How Much Sunlight Do Seeds Need to Grow?

Sunlight's First Job: Warming the Soil

Seeds don't respond to a calendar. They respond to soil temperature.

When a seed sits in warm, moist soil, it detects conditions are right and begins to germinate. When soil is cold — even if the air feels warm on a nice afternoon — seeds often sit and wait. Some will eventually rot if cold and wet conditions persist for too long.

Sunlight warms the soil. That's its most important role before a seed has even sprouted. A position that gets good sun during the day will have noticeably warmer soil than a shaded spot, and warmer soil means faster, more reliable germination.

This is why planting in the right season matters so much. In spring, as days get longer and the sun gets stronger, soil warms up and seeds respond. Plant too early and the soil hasn't caught up yet. Plant in the wrong spot and you might be waiting a long time.

Sunlight's Second Job: Powering Growth

The moment a seedling breaks the surface, everything changes. It's no longer running on the stored energy inside the seed — it needs to start making its own. That's what photosynthesis is: the process plants use to convert sunlight into energy.

From the moment a seedling appears above the soil, it needs light. Not eventually — immediately. A seedling left in low light even for a few days will start stretching upward trying to reach a stronger source, producing long, thin, weak stems that struggle to support themselves. This is called going leggy, and it's one of the most common and preventable problems beginners encounter.

The fix is simple: move seeds into a good sunny position as soon as they germinate, or better yet, have them in the right position from the start.

Sunlight, Heat, and Why They're Linked

Here's something that seems obvious once you say it, but catches a lot of beginners off guard: more sunlight means more heat, and more heat means faster everything.

More sun means the soil warms up faster, which means seeds germinate sooner. Once seedlings are up, more sun means faster growth. But it also means the growing medium dries out faster — sometimes dramatically faster in the Australian summer.

A seedling in a small container in full summer sun might need watering twice a day. The same seedling in a shadier spot might be fine with once every two days. Neither is wrong — they just need different management.

The practical takeaway: when you increase sun exposure, increase how often you check the moisture level too. Don't assume the watering routine that worked in a shaded spot will still work once you've moved things into full sun.

Morning Sun vs Harsh Midday Sun

Not all sunlight is equal, and in Australia the difference between morning and midday sun is significant.

Morning sun — roughly from sunrise until around 10am — is gentler, lower in the sky, and less intense. It warms seeds and seedlings steadily without the stress that comes from peak-of-day heat. Most seeds do well with morning sun from an early stage.

Midday and afternoon sun in warm weather is much more intense. It can heat small containers rapidly, scorch young leaves that haven't toughened up yet, and dry out growing medium far faster than you'd expect.

Start with morning sun. Protect from harsh afternoon exposure, particularly through spring and summer. As seeds grow stronger over one to two weeks, gradually increase how much sun they're getting.

Warm season vs cool season

The right amount of sun changes with the season.

In warm weather, start with just one to three hours of gentle morning sun and build from there. The risk is heat stress — small containers in full summer sun can go from fine to stressed very quickly.

In cool weather, the sun is less intense and seeds can handle more exposure earlier. More sun also helps warm the soil, which is exactly what you want when temperatures are lower. In cool conditions, avoid positions that stay in full shade — cold, dark soil is slow to germinate.

The Problem with Windowsills

A windowsill seems like the obvious place to grow seeds indoors. It looks bright, it faces the sun, and it seems like a natural nursery. In practice, it's one of the least reliable spots you can use.

The issue is that light through glass is significantly weaker than direct outdoor light — often only a fraction of the intensity. It also comes from one direction only, so seeds lean toward it. And in cooler months, the glass doesn't let through enough warmth to properly heat the soil.

The result is usually leggy, leaning plants that struggle to support themselves — not because anything went wrong, but because a windowsill simply isn't a strong enough light source for most seeds.

If a windowsill is your only option, choose the sunniest one available. In Australia, a north-facing window gets the most light. Rotate trays daily to stop leaning. And move seeds outside as soon as weather allows — even a sheltered spot outdoors will give them significantly more light than any window.

How to Tell If Your Seeds Are Getting the Right Amount of Sun

The signs show up in how the plant grows — not in the soil. Here's what to look for:

Most light problems are fixable. Leggy seedlings haven't failed — they're just reaching. Move them to stronger light and future growth will be sturdier, even if the stretched stems stay stretched. Scorched seedlings need a few days of shade and a gradual reintroduction to sun.

Moving Seeds Outside: Take It Gradually

If seeds have been started indoors or in a sheltered spot, moving them straight into full outdoor sun can shock them — even if they look healthy. They've been in protected conditions, and full outdoor sun, wind, and temperature swings all hit at once.

The solution is a gradual transition called hardening off. Start in a sheltered spot with gentle morning light. Each day, increase their sun exposure a little. After seven to fourteen days, they should be comfortable in their intended outdoor position.

If you've already been giving seeds morning sun and slowly increasing it over time — you've been hardening them off. You just may not have had a name for it.

For more on this, check out Transplanting Seedlings: When to Move Them and How to Do It Without Shock

Common Questions

Do seeds need sunlight to germinate?

Not directly — most seeds germinate underground in the dark. What they need from sunlight at this stage is the warmth it provides to the soil. A sunny position will have warmer soil, which triggers germination faster and more reliably. A small number of very fine seeds need light directly on them to germinate and should be sown on the surface of the mix — your seed packet will say 'surface sow' if this applies.

Which seeds need light to germinate?

Fine seeds that are sown on the soil surface rather than buried typically need light to germinate. Common examples include lettuce, celery, snapdragons, petunias, and some herbs like oregano and thyme. These are pressed gently onto moist mix without covering. If your seed packet gives a planting depth, the seed can be buried and doesn't need direct light to sprout.

Which seeds need darkness to germinate?

Some seeds germinate better when covered and away from light — including onions, leeks, calendula, and some pansies. For these, make sure they're buried to their recommended depth and the tray isn't exposed to direct light until they sprout. The seed packet will usually note this.

Why are my seeds going tall and thin?

Tall, thin stems — called leggy growth — almost always mean not enough light. Seeds are stretching upward trying to reach a stronger source than they're currently getting. Move them to a brighter position with more direct morning sun as soon as possible. The stretched stems won't shorten, but new growth from that point will be sturdier. This is one of the most common and fixable problems in seed growing.

Can seeds get too much sun?

Yes. Signs include scorched or crispy leaf edges, soil drying out extremely fast, and wilting during the hottest part of the day even when the mix is moist. This is most common in Australian summer when midday sun is intense. Move to a position with morning sun only, or provide afternoon shade, and reintroduce full sun gradually once seeds have recovered.

Do seeds need 24 hours of light?

No — and it's not beneficial. Plants need a period of darkness as part of their natural cycle. For seeds growing outdoors or on a windowsill, the natural day-night cycle is exactly right. If using grow lights indoors, around fourteen to sixteen hours on and eight to ten hours off is a reasonable schedule.

What's the best window for growing seeds indoors?

In Australia, a north-facing window gets the strongest light. East-facing is the next best option — it gets morning sun, which is gentler and more consistent than afternoon sun. West-facing windows get afternoon sun, which is hotter and less reliable. South-facing windows get the least direct light and are generally the least suitable for seed growing.

Next Step: Getting the Timing Right

Sunlight sorted — the next piece is timing. Planting in the right season for your climate makes everything else easier, including managing sun and heat.

 When to Plant Seeds in Australia: A Practical Guide

For the complete guide from sowing to transplanting:

Growing from Seed: A Complete Beginner's Guide