Tips for Sowing Seeds Indoors

Sowing seeds indoors is one of the most satisfying ways to get a head start on the growing season, especially here in Hampshire, where spring can feel like it’s arriving one minute and disappearing behind a cold easterly wind the next!  While outdoor sowing definitely has a place, indoor seed-starting gives us gardeners much more control over warmth, moisture and timing.

If you’ve already tried growing from seed before, you’ll know the basics. The real difference between patchy results and strong, healthy seedlings often comes down to a few simple indoor techniques.  Building on our general advice on sowing seeds, here are our five top tips for indoor sowing.  And we’ve currently got a lovely selection of seed sowing kits in stock (see the images below) if you want any easy way to get started on something – and inspire the next generation of growers!

 

Choose the Right Space (It’s Not Always the Warmest)

It’s tempting to put seed trays straight onto a radiator or in the hottest room of the house but steady conditions matter more than high temperatures. Most seeds germinate happily at around 18–21°C (check the packet for specifics).  Too much heat can lead to weak, leggy growth instead of more robust seedlings.

A bright windowsill is the classic option but in late winter and early spring, light levels in the UK can still be surprisingly low so south-facing windowsills are best.  Remember to turn your seed trays every couple of days so your seedlings don’t all lean over in one direction (towards the light!).

 

Use Fresh Compost and Clean Containers

One of the biggest mistakes is reusing old compost or sowing in unwashed pots. Seedlings are vulnerable in their early stages and stale compost can harbour pests, mould or fungal diseases. Use a fine seed compost for best results and always start with clean trays or modules. It’s a small step that makes a big difference in giving your new plants the best possible start.

 

Don’t Overwater: Moist, Not Soaked

Indoor sowing failures often come from too much enthusiasm with the watering can. Seed compost should be evenly moist but never waterlogged. Overly wet conditions encourage fungus and discourage strong root growth.

We like to water our pots of compost well then let it drain before sowing.  To keep the compost moist after that, a spray bottle or gentle watering from below (standing trays in a shallow dish) works well. In Hampshire homes, where central heating can dry the air, check trays regularly to see if they feel damp, rather than just soak them out of habit.

 

Sow Thinly and Label Everything

Even experienced gardeners underestimate how quickly seedlings fill a tray. Sowing thinly reduces competition, wastes less seed and makes pricking out much easier later.

And labels are essential. It’s amazing how similar tomato, cosmos and salvia seedlings look when they first emerge! A simple waterproof label saves a lot of guesswork in April.

 

Timing Matters More Than You Think

Indoor sowing isn’t about starting as early as possible; it’s about starting at the right time based on temperature and light levels. Seedlings started too soon often outgrow their pots before they can safely go outside, leading to stressed plants.

Use our sowing calendar as a guide and remember our Hampshire gardens can usually begin hardening off in mid to late spring, depending on local frost pockets.

 

Harden Off Slowly for Outdoor Life

The final step is helping seedlings adjust to real garden conditions. Before planting out, give them a week or two of gradual exposure to cooler air, breeze and daylight. This reduces shock and produces sturdier plants.

 

Best Seeds to Sow Indoors (That Really Benefit From It)

Not everything needs an early indoor start, but some plants are definitely worth the effort - especially if you want a longer growing season or earlier flowers.  If you’re ever unsure, a good rule of thumb is to sow indoors for warmth-loving plants, slow growers or anything you want to protect from early weather and hungry wildlife.

A few of our favourites that do brilliantly when germinated indoors here in Hampshire are:

Tomatoes
Probably the classic indoor sowing success story. Our summers can be a bit hit-and-miss, so giving tomatoes a head start indoors means you’re far more likely to get a decent crop before autumn rolls in.

Chillies and peppers
These are slow starters and really appreciate the extra warmth early on. If you fancy growing them, sowing indoors is almost essential.

Cosmos and annual flowers
Cosmos, zinnias and sweet peas are great if you want colour early in summer. Starting them inside means you get strong plants ready to go out once the weather settles.

Basil and tender herbs
Basil in particular loves warmth and it’s much happier on a sunny windowsill than outdoors in March.

Courgettes, squash and pumpkins (later in spring)
These grow fast so you don’t want to sow them too early but starting them indoors in late April gives them a nice boost before planting out.

Brassicas for the veg patch
Cabbage, kale and broccoli can be started indoors to avoid pests later on - and it helps you control spacing from the beginning.

Common Indoor Seed-Sowing Mistakes (We’ve All Done Them)

Even with a bit of experience, indoor sowing comes with a few classic traps. Honestly, most of us have made at least one of these mistakes over the years!  Here are some to avoid:

Starting too early

It’s so tempting when you see packets in the garden centre in February… but seedlings can quickly become tall, floppy, and pot-bound if it’s not warm enough to move them outside yet.

Not enough light

This is probably the biggest issue on UK windowsills. Seedlings stretch towards the light and end up thin and wobbly. A bright spot and regular turning helps a lot.

Overwatering

Seedlings don’t want to sit in soggy compost. If the surface looks wet all the time, it’s too much. Moist is good - swampy is not!

Sowing too thickly

It feels like more seeds = more success but overcrowding leads to weak plants and a fiddly job when it’s time to prick them out.  (Less is more, once again!)

Forgetting to label trays

We all think we’ll remember… and then suddenly everything looks like “small green leaves.” Labels save so much confusion later.

Skipping the hardening off stage

This is the big one. Plants raised indoors have lived a very sheltered life. Putting them straight outside into wind and chilly nights is a shock. A gradual transition makes all the difference.

Finally, a Little Hampshire Reminder…

Down here, we’re lucky with a slightly milder climate than further north, but we can still get cold snaps well into April. Indoor sowing is a brilliant way to get going early - just don’t rush the final step of getting plants outdoors.

A slow and steady approach now means stronger plants, better crops, and far fewer disappointments later.  Happy sowing!