Direct Sow or Transplant?

Before you begin sowing you'll need to decide what method of sowing you're going to use. There's not necessarily a right or wrong way, it is possible to start most seed via both methods but you will find certain species definitely do lend themselves to one method over the other.

I personally choose to transplant almost all of our seeds, as that gives me just a tad more control of their germination. I do then make sure to take extra care not to disturb the roots of any plants that do prefer to be sown directly - in fact soil blocks are the perfect inbetween of direct sowing and transplanting, as their cells greatly reduce transplant shock.

These seeds are the best candidates for sowing directly into your garden beds, exactly where you want them to grow right from the get go. They generally are a little more sensitive to root disturbance than other seeds, but they can alternatively be sown into soil blocks as they're kinda like a hybrid between direct sowing & transplanting (so long as you don't leave them in their blocks for so long they eventually grow into each other, thereby undoing their less root disturbance advantage).

Slugs absolutely love seedlings, so direct sown seeds are very vulnerable to becoming slug & snail fodder, so you will need to scout or use slug bait to keep your seedlings safe.

Direct sowing I find is best suited to larger seeds, and seeds that don't mind a little covering (like sunflowers, calendula, zinnia) or seeds that self sow readily, and don't really love their roots disturbed (nigella, larkspur, orlaya, bupleurum, orach).

Seeds that need light are only worth sowing when they are cheaper (like some poppies, and amaranth) as you will inadvertently sacrifice some germination % by direct sowing them over sowing into cells/blocks. Also as they need light a little extra care is needed to make sure they don't either dry out in the sun or wash away in the rain.

These seeds can be a little more finicky to germinate so do benefit from being sown into cell trays or soil blocks inside, so you can really hone in and give them their exact conditions they need to get them to germinate their best selves. They'll then need to be grown on, hardened off (acclimatise from sheltered inside to more exposed outside conditions) then planted out in their beds.

Sowing into dibbles (little 10mm-ish) indents made with your fingers or a pencil is a very handy tip to help improve your germination in cell trays. It gives you hand space to add a covering over if need be, or if the seeds need light, they can hang out at the bottom of the dibble, get the light they need but still be partially shaded and slower to dry out than if they'd been surface sown.

One of the hardest parts of growing seeds in cell trays or soil blocks is mastering their watering. They have no connection to the ground and any natural moisture so they are fully reliant on you for their watering. And mastering the line between too much water and not enough can be particularly challenging as no day's weather or soil moisture requires are ever truly alike. Having young seedlings that are very prone to moisture swings somewhere that receives a couple hours or so shade does help reduce them drying out too fast, which does make them easier to manage. 

It is also however worth noting that most windowsills do not receive enough light for most seedlings, which will make them stretch and get very tall, lanky and weak stems aka "leggy". So once seeds have germinated they will need as much light as you can give them.

Cell Tray (50) vs Soil Blocks (35s). 

Ranunculus seedling grown in a cell tray. 

Wallflower seedlings in soil blocks (35s) – we often double sow (sow two seeds per cell) so we always have a very high percentage of seedlings per tray (so we don't waste valuable space).