You should be able to stick virtually anything outside by the end of May, bung it in but it might not flower or get very big. Lots of plants have a kind of locked in growth period linked to daylight length, meaning that they store energy up for flowering at the optimum time for the flowers to get pollinated and set seed before autumn. Sow/plant too late and they just go with what energy they have stored even if they are small.
Thanks for that, very helpful