I want to plant some flowers/small plants that are perennial and beneficial to insects (therefore beneficial to birds)
what species are best and
when do I plant flowers/plants that grow in summer?
The answer to this is as long as the preverbial piece of string! If you're a starter with gardening for wildlife, you can't really go wrong with loads of herbs allowed to run to flower and seed in planters/tubs or in the ground if you have good enough ground.
I have sage, which is absolutely loved by bumble bees, rosemary, thyme, marjoram (honey bees and all kinds of insects love that, it's always covered) coriander (replanted through the summer), nasturtiums and mint. You can buy pots of rooted herbs in the supermarket for around £1.00 or so and these make great summer flowering plants when you pot them up in a bigger pot and leave them outside.
Some you have to replace each year and some will establish and thrive on their own. When you buy them depends on their own season but with most herbs you can plant them out when the frosts are over (mid May is my usual bet, but I sometimes risk it before then).
I would also consider a buddliea, they grow into quite large bushes but you cut them down annually. They are superb for butterflies, moths and bees. Growing sunflowers (multiheaded type is good) is usually rewarding as well - insects in summer and birds feed on the seed heads in winter.
There are tons of other native shrubs and trees that you could consider if you are looking for something permanent, but I've typed enough for one post!
:t:
P.S. I forgot to mention that nasturtiums usually attract white butterflies to lay eggs and produce lots of juicy caterpillars for the birds! Your nasturtiums end up a bit tatty but it's benefit all round