All About Potted Plants, Compost and Additives

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By Josie Jeffery | Feb 28, 2017

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Repotting plants is an art that can make or break the plants ability to grow.
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"Seedswap" by Josie Jeffery gives seed activists all the tools necessary to save and swap seeds!

With the rise in genetically modified seeds many are looking to the past, to the days of seed saving and seed swapping. Swapping and saving seeds insures our planet’s plant heritage, and the movement is gaining popularity. Seedswap (Roost Books, 2014), by Josie Jeffery, is the perfect introduction to saving seeds, with illustrations, easy to follow instructions, a glossary of plants and more. This excerpt comes from chapter 7, “Raising Seeds.”

You can purchase this book from the Capper’s Farmer store: Seedswap.

Before you sow your seeds, you need to have the right potting compost, additives and containers.


Use peat-free seed compost, which is eco-friendly and finer than normal potting compost. It makes better contact with the seeds. Seed compost also contains fewer nutrients, since they are not necessary for germinating seedlings, which use the food supply stored in the seed. Potting compost can be used for transplanting. To help with drainage, aeration and water retention, add one part horticultural perlite to two parts compost.


Plastic trays Use these for small seeds. They are cheap, retain warmth, and are washable, re-usable and strong.
Wooden trays These are costly but will last for years if looked after. They retain warmth and the strong aroma of cedar wood trays can deter pests and diseases.
Modules and 7.5cm pots Use these for large seeds that are sown individually or in twos or threes, such as sweetcorn or broad beans. This limits transplant shock.
Recycled containers Re-use food tubs, tins, buckets, bags and old boots etc. Clean them and punch in drainage holes before use.
Accessories Clear plastic covers can be used to keep seedlings warm, and capillary matting is a synthetic fibre that is placed under containers to supply them with greater reserves of water.

Bottle Pots

Recycle a plastic bottle to make a self-watering pot by cutting it in half. Punch drainage holes with a hot needle in the spout end and remove the lid. Place the top end of the bottle upside-down into the bottom half, making sure the spout touches the bottom. Fill the top end with compost, packing it firmly into the spout neck. Sow seeds and water. The water will collect in the bottle bottom, which acts as a reservoir, and will be drawn up through the spout by capillary action.

Seed-Sowing Techniques

Seeds can be sown indoors or directly outdoors, depending on the type of plant and the time of year. Note that some seeds need specific sowing treatments, such as scarification, stratification or light-exclusion before they will germinate. Make sure you do your research before you sow.

Sowing Indoors

Fill the container with seed compost, level, firm gently and then water well.

For dust-like seeds, such as foxgloves, mix with fine, dry sand, before sowing in shallow seed drills, to aid even distribution.
For small seeds, such as cornflowers, thinly sow on the compost surface and sift a thin layer of compost over them.
For large seeds, such as pumpkins, sow in modular trays or 7.5cm pots. Use two seeds pressed 2cm deep and 2cm apart per module or pot.

Label the container with the name of the plant sown, and the date. Use a Popsicle stick, or equivalent, as a label, and write using a permanent marker or pencil. Water the seeds lightly and cover the container with clear polythene, a sheet of glass or cling film or trays or pop bottle cloches for pots. Place somewhere warm and well lit, and check the seeds regularly. Keep the compost moist and transplant the seedlings when the first set of true leaves develop. It may be necessary to turn the container from time to time if the seedlings start to grow unevenly towards the light.

Sow Directly

Prepare the soil a few weeks before sowing by breaking up any clumps and removing rocks and weeds. Dig or rake in any soil improvers and level the surface. Weeding will need to be done again just before sowing, and the soil will need to be raked once more to create a fine, crumb-like texture on the surface. You can then broadcast your seeds and rake them in. Alternatively, you can sow seeds in rows, which are called seed drills.

Mark where your seeds are, so you don’t accidentally pull them up when you weed. It helps to research what your seedlings will look like. Water well with a fine spray.

Top Dressings & Coverings

Top dressing is where something is spread onto the surface of the soil, without it being dug in. In relation to seed growing, it usually applies to a layer of horticultural sand, fine grit or vermiculite. Top dressing isn’t a necessity, but it is a helpful technique for seed sowing because it helps retain moisture and prevent nutrients leaching out. It also stops smaller seeds from washing away when you water your pots or trays. The thickness of the dressing varies according to seed type, but the larger the seed, the thicker the dressing can be.

Greenhouse Effect

To hold in heat and moisture, simply lay a sheet of plastic over the container or place it in a plastic bag. Remove the covering as soon as the seedlings emerge.

Pop Bottle Greenhouse

Plastic bottles, cut in half, can have much the same effect, and are often used outside to protect developing seedlings from pests and cold weather in spring. Use the top half of the bottle with the lid removed, and push it 5cm into the soil over individual plants.


Seedlings and tender plants can be protected with tunnels made with pipes or wire bent into semi circles, covered with horticultural fleece or clear plastic. Push the ends of the wire or pipes into the soil for stability. Ready-made tunnels are readily available online or in garden centers.

Watering Systems

After sowing seeds, the compost needs to be watered daily in order for seeds to germinate. The seeds need to take on water and swell, breaking the seed coat so they can emerge. A steady water supply will also be needed for the developing seedlings. In both cases, keep the compost moist but not so wet that it becomes waterlogged.

Watering Seedlings

Irrigate little and often as overwatering can cause diseases, such as damping off. When watering, use a watering can with a rose for a fine spray or a mist sprayer, so the compost surface is disturbed as little as possible.

Pop Bottle Drip Irrigation System

When your plants have been planted out, the rain will be their irrigation, but if there is prolonged dry weather you need to water too. These pop bottle drip irrigation systems are perfect. Drip irrigation slowly delivers water into the soil directly around the roots. Watering spikes can be purchased from garden center, but why not recycle and make your own for free?

1. Drill eight small holes into the cap of an empty plastic drink bottle.
2. Cut off the bottom of the bottle to create a funnel for filling and catching rain water.
3. Bury the top third of the bottle by the plants, so that the spout end is buried, and fill the bottle with water. Check the bottle daily and refill if needed. It can also be used for administering liquid fertilizer when necessary.


Baby plants, known as seedlings, need a high level of care as they face many threats like mechanical damage, overwatering, drought or pests and diseases. Seedlings have tender roots and shoots and to ensure they survive, they must be handled gently until they are planted out in to the garden.

Pricking Out

Pricking out is usually done when the second set of leaves (‘true leaves’) develop. Overcrowded plants will compete for space, water and nutrients, so it will be necessary to thin out the weaker seedlings to allow the stronger ones to flourish.

1. Fill a modular tray with potting compost and poke a hole in each module with the blunt end of a pencil. Next, use the pointed end of the pencil to loosen the soil around one of the seedlings.
2. Hold the seedling by one of its seed leaves, and gently prise the seedling out. It doesn’t matter if you accidentally damage the leaf slightly, but if the roots break the seedling will perish. Lower the roots into the hole that you previously made in the modular tray, and use the blunt end of the pencil to bed the seedling in.
3. Plant one seedling per module to give it room to grow, then water it with a fine spray, and label.

Feeding & Potting On

A pricked-out seedling will need feeding fortnightly with a balanced liquid fertilizer, but check first, in case the potting compost you are using already contains added fertilizer. Before you sow seeds straight into modules, you can mix slow-release fertilizer granules with the compost at the bottom of the module to provide nutrients as their roots grow down into the compost.

Potting On

Once the seedlings begin to outgrow their pots or modules, it will be necessary to pot them on into larger pots, unless they are already large enough to be planted directly into their final positions outdoors.

1. To remove a plant from its module or pot, turn it on its side and gently squeeze the sides and bottom to loosen the root ball, allowing it to fall out whole.
2. Put a thin layer of potting compost in the base of a 7.5–9cm pot and place the root ball in whole.
3. Backfill with more compost and gently firm in, allowing a 1cm gap between the top of the pot and the compost for watering. Label and water in.

Young Plants

Your indoor sown seedlings have grown from babies into juvenile plants. Like all juveniles, they need guidance and a helping hand to prepare them for ‘flying the nest’ or, in this case, uprooting from the warmth of a heated glasshouse into the garden where they will have to, for the most part, fend for themselves.

Planting Outdoors

The plants will still need some protection from potential threats ahead of them, and they will need to be hardened off.

Hardening Off

Your pampered indoor plants need to adjust to outdoor conditions prior to planting out. A one-to-two week spell in a cold greenhouse or a cold frame toughens them up by thickening the cuticle on the leaves and stems, making them stronger and more drought tolerant.


1. Water the plant the day before you transplant, and once again before you remove it from its pot.
2. Dig a hole slightly bigger than the root ball, and water the hole before you plant out.
3. Place the root ball in the hole and fill halfway with water. Allow the water to settle around the roots. Fill the hole and lightly firm the soil in around the plant. Water again, and then daily for the first couple of weeks.

Caring for Plants

Established plants still need care. Some may need support stakes and climbing trellises or trimming and pruning, and there is always weeding to be done! Regular weeding will stop weeds from competing with your plants for water and nutrients. The more weeds there are, the more your plants will struggle. Gardeners often remove spent flowers, because this can promote further flowering and sometimes the old flowers spoil the display. This technique is called deadheading, but don’t deadhead your seed-saving plants as the seeds will be developing. Check the plants for any pests, diseases and nutrient deficiencies. Tell-tale signs are changes in foliage color, spotting and holes on the leaves, wilting and even total collapse.

Healthy, strong-growing plants are much less prone to pests and diseases, and one way to keep them healthy is to make sure they are well fed. The three major nutrients most plants need to thrive are nitrogen (N), which promotes leaf growth, phosphorous (P), for the roots and shoots, and potassium (K), for flowers and fruits. You will see these elements listed on packets of fertilizer, usually in different balances depending on the purpose of the fertilizer. Look for the NPK ratio. Some fertilizers also contain trace elements, such as magnesium and iron. Fertilizers are either natural in origin (such as ‘fish, blood and bone’ products) or entirely synthetic. Natural fertilizers tend to be slower acting, and if you are a ‘natural’ gardener they will fit better with your gardening ethos.

Keep plants well supplied with nutrients by preparing the soil well before planting, by digging in well-rotted manure to boost its fertility, or by growing a green manure on the patch before sowing, such as buckwheat or clover.

Mature Plants

Your plants have worked really hard so far and are now on their last push to complete their growing cycle. The flowers are about to bloom (an eagerly awaited moment by all), and this is the time to decide what steps to take to save ‘true’ seed.

Letting it Go to Seed

Mark off some healthy plants for isolation. You should preferably choose plants that are in close proximity to each other, especially if you are building a cage around them. But if you are bagging self-pollinating flowers, proximity isn’t necessary. Your ‘true seed plants’ will need monitoring, and you will find specific requirements under the entries in the Seed Directory.

Allow the remaining plants in your garden to accept the full flurry of hungry insects, bees and butterflies. These pollinators will be hard at work, and the fruit and seeds that result will in turn give food to a whole other layer of garden wildlife, as well as produce for your kitchen table.

Your plants have completed their life cycle, from seed to flower and back to seed, and you have helped them every step of the way. Feel proud.

From Seedswap by Josie Jeffery, © 2013 by Josie Jeffery. Reprinted by arrangement with Roost Books, an imprint of Shambhala Publications, Inc. Buy this book from our store:Seedswap.

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