There are two issues here:
1) Can you direct seed
2) Can you save vegetable seed

Tomatoes take direct-seeding in the ground quite well. Also, open pollinated varieties usually produce true to type with no special precautions, but you should read some of the info in the seed savers forum for how to harvest and treat the seeds. It's not difficult at all.

Grapes can be grown from seed but it is hard to get them to germinate and do not produce true to type. Even if you get seeds from that lovely table grape to grow, when they produce grapes in several years they will not taste like their parents. Grape vines also get huge and are difficult to keep as container specimens. If you want to try the container route, take a cutting from an established plant for which you like the grapes, and then root that.

I don't grow onions or nettles, so I can't speak to those plants.

Okay. I read your question differently. Maybe one of us will answer your question. If you don't have grow lights and a set up for starting inside, you want to know how to do it outside, right?

I do this sometimes. In the spring, as the weather is beginning to warm, I fill small pots with potting soil and sow the seeds into them. Not lots of seeds, maybe 2 or 3 of a kind into each four inch pot. (Be sure to label the pots with the name of what you sowed.) Water the pot well and let it drain for a good hour or so. Then, depending on the type of seed it is, I enclose the pots in a ziplock plastic bag, set it under a glass jar, or something of that sort, set it in a shady spot and watch it carefully. Usually in three to seven days, the seeds will show germination. At that point, I take them out of the enclosure and set them in a semi shady spot where I can continue to watch them. If the weather man calls for frost, I bring the pots inside, but take them back out when it warms back up. I fertilize the baby plants with half strength fertilizer every week or two and once they have 2 sets of true leaves, I cull out all the weaker ones and leave the one strongest plant. (Cut them out at the soil line. Don't pull.). When the plant is large enough to fend for itself in the ground, AND when the ground is warm enough for the plant to grow there, I put it into the garden. The entire process takes about six to eight weeks, depending on the type plant. It is, of course, imperative that the plant never be allowed to dry out.

Last year, I waited until my last frost date and sowed all my tomatoes outside on that date. They grew outside from day one and didn't need to be hardened off. They caught up to the storebought transplants in a hurry. The ground wasn't warm enough to receive them on the first "safe" date anyway.

If you have a seed that needs scarification (damp chill), you can sow the seeds into pots during winter and set them outside and they will come up on schedule in the spring, as long as the soil stays moist. My winters are usually wet enough that I do not have to water. Alot of flowers and ornamentals require a scarification period. Case in point: cleome.

These outside procedures will work for lots of things. Probably everything, if you are patient. Plants like peppers and eggplants have to have very warm soil and air to germinate, and the peppers take longer than tomatoes to grow to a transplantable size. This could be a problem for you if your growing season is short. I start my peppers inside 10 to 12 weeks before my last frost date, because they need that long to reach transplant size.

You are correct. Seed in nature falls to the ground and just comes up. However, you must remember that Mother Nature sows LOTS and LOTS of seed so that a small percentage of them will actually make plants. Since we never have that big of a supply of seed, we have to hedge our bet by learning what conditions are favorable to each plant and then try to provide those conditions.

I would advise you to read as much as you can about each plant you want to grow...and then just try. There is no replacement for individual experience: your climate, your seeds, your soil, your technique. You'll have some failures, but you'll have successes too. Each one will teach you something if you pay attention. Start with "easy" and inexpensive seeds at first. Be vigilant. Take good notes, and you will learn how to grow things.

If this is not the answer you need, or if it's so elementary that you already know this, I apologize ahead of time. Hope it helps.

really interesting read...

glad to know tomatoes are direct sow Nicole they are the first seeds Im experimenting with.

Hey Donna you sum up my attitude for the moment: "Be vigilant. Take good notes, and you will learn how to grow things."

Short post because late will keep updated...

semiotically, you can direct sow tomatoes or start indoors and transplant. My experience has been the same as Donna's -- I don't gain any time by messing with transplants and save myself much work. However, we are both in warm climates so your mileage may vary.

Hey Nicole are you still in US right (Nicole's 'mypage'). I'm in Southern Portugal, Algarve it's pretty warm here & doesnt frost so I'm being overly cautious - time to get going!

Just remember that "wild" plants are much more aggressive than "cultivated" plants. Most of them do not compete well with natives under natural conditions. If they did, you would not have to worry about growing them.

smeniotically - yes, I'm in the US. Your winters are milder than mine since we have several months of potential frost and hard freezes here in Alabama, but I think our summers are probably very similar -- hot and humid with warm nights.

Tomatoes do need warm soil (>60F), so if you aren't that warm, hold off for a bit and plant some cooler weather crops instead.

Has anyone answered your questions?

Semi, I think you are not a native english-speaker? Chances are you can read and write well in spanish, and there is a good spanish gardening site: infojardin.com. I participate there sometimes, and many there are quite tolerant of my poor literacy in Spanish. Minha lingua Portuguesa eh um pouco melhor.

I think perhaps you are asking about cultivated vegetables that can naturalize and become self-sowers. There are many such, and they vary a lot by climate and geographic region. I suspect many brassicas will naturalize in your climate, chards and spinach and beets, maybe carrots.