Sowing Seeds: Indoors

If you’re just starting out, starting seeds indoors is a great way to learn. This method works best for seeds that need warmth to grow and germinate, like tender and half-hardy vegetables and flowers. By sowing indoors, you can give plants the protection they need and get them off to a good start early.

Many spring-sown plants (annuals, bedding plants, vegetables and tender perennials) can benefit from the following information. Please do not be discouraged if the first batch of seeds you sow does not turn out perfectly because sowing seeds can be moderately challenging.

Sowing seeds indoors
Sowing seeds indoors

Why sow seeds indoors?

Sowing seeds indoors is easy and fun, but they do require regular care for several months. You will also need space in a greenhouse or on a sunny windowsill to keep them until they are ready to be moved outdoors. 

By sowing seeds indoors, you are providing them with a frost-free environment; some seeds, like tender and half-hardy flowers and vegetables, require warmer conditions to germinate and grow. You are also able to provide them with additional protection from slugs or damp weather. 

Sowing seeds indoors also provides them with a head start and faster, more reliable germination, for earlier crops or flowers. It also provides slow-growing plants the additional time they require to flower or crop. 

What can be sown indoors?

There are a wide variety of seeds that can be sown indoors including:

– Tender crops like chillies and tomatoes

– Half-hardy annuals 

– Hardy annuals and veg such as sunflowers and broccoli

– Annual climbers 

– Perennials (although they may not flower in their first summer)

– Tender herbs like basil and parsley

– Slow-growing crops

Where to start sowing seeds and grow on

Small pots, modular trays, seed trays, recycled containers like fruit punnets, juice cartons, and yoghurt pots with holes in the bottom, as well as pots made from newspaper strips or toilet roll middles, are all viable options for sowing. Ensure that the seed trays and pots from the previous year have been thoroughly cleaned before you reuse them.

The specifics of whether seeds should be sown indoors or outdoors can be found on seed packets.

Place the pots or trays in a heated or frost-free greenhouse, on a bright windowsill, in a conservatory, on an enclosed porch, or in any other frost-free location that receives ample sunlight after sowing. A heated propagator can also be useful for seeds that need a lot of warmth or a certain temperature to germinate and germination can be sped up.
The required temperature for germination can be found in the seed packet.

When to start sowing seeds

For each plant, sowing times vary so it is always best to check the seed packet for their recommended sowing months. Sow tender and half-hardy vegetables and flowers in the spring for late spring and early summer planting. Biennial flowers such as foxgloves and fast-growing vegetables can be sown in summer. Salads and vegetables can be sown in autumn for indoor cropping over the winter or early spring planting. Finally, tender and half-hardy flowers and vegetables that need a long growing season can be sown in late winter. You can also sow any fast-growing microgreens and sprouting seeds at any time of year on a kitchen windowsill. 

Make sure to check the ‘use by’ date on any old seed packets before sowing. Germination rates slow over time so you may get disappointing results from old seeds. 

A guide to sowing seeds

1. Fill a seed tray, module, or small pot with peat-free seed compost or sieved multipurpose compost and gently compact it until it reaches just below the rim. Seed compost is preferred by smaller seeds due to its finer texture than multi-purpose compost. Additionally, it has fewer nutrients, which is good for young seedlings.

2. Scatter smaller seeds as thinly as possible on the surface. Larger seeds can be pressed into the compost individually, at the spacing distance recommended on the packet. 

3. The majority of seeds ought to then be encased in a thin layer of compost. To determine whether and how to cover the seeds you are sowing, check the seed packet.

4. Water gently using a watering can with a sprinkler head to avoid dislodging the seeds. Alternately, place the container upside down in a tray of water to absorb water from below until the compost is completely damp. 

5. The containers should be labelled so that you can remember what was sown and when.

6. To maintain a humid environment, place the container in a propagator with a clear lid or cover it with a transparent polythene bag. Most seeds need a temperature of 18C for germination unless the seed packet states otherwise.  

7. Water regularly, aiming to keep the compost moist at all times. 

8. As soon as seedlings begin to increase ventilation, remove the bag or lid. Typically, germination takes two to three weeks.

9. To ensure that they grow steadily and evenly, make sure they get plenty of sunlight and water frequently.

Note: Whilst most seeds can be sown direct from the packet, a few with tough seed coats may need to be pre-soaked, scraped or nicked (with sandpaper or a knife) to aid germination – seed packets will provide details. Others require a period of cold or warmth before they will germinate

How to take care of your seedlings

When your seedlings have at least two pairs of leaves, move them into their own modules or pots. This, known as pricking out, keeps them from getting too big and gives them room to grow. Seeds sown singly in modular trays don’t need pricking out, simply move each seedling into a 9cm pot once its roots have filled its module. 

Overcrowded seedlings are more susceptible to fungal diseases like damping off, so do not delay pricking out. It becomes even more challenging to separate them without causing damage as their roots become intertwined. They can also become spindly and weak as they strive upward for more light and space. 

Pricking out seedlings

Fill several modular trays or 9-inch pots with peat-free multipurpose compost to prick out seedlings. To loosen the compost around the first seedling, use a stick or pencil. While supporting the roots with the pencil, carefully lift it out of the compost by holding it by a sturdy leaf rather than the stem. Keep as much compost as possible around the roots. Make a hole with the pencil for the seedling in its new compost pot, then lower it in place. Water in as you gently press the compost around the seedling without damaging or dislodging it. In the new pot, bury the seedling slightly deeper (up to the first pair of leaves) if it has a long, weak stem.

Growing on your seedlings

Seedlings should be watered regularly, aiming to keep the compost slightly damp but not completely dry. If there isn’t enough water, seedlings can quickly die, so it’s important to keep an eye on them. Feed every fortnight with a liquid fertiliser that is balanced for strong growth. Seedlings may lack nutrients if they develop pale lower leaves. Pinch the tips of long shoots on flowering plants to encourage branching; a bushier plant will produce more flowers. When plants become slender and tall, it usually indicates that they require more light.

How to plant out

Your seedlings should have grown strong and sturdy by April or early May when it is time to plant them in the garden. Seven steps are involved in this:

1. A week or two before planting out, harden off. They will continue to develop as a result, and once outside, they should adapt. To accomplish this, take the plants out of the propagator or greenhouse. To avoid wilting, this should be done in the shade on a cloudy day. Move them into a cold frame with the lid slightly ajar during the day if you don’t have a greenhouse.  

2. Get the site ready to plant. Check the seed packet to find out the planting conditions needed. Make sure to thoroughly weed the area before planting. 

3. Water your plants an hour or so before planting so that the compost it thoroughly damp. 

4. Water the base of the plant in a hole that is the same size as the plant’s container.

5. Make sure the compost’s surface is level with the soil as you carefully insert the plants into the hole.

6. Soil should be used to fill in any spaces around the plant, then the soil should be firm and the plant should be lavishly watered. 

7. Repeat the process with your other plants in the same way, spacing the plants at the distance specified on the seed packet. 

Updated on April 12, 2023

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