If there is one guide in this masterclass that will save more of your seedlings than any other, it is this one. The single most common reason home-started seedlings fail - pale, stretched, floppy things that keel over or limp into the garden and sulk - is not disease, not bad seed, not a black thumb. It is light. Not enough of it, from the wrong direction, for too few hours. Fix light and half of all seed-starting problems simply never happen.
Why the windowsill betrays you
The romantic image of seedlings on a sunny sill is, in late winter, a trap. Here is the physics. Sunlight outdoors on a clear day delivers a colossal amount of light energy. A window - even a big, south-facing one - throws most of that away. The glass reflects and absorbs a chunk. The light arrives from one side only, so half the seedling is in shade and the plant bends toward the pane, straining. And crucially, the late-winter and early-spring sun, when you are starting most seeds, is low, weak and often behind cloud for days. The seedling is receiving a small fraction of what it evolved to expect the instant it broke the surface.
A germinating seed does not care about light. A seedling cares about nothing else. The moment those first leaves unfold, the plant switches from spending its packed lunch to earning its own living by photosynthesis, and it needs bright light immediately and continuously to do that. Give it too little and it does the only thing it can: it stretches, pouring its limited resources into a long, thin, desperate stem reaching for a sun that never gets closer. That stretch is called being leggy, and a leggy seedling is a weak seedling - thin-stemmed, prone to flopping, damping off, and transplant failure. You cannot un-stretch it. Prevention is the only cure, and prevention means real light from day one.
Signs your light is too weak: seedlings taller than they “should” be with wide gaps between leaves, stems that lean hard toward the window, a pale rather than deep green colour, and seedlings that fall over under their own weight. If you see these, the answer is almost never water or feed. It is light, and usually more of it, closer.
What “enough light” actually takes
The good news is that fixing this is cheap and easy, and does not require expensive specialist equipment. For years the gardening world has quietly known a secret the marketing does not advertise: ordinary fluorescent or LED shop lights work brilliantly for seedlings. The plain white “shop light” fixtures sold for garages, in the standard cool-white or daylight colour, put out plenty of the light young seedlings need, at a fraction of the price of anything with “grow light” on the box.
The reason is that seedlings are not demanding about the colour (spectrum) of light the way a flowering, fruiting plant is. They mostly want quantity and proximity. A cheap 4-foot LED or fluorescent shop light, or even a bright LED desk lamp for a small batch, held very close to the seedlings, beats a weak windowsill every time. “Full spectrum” and purple “grow” lights are not wrong - they work fine - they are just frequently a premium you do not need to pay for the seedling stage. Spend the difference on a second plain fixture and cover more trays.
The rule that makes it work: get close, stay close
Here is the technique that matters more than which light you buy: hang the light just a few inches above the seedlings, and raise it as they grow to keep it that close. Light intensity falls off dramatically with distance - a light a foot away delivers a small fraction of what the same light delivers three inches away. This is why a lamp “on” in the room does almost nothing, and the same lamp two inches over the leaves does everything.
For most LED and fluorescent shop lights, two to four inches above the seedling tops is the target. LEDs run cool, so you can keep them genuinely close without scorching; older fluorescents run warm, so give a touch more room and check the leaves are not too hot to hold your hand near. As the seedlings grow, they climb toward the light - so the fixture must climb too. A shelf setup with the light on adjustable chains or a simple prop that lets you raise it a little every few days keeps that critical few-inch gap constant. Set it once and forget it, and within a week the seedlings have grown into the light and started to stretch again.
If your only option truly is a windowsill, stack the deck: choose the brightest window you have, add any lamp you own held close as a supplement, and rotate the trays a quarter-turn every day so the bending averages out and no side is starved. It is a compromise, not a solution - but it is a much better compromise than an unassisted sill.
Hours matter as much as intensity: the 14-16 hour rule
Light has two dials, brightness and duration, and beginners usually forget the second. Seedlings do best with 14 to 16 hours of light a day. That is far longer than a late-winter day naturally provides, which is another reason the windowsill underperforms even on sunny days - the days themselves are too short. Under artificial light you control the day length directly, and a long day drives the steady, sturdy growth you want.
The easiest way to manage this is a cheap plug-in timer. Set it for, say, on at 6 a.m. and off at 10 p.m., and never think about it again - no forgetting to switch lights on before work or off before bed. A timer costs a few dollars and removes the most common human-error failure in the whole setup.
Do not, however, run lights 24 hours. Seedlings need a dark period - it is part of how plants regulate their metabolism, and continuous light can stress them. Sixteen hours on, eight hours off is a sound default. More is not better past that point.
Putting it together
The whole of this guide reduces to four sentences. Do not rely on a windowsill for late-winter seedlings; it is too weak, too one-sided and the days are too short. Use a plain, cheap white LED or fluorescent shop light - you do not need to pay for “grow light” branding at the seedling stage. Hang it two to four inches above the seedlings and raise it as they grow to hold that gap. Run it 14 to 16 hours a day on a timer.
Do those four things and leggy seedlings, the single most common home-starting failure, largely disappear from your life. Your seedlings come out short, thick-stemmed, deep green and tough - exactly the kind that shrug off transplanting and take off in the garden. It is genuinely the highest-leverage change most beginners can make, and it costs less than a couple of trays of nursery transplants. With light sorted, the next question is what these seedlings should be rooting into - the seed-starting mix, which has its own rules that trip people up.