Planting Annuals in Greenhouse - timing/methods

men8ifr

Quite a lot of questions here for a beginner:

1) What is the reason to plant seeds in a tray and then pot on later? Obviously potting on is quite a lot of effort, why not just plant in decent sized pots 

2) Do you find with modern seeds that planting 3, throwing away 1 actually gives you better plants? Usually all seedlings (that sprout) seem to be healthy and a similar size... 

3) What is the earliest that annuals for next year could be planted from seed given a greenhouse? Is earlier better or will it cause problems, is it only frost that is a threat or the general low temperatures as well? 

edhelka

1) Not every seed germinates, not every tiny seedling survive to maturity. Bigger pots take space. And it is much easier to overwater (and kill) a tiny seedling in a too big pot.

Fairygirl

Hi @men8ifr. Seeds need very little medium in order to germinate, so a tray is often a more economical way of sowing. It also means less chance of them being drowned [or affected by other problems] by overwatering, as there's less soil to moisten. 
It does depend on the seed/plant though. 
The seeds sowing in a greenhouse depends on the seed/plant too, and your own general conditions and climate. I rarely do it [the only annual flowers I do are sweet peas though] because by the time I can plant out and have them growing well, spring sown seed germinates and catches up. If conditions allow, sowing in autumn can give you a head start with plants, as they're ready for planting out in spring, and normally early autumn is the best time. Hardy plants need very little other than a bit of shelter from the worst of the elements. Half hardy plants need more care. 
I rarely throw any away. Again - it would depend on how you're growing, and what you intend to do. It's usually for veg growing that thinning is required, and is beneficial, to allow plants to reach their maximum strength. Not all seedlings do well when transplanted either.
Not thinning means more competition for nutrients and water etc, as well as problems created by plants being too crowded, like poor airflow leading to more pests and diseases. 

Astro

To add seeds are better started in a low fertility medium, once they are away they may require something richer.