I suspect you come to a garden blog to see lovely photos of plants, not the above! This is my so far unsuccessful attempt to germinate seeds of the giant milkweed, Calotropsis gigantea. But, I digress, and I haven't even started yet!Over the years, I have used many methods of seed starting. The first is direct sowing. Some plants, especially ones with large seeds like beans and squash, generally do well when sowed directly in the ground after the last frost in your area. Side note: how to figure out your last frost? Dave's Garden has this handy search box- just enter your zip code and get your frost dates:
https://davesgarden.com/guides/freeze-frost-dates/
and the USDA has this information in map form
https://images.app.goo.gl/xSVZwnm5MJw5mXGi7
Why direct sow? Can't you get a jump start on growing these seeds, too? Yes, you can. But if you are like me, with limited room for seedling flats and lights, it is not a great use of space. These seeds do well on their own in the garden (except for rabbits at times, but that's another blog). The seed pack will tell you if the seed is good for direct sowing and when.
Direct sowing is also a good strategy for cool season crops, or plants that do not transplant well: peas, lettuces, greens, and root crops. Some people do get ahead start on peas, lettuces and greens indoors, but I find it is typically not worth it, again due to space limitations.
Starting indoors: some plants have a longer growing season or you may want to harvest their fruits earlier (think...tomatoes, the number one vegetable plant for home growers). Yes, you can start them outdoors, after the last frost...and wait a month or two longer for fruits than with transplanted seedlings. The other problem is that some of the most beloved veggie plants (the solanaceae: tomatoes, eggplants, peppers) need really warm soil to germinate, hence the need for bottom heat germination using seedling heat mats. In the photo above, you can see a heat mat under my milkweed seeds. I would not try to start members of the solanaceae without one. They cost around $25 and up for larger sizes, but mine have lasted for years. I have seen advice to use an old heating pad, the kind that does not automatically turn off, but I would avoid this option as they are not made for this use and may overheat. Some people also put planted peat pots directly atop water heaters or furnaces, but the pots can dry out quickly and you may damage your appliance.
Top or surface sowing: Some seeds, like petunias, need light to germinate. These seeds are often tiny, and have little nutrient reserve, so need moist soil and light. I typically scatter them on the damp, seed starter-surface of a flat or part of a flat, and hang my T-5 shop light as close to the soil surface as I can. Check regularly to make sure they do not dry out. As soon as they have two true leaves (the first leaves are seedling leaves, wait for the next set) I gently transplant them into individual peat pots filled with damp seed starting mix, NOT regular potting soil (use seed starting mix for ALL your seed starting). Petunia seeds, by the way, are like dust and you need work with them carefully. One sneeze and that's it!
Stratification: Sometimes you may buy seeds (especially from an independent grower or hobbyist, or that you have collected yourself) may need cold stratification (less common is warm stratification). Many seeds (common milkweed, many north American tree seeds like the pawpaw or persimmon) need to be chilled for 6 weeks to several months. This is typically achieved in the fridge. Seeds are placed in a damp paper towel or in slightly dampened peat moss, in a zipper bag, and put in the fridge for the needed period of time).
Other oddball methods: There are other methods for specific plants. Some tropical seeds may need supplemental heat and high humidity (you can put the seedling pot in a plastic bag, on a heat mat, checking frequently for mold). I am trying to start the giant milkweed seeds above using the water method, which I fear is not working. I am soaking the seeds in dechlorinated, 80 degree water, changing it out twice a day, on a heat mat. They typically start to sprout in 7 days, but it has been 7 days, no joy. I also soaked some of the seeds overnight and put them directly into peat pots on a heat mat with cover...fingers crossed!
Some plants are best started (or more easily started) from rooted cuttings (African blue basic, some houseplants like rex begonias) or rhizomes (tulips take 7 years to flower from seed, so unless you are a breeder...)(ginger and turmeric- I will update you on this year's attempts soon).
When to start seeds? This is typically listed (along with how to start) on the seed pack (e.g. after last front date in your region, 4 weeks before last frost day, 8-12 weeks before last frost date, etc). Seeds packs give all sorts of good growing info: is the seed annual, biennial, perennial; what zone does it grow in; what are soil and other requirements...etc.
Happy Seed Starting!