The Ultimate Guide to Sowing Seeds: What, When & How

The Ultimate Guide to Sowing Seeds: What, When & How

Buying seedlings or fully-grown plants from the nursery can soon add up and growing your own plants from seeds is a cost effective and rewarding alternative. Depending on the plants you are wishing to grow, your seeds may not be hardy enough to raise outdoors, so will have to be raised in a greenhouse or on a windowsill. Other plants can brave the UK weather, but can only be planted at certain times of year to prevent them being affected by frost and other issues.

Seedlings require specific care, so novice gardeners sometimes find that they struggle with seedlings rotting, failing to emerge, or showing stress due to lack of feed. Knowing when to sow your seeds, how different seeds should be cared for, and when to move your seedlings outdoors are all essential to ensuring that you end up with healthy plants. Don’t worry if you aren’t totally confident in your seed sowing abilities though, you’ll be raising prize-winning saplings in no time with our ultimate guide to sowing seeds.

When to Sow

Choosing when to sow your seeds will depend on the type of plants you are growing and whether you will be raising them indoors or outdoors. Some plants grown indoors or in the greenhouse can be planted as early as January, whereas plants raised outdoors must be sown in early or late summer.

Small sapling sprouting in the greenhouse.


There are lots of plants that are suited to being started off indoors, particularly vegetables, annual bedding plants, and some of the more tender perennials. Vegetables like tomatoes and runner beans can be started off indoors and then transplanted outdoors when the risk of frost is low. For vegetables that need a longer growing season, you can start plants such as celeriac, radish, chicory, and sweet peas as early as January when growing indoors.

Annual bedding plants will also benefit from being started inside, to allow them to reach maturity in time for beautiful floral displays in your summer beds. Otherwise, plants can be started indoors around February/March time to be planted outdoors after the risk of frost has passed in early summer.


Lots of plants can be sown directly outdoors in their final growing position, as long as the soil is soft and warm enough for it to germinate. This usually means sowing around mid-spring to early-summer, when the soil has had chance to warm up. If you are planning to sow seeds at the height of summer, be aware that they will require regular watering to be successful. Sowing times do vary, so be sure to check on the seed packet when you should be sowing each plant. Vegetables, annuals, biennials, and herbaceous plants can all be grown outdoors with the right care. Sowing outdoors gives you more freedom of design, allowing you to sow in free drifts that create natural-looking distribution or the kind of regulated rows that make weeding and thinning much easier.

Woman and child girl, mother and daughter, gardening together planting strawberry plants in the garden


If you aren’t planning to sow seeds directly into their final growing position, you will need to raise your seedlings in containers. Small seeds are generally sown in shallow seed trays and transplanted to larger pots once they have reached the seedling stage. Slightly larger seeds can be sown into a modular plug tray, to limit any transplant shock, and large seeds can be sown individually into 9cm pots. If you are planning to use old containers, be sure to clean them thoroughly before use to avoid cross-contamination of any kind. Containers should be filled with a standard soil-based or soil-less seed compost, as they are finely milled and don’t contain nutrients that could harm seedlings.


Some seeds require specific sowing treatments, such as light-exclusion or a nick being made in the seed coat. Instructions for any special requirements such as these can usually be found on the seed packet, so check for this before sowing. Small seeds can just be scatter-sown over the surface of the compost, but larger seeds must be station-sown by pushing each seed a couple of centimetres into the compost. Usually, you will need to maintain a temperature of around 18°C under a cover, unless the seed packet states otherwise. Polythene, glass, or a heated propagator with lid can all be used to create the warm temperatures needed to raise your seeds.

Farmer transplanting corn salad seedlings from a plastic nursery tray into an open bed covered in mulching cloths with holes for the plants


Once germination has occurred, you should remove the cover on your seeds to increase the ventilation around the new shoots. The compost should be kept slightly moist at all times during this period in order to maintain a steady growth.

Pricking Out

Seeds grown in seed trays will need to be transplanted once they reach the seedling stage and this is called pricking out. You can do this by loosening the compost around the roots and lifting the seedling out of the tray holding the true leaves that replace the seed leaves.

Growing On

Your seeds will need attentive care if they are to grow into strong seedlings, so it’s important to plant them at a time when you’ll be able to tend them regularly. Small seedlings will need to be watered regularly with a fine rose, to avoid damaging their delicate shoots. Larger seedlings can be watered using a course rose to ensure the entire depth of the compost is fully soaked. Once they have been transplanted, seedlings should have a balanced liquid fertilizer fed to them every couple of weeks, although larger plants may require a weekly feed.

Moving Outdoors

Once the weather is warm enough and your plants are big enough, they will be ready to begin the hardening off process required for planting outdoors. Congratulations, you raised your own plants from seeds!

Come and visit one of our garden centres near Alexandra Palace, Wandsworth Common, or Berkhamsted to stock up for your garden and get some helpful advice from our Plant People. Alternatively, head over to our homepage for more inspiring garden ideas.

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