Poppy

Collect seedpods when they are dry in late summer, and scatter them on tree bases and path verges. You can also collect them in jars or envelopes and scatter them in spring. The flowers will keep coming back every year. 

These are field poppies, but other kinds work also work, for example opium poppies, which are taller.

Nigella

Also called love-in-a-mist, they have beautiful blue flowers and a longer flowering season than poppies. Exactly like poppies, collect seedpods when they are dry in late summer, and scatter them everywhere you go. They will come up every year, and spread around the general area. Bees love them.

Like poppies, the seeds are edible and used in Indian cooking (kalonji)

Hollyhocks

Once one of these plants is established somewhere, it will come back every year and also tend to seed itself around the general area. It has tall flower spikes visible from far away, from July to September. The only catch is that they fall over easily when unsupported or unsheltered, so it's best to grow them in a place where they have a fence or other bushes to lean against, or be prepared to stake them or cut them when they get in the way.

It's easy to collect the seeds from the dry flowerheads, and scatter them where you'd like them. If starting from scratch, it may be best to start with a potted one.

Forget-me-not

One of the earliest things to flower in Spring, an absolute joy after the dreary winter. A single little plant one year will turn into hundreds by the next, and the abundant seedlings are extremely easy to pull up and plant anywhere you like, in winter when the ground is wet. They'll grow in the most difficult spots. 

After they finish flowering they can look untidy, so it's best to pull them up, but they come up very easily.

Marigolds

Once you planted just one of these in spring, they will pop up everywhere in the general area later the same year, growing more numerous every year after that. Their bright orange flowers keep going all the way from spring to late autumn. Careful, they can crowd out other small plants. 

Similar to forget-me-nots, the abundant seedlings can easily be lifted and replanted wherever you want them.

Buddleia

Also called butterfly bushes because pollinators love them, they get a bad rap because they like to grow on waste ground so many people consider them weeds. But planted in a sensible place where it won't get in the way, and pruned occasionally to keep it tidy (it gets quite bushy), it looks lovely and requires no attention at all.

Cut off a small branch and stick in a plant pot with damp soil to make a cutting, then plant it out when it starts putting out new branches.

Verbascum

Tall sturdy yellow flower spikes, once you've planted one you can expect to see more come up in the general area the following year. When dry, break off the spike and sprinkle the seeds elsewhere. If the seedlings come up in inconvenient places, just lift and move them to a more suitable spot.

Verbena

One of the best flowers for pollinating insects, they have a very long flowering season. They will also seed themselves around the general area, often in paving stone cracks. They're quite tall and spindly, so work best in an area where they can lean against other bushes or fences. Drought-resistant, they like it sunny.

You need to cut them down in autumn to keep them tidy, and that will also give you seeds to scatter elsewhere. You probably need to start with a potted plant.

Mexican daisy

You have to buy these as small plug plants, I've not had success with seeds. They take a little while to establish and they're quite fragile until they do. But once they've established, they'll be there every year, need no maintenance, and gradually spread wider. They love filling gaps between bricks and paving, so you can grow them even where there isn't visible soil.

Black-eyed susan

A little more work and expense than the others. You need to buy plug plants, you can sometimes get half a dozen for £10 or so. And then they need regular watering to get them started. But once established, from August to October, they grow strongly and just keep on flowering. The bright colours are visible from far away and they can really cheer up an ugly location.

Nasturtiums

Bright cheerful colours and edible in salads. The hard pea-like seeds are cheap to buy, and once established you can harvest many more than you started with. Stick them in the ground in tree bases and other neglected areas. But choose areas with damp soil if you don't want to have to water them. 

In areas where they are very happy they'll grow rampant, but in my experience they usually stay fairly small. 

Lemon balm

From the mint family, this will completely take over an area once established, and impossible to get rid of once it has, so consider carefully where you put it. But it looks attractive for a very long time, smells nice, and is good for insects. And it will totally crowd out all other weeds. 

Grows easily from small cuttings. I prefer to cut it down to the ground in late autumn, which is a bit of work.