You didn't give us soil info, moisture, exposure, sun? That would help.

Cleome and poppies reseed most reliably in my zone 6 garden, but only since I learned to recognize the seedlings and eliminated mulch/leaf cover in the beds.

Cosmos, California poppies, breadseed poppies and marigolds reseed freely here.

Columbines, cosmos, cleome, foxgloves, dame's rocket (to the point of invasiveness here), lunaria

Lunaria is my most reliable reseeder. I can always count on a new patch of them, and if you plant white bleeding heart where they grow, they will bloom together and make a very pretty spring picture. Then the bleeding heart will go into dormancy as other plants take their place. I am also careful not to mulch in that bed. Columbines, cosmos, cleome, are very good here too. Reliable. Larkspur, California poppies I usually find here and there, but not heavily. And the California poppies peter out after I haven't sown any in a few years.

I sowed a LOT of alyssum around the edges of a bed a few years ago and the past two I haven't had to plant any new seed, it seems like the whole edging reseeded. Sunflowers will often reseed for me, if I let the seed drop, although the birds will often eat most of them. Portulaca is another edger that comes back for me, but peters out if I don't reseed for a few years. Or maybe I'm pulling out the seedlings. Morning glory is also very dependable and I leave the seedlings where they can climb. Up into shrubs, at the base of sunflowers, near a fence. They are not invasive here. Foxglove will sometimes reseed for me but not heavily enough to suit me.

Morning glories reseed to the point of being invasive, and in fact they are illegal in Arizona for that reason.

corn flower . aka annual bachelor button .. aka Centaurea cyanus

https://duckduckgo.com/?q=corn+flower&t=ffcm&ia=images&iax=images

mickey mouse viola .... the one with blue ears

https://duckduckgo.com/?q=Viola+cornuta+%27Rocky+Mickey%27&t=ffcm&ia=web

plus all the ones mentioned above .....

ken

ps: you dont mention height ...

ken

Larkspur! Everywhere in my yard. It hangs out best in the mostly sunny, very moist part of the garden. It probably does well other places, but that's where mine is sited. And I love the ferny foliage, blue (I guess there are other colors, but I am a blue freak), tall...

dbarron, it is a location which consists unamended clay soil, getting no supplemental water or weed control and receives about 5 hours of direct sun.

(I would surmise that our location is maybe a USA zone 5.)

Coreopsis tinctoria (dont know if you have the water for it), rudbeckia triloba, coreopsis lancelota (short lived perennial). Gaillardia pulchra (if it will grow there), it does grow on caliche and clay here.

Chances are not all will do, but I don't know your region enough. I can't grow larkspur to save my life, so though I like some of the other suggestions..

(dbarron, I can confirm that rudbeckia triloba has been successful as a reseed),

This is the only sort of 'gardening' I do in the wood...but I have found a few reliable reseeders which can compete with grass and weeds, require no extra water and will generally perform under less than stellar conditions. Top of the list is hesperis matronalis, sweet rocket, which will cheerfully partner alchemilla, meconopsis cambrica (there is a rather nice red form - 'Frances Perry' if you can find it). Centaurea, gypsophila, ammi visnaga, ox-eye daisies, vaccaria, silene, saponaria, dianthus carthusianorum, blue and red flax, ridoplhia segetum, mollucella laevis, euphorbia oblongata (although classed as an invasive in some states). Prairie Moon, with whom I concur, suggested lunaria...so I must endorse the variety 'Corfu Blue (seeds available stateside from Special Plants) - a truly outstanding variety.

I have had some success with biennials - campanula, digitalis, myosotis cynoglossum amabile, while perennials such as hellebore, stylophorum, salvia pratensis and aconitum have been generous re-seeders, while my most fervent hope is for tulipa sprengeri along with t.sylvestris, the only tulips which will tolerate some shade) to colonise the understorey. It readily reseeds but I have had a series of catastrophes so still only in year 2 of the sprengeri campaign.

My best reseeders, a combination of annuals, biennials, and short-lived perennials include Nicotiana, worth growing for the wafting evening scent as well as the flowers that range from lime green through white and pink to deep red; breadseed poppies; foxglove/Digitalis; and native columbine/Aquilegia canadensis. Five hours of sun would work for any of them, but IME they don’t like having soil surface competitors, and so the surface would need to be cleared in fall or early spring in order to get good germination.

Nasturtiums reseed fairly easily. And as an added bonus the flowers are edible (I think the leaves are too but not 100% sure).

Nasturtiums self sow for me too and the leaves are a regular salad ingredient for us. The flowers tend too. Curiously they seem to be able to self seed even when frost catches them while the seed is still green.

I haven't had much luck with Nasturtiums coming back on their own. I have trouble getting them to germinate even when I pre soak them. [g]. And then when they do, they don't always do well until the fall. They grow poorly most of the season and then when the weather cools in the fall they grow gangbusters. It's very odd. I know they are a cool season plant, but it's always a toss up whether I'll bother with them. I did find last season, that when I put rain barrels up on cement blocks, I used the holes to sow some Nasturtiums to disguise the blocks, along with verbena and alyssum. Only the Nasturtiums did well, so that will be a position I will repeat next season.

NHBabs - I usually intend to add Nicotiana, but I rarely do. And I'm always looking for fragrant flowers. Maybe next season I'll give it a try. Which Nicotiana seed do you use? And they reseed in your zone 5?

PM2, it has been so long since I planted my Nicotiana, I no longer remember their variety, but it was a single six pack of plants, most likely hybrids, that have perpetuated themselves for 20 years or so. They are easy to pull where they aren’t wanted and bloom from mid-summer until frost. They are nice planted near a window or an outdoor seating area, but with the size of your property, just a few will be enough to scent the air for the whole area in the evenings. I imagine that some of the hybrids may have had scent bred out of them if they were concentrating on flower size or color.

Two more I forgot: old fashioned hollyhocks and wild type Rudbeckia, both biennials, seem to self-seed regardless of how crowded the space is. And annual Cosmos reseed reliably as well.

My verbena reseed.

Down on the allotment just now hoeing out the Love in a Mist. That's another crazy reseeder. Not annuals, but hollyhocks and Matricaria are also.

Kitasei what zone are you in and what kind of soil do you garden in out of curiosity? I've never grown nicotiana but if they truly reseed that easily I might just give them a shot. Also, after reading reviews on davesgarden, several people mentioned that nicotiana alata and nicotiana langsdorfii reseeded for them. I think a lot of flowers will reseed but ONLY if conditions are favorable (if the soil temperature is what they need, if there is enough rain, etc).

Other plants that reseed are columbines, sweet william (dianthus barbatus), and borage. I've only grown columbines, but my neighbors have grown sweet william and borage in the past and they seemed to reseed fairly easily for them.

Perennialfan, I know you asked Kitasei, but my Nicotiana reseed readily in sunny spots in my moist, well-drained acid sandy loam on the northern edge of zone 5.

Mine reseed in a warm, south facing, unmulched site in zone 6b New York. Soil is acidic loam, not fertilized or irrigated particularly...

I thought of one more perennial that reseeds freely, regular fern-leafed bleeding heart, Dicentra eximia.

Add rose campion to the list also.

My annuals that reseed are Monarda punctata, and M. citriodora, Ipomosis rubra, Palafoxia, Bluebonnets, Callirhoe leiocarpa (annual winecup), Blanket Flower. My sweet peas are coming up all over. I did not plant them this year. It has been a wettish dampish winter. No floods but lots of tiny gentle drizzle..

Nicotiana sylvestris reseeds here but sometimes it goes away for many years and then will come back out of the blue.

The most invasive reseeder here is tall phlox. I like it but it takes up a lot of real estate.

Other good reseeders: larkspur, lunaria, rose campion, obedient plant (invasive), rudbeckia, Zebrina hollyhock, hollyhock, impatiens, vinca, and I usually get one mullein plant in a certain gravelly area. Have had thunbergia reseed. Have too many morning glories. Used to have some poppies that reseeded.

Silene coronaria (rose campion) intense bloomer, can reseed pretty much on anything, almost invasive. Biannual.

Did anyone mention forget me nots?

Petunias here!

snapdragons, forget me nots, moss rose, hollyhock, zinnia, johnny jump ups

cerinthe, linaria, eryngium, omphalodes linifolia, legousia speculum-veneris. molucella laevis, lathyrus tingitanus.

Lobelia siphilitica & L. s. 'Alba' are perennial but seed very well, almost too well if not deadheaded. Typically it likes moist soil but I have found it to be happy where ever it sets down. Easy to pull if it is in the wrong spot.