I heard from a friend that Lunaria annua seed should be started now (July-August) (directly in the ground) since this is their natural time for starting (without human intervention). She actually says that this is the only time of the year one can successfully do it. Is there anybother plant whose seed can and should be started during summer?
Assuming that you are in a temperate region of the Northern Hemisphere, here are three examples of seeds you can sow in summer: kale, wallflowers, asparagus.
Kale is the member of the cabbage family which does not form a head or flower to provide the edible part; we focus on the young leaves. It is very hardy but does not like heat which can force it into flower and terminates its productive usefulness. So in this case we sow late to avoid summer heat but not so late that it does not have a chance to size up well. Kale comes into its own when the cabbage, broccoli and cauliflower are finished.
Wallflowers are a biennial that produces nicely scented flowers in the second year. Sowing too early might result in part of your crop trying to flower as the first winter approaches, so we delay a bit to ensure that this does not happen. Wallflowers are not very hardy and will be killed by a moderately cold winter if unprotected.
Asparagus is an extremely hardy perennial and can really be sown anytime; it will just go into the winter at the size it has achieved, go dormant and then carry on next spring. In this case timing is dependent on when you want to plant out in permanent locations and what size pot will be ideal for that operation. For a fall planting out from 50-plug trays (45mm.) a mid summer sowing fills up the plugs nicely without exhausting the resources.
Lunaria annua is, as its name suggests, an annual/biennial, and it's hardy. Plants classed as hardy annuals can be sown in late summer/autumn, or at whatever time they would naturally develop ripe seed if left to their own devices. The nature of an annual is to grow, flower and set seed in one year; the 'hardy' part means the plant produces seeds that will overwinter outdoors, or sometimes, if the seed ripens early and there's time, the seed will actually germinate in the same year, form a small plant, which then overwinters and flowers the following year (which is why it's classed as hardy annual/biennial rather than just annual). Forget-me-not (Myosotis arvensis) is another example of this type of plant.
In the natural order of things, the seed, once ripe, will fall to the ground once the seed case dries out and opens; the seed then germinates (if conditions are right) and the cycle repeats itself. However, many people collect the seeds and sow them where they want the plants to grow rather than leaving it to nature. They can be sown around May as well as in autumn; most sown in spring will flower later in the same year https://www.crocus.co.uk/plants/_/lunaria-annua/classid.2000014788/
If you want to know all the plants that you can sow this year or in spring to flower next year, you need to look at what group they're classed as - if they're half hardy annuals, they will need to be sown early in the year somewhere warm (like a greenhouse) before planting out in early summer. Hardy biennials are sown one year, and will overwinter and flower the following year; Digitalis purpurea (common Foxglove) would be an example of those.
The more you know the more you understand a plant, any plant the more likely you will be able to suss out what that plant needs to grow well, with minimal intervention.
Any seed can be started in the summer. Any seed can be germinated in the winter. One needs to understand a far larger body of intertwined processes to be able to artificially grow a single plant.
We now have artificial REAL grow lights. With the proper spectrums we humans can learn to control different phases of growth; vegetative or reproductive. The proper daylight hours/night time hours to induce reproductive growth. The growing medium absolutely necessary for potted plants. Watering techniques and timing to better simulate a plant's 'natural environmental needs developed by evolution'.
Every single thing we humans touch is made artificial. The definition of artificial. Yet the more we know about how things worked before we humans butted in will certainly help us to mimic those conditions and grow better healthier happier plants and our food.