How do I know when my young plants (from seed) are ready to go outside?

alexemmersonuk

Help! I still don't know what I'm doing!!! Haha!

I sowed some zinnia seeds, sunflower seeds, tomato seeds, cosmo seeds and felicia seeds this year and I have NO IDEA how to tell if / when they are ready to go outside. 

I did ask a similar question last month, and I think the main reply had to with frost.

But I am confused: should all my young plants go outside the minute there is no more chance of frost? Is that the main rule here? 

Or does it also depend on how young / tall / sturdy they are? Is it a matter of common sense / trial and error or are there some steadfast rules I can follow to know when they're ready? 

I have a (very) small greenhouse with a couple of shelves but the plastic sheet around it has a tear. 

I also read that it's a good idea to put the plants outside for a while, acclimatise them and then bring them back in in the afternoon. How long would you do this for? A week? Two? 

Any advice on this appreciated as I think my little plants are doing quite well and I don't want to damage them. 

Pictures coming up too  

Thanks

alexemmersonuk

Here are the pictures:

JennyJ

The main risks are frost (tender plants like your tomatoes and cosmos will be clobbered no matter how big they are) and slugs/snails (smaller plants will be less able to cope with damage than when they've got bigger and stronger).

I think only the sunflowers are hardy in most parts of the UK, but even so they will be checked if they go straight from a warm windowsill to the outside. Yours look big enough to start that process if mild-ish weather is forecast where you are.How long the hardening-off takes depends on the weather to be honest. Maybe a couple of weeks but there's no definite rule that will work everywhere in all conditions.

I don't put my tomatoes outside until late May (they were only sown last week and have just started to pop up). By then it should be mild enough that they can just be fleeced at nights for a week or so. Same for cosmos - I sowed those yesterday.

carletonexotic

The main consideration for me is frost, but also the smaller the plants, the more susceptible they are to pests like slugs. I'd hold off for a few weeks, let them get bigger to give them a better chance outdoors.

punkdoc

Zinnias are very tender, I haven’t even sown mine yet. They won’t go out until June.