Seed packets should always be read as they often provide a wealth of information. In rare cases, you will find that some seeds need special treatment such as cold stratification in the fridge or to be placed in moist soil in a sealed container in the fridge. If so, then this information will be on the packet.

  2 Responses to “How to plant seeds in punnets”

  1. Thanks very much for this article – very helpful.

  2. Excellent advice!!

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