Not planting the whole packet of seed is usually a good idea. Some special, expensive to produce, seeds don’t have lots in the packet, but most have plenty, a packet of Little Gem lettuce I bought has 2,000 seeds, that is extreme, but most have far more than you need.
Take planting runner beans, I will want sixteen plants, two ‘tepees’ of eight each. Very early I planted about two dozen in pots in the greenhouse, there will always be some that don’t germinate and the odd runt. When they were about nine inches tall I planted the same again, plus some to give away. Runners won’t take a frost, so to be sure it is unwise to plant them out before June, but sometimes we get an unseasonably mild spring, and beans planted early crop early. If my first lot, planted out in mid-May, don’t make it because of a late frost, no matter, I shall simply replace them, If they make it then I shall have early beans, and those I plant out later will extend the season.
This policy of spreading your chances is a good way to be sure something works, and to learn what does, though what works one time may not work the next and, conversely, what doesn’t work one time may work the next. You can vary many things, planting times, watering regimes, compost mixes, depth of planting, and it is often worth doing a few each way. Those extra seeds can be useful and instructive. Remember also that most seed will keep over, some for years, even if it says ‘best before last year’ on the packet it is often worth giving it a go, “Damn, germination is down to 10 % and I only have three hundred seeds left, what will we eat when we finish these thirty cabbages?”
Some seed, like lettuce, are best sown in situ when it is warm enough, but sowing indoors I sow a half tray as thinly as I can manage. I will probably get at least a dozen usable seedlings. With small seed like basil or lettuce the instructions usually say something like ‘cover finely with approx 1/16th inch of earth’, it is just about impossible. What I do is scatter seed thinly over part of the tray, then muss the surface up with the tips of my fingers to spread them all over. Some get buried a bit deep, some sit on the surface, some are just right. What do I care if only a third germinate? It will still be plenty.
After germination comes potting on, where having seedlings thinly sown pays; seedlings that are on their own can be lifted with no root disturbance at all sometimes. Take the seedlings as soon as you can handle them, so the root is still separate, and always handle the seedlings by the leaves. Roots have tiny, fine, fibres that are easily damaged, and plants have only one stem, which is vital, leaves they will grow more of, and they carry on working even if a slug eats a chunk out of them, leaves are tough, relatively.
If you have several seedlings growing together it may be possible to separate them, but if not, never mind, you don’t have to save them all, plant a bunch of three or four and pinch out all but the best so you don’t disturb its root. Never bother with anything that looks weak, be merciless. The beginners most common mistake is trying to grow everything; even I grow too many of most things, but I give them away early or compost them, I don’t try and grow them all on. However, having a few too many at any stage is an insurance if you spread them. Allow for the unexpected; when those in one place get slugged, or mildewed, or the cat scratches them up, you still have those in the other, if nothing goes wrong you can keep the best and lose the rest.
Have your pots ready, loosely filled, poke a hole big enough for the root to drop in so it’s out of the earth for the minimum time. Holding the seedling by the leaves with one hand drop the root in then fill the hole with the other hand, letting soil fall in so it does as little harm as possible to the root. I fill the pot concave then tap it to settle the earth in, you will quickly learn how deep you need to plant, then water in to settle everything and get the plant into direct contact with the earth.
People often feel bad about rejecting seedlings. Think of this, an oak tree will be mature enough to produce acorns within fifty years. If it lives its life out it will go on producing them by the thousand every year for another three hundred plus years. To maintain the number of oak trees one of them needs to grow. That is the extreme case, but most plants approach this, there is only room in the world for the plants you really want, the best ones.