Growing lilacs from seeds isn’t easy, but it can be done. It can take up to three or four years for a seed-propagated lilac to bloom, though, so you’ll have to be patient and just enjoy watching the bush grow!
From Garden Guides:
When growing seeds from a lilac, two things are of utmost importance. First, make sure you’re not harvesting seeds from a hybrid, as they are often sterile, and even if not, they generally won’t retain the characteristics of the plant from which they came. Second, never use seeds that have fallen to the ground.
You’re correct in assuming that harvesting is done from the flowerheads, but the seeds need to be whole, not broken or partially eaten by birds. Wait until the flowers dry to get the largest possible number of seeds.
A period of cold stratification is essential for the purpose of imitating winter conditions, a time during which the seeds would be moist and dormant if left outside. Your type of lilac seeds should be stored for approximately two months.
Begin by soaking the freshly-harvested seeds overnight in a bowl with enough room temperature water to completely cover them. In the morning, mix them with a handful of dampened peat or other sterile seed-planting medium in a ziploc-type baggie. Form the mixture into a clump and squeeze out almost all of the water, leaving enough so that they're uniformly damp, but not wet. You shouldn't be able to squeeze much more than a few drops of water out of the mixture. Seal the baggie so it’s airtight.
The best place to stratify the seeds is the vegetable drawer of your refrigerator, as it provides the ideal temperature of between 35 and 42 degrees fahrenheit. It also keeps them easily accessible so you can check on them every few weeks and lightly mist using a spray bottle with tap water if they start to dry out. Some people keep them in a garage, which is fine too. You may want to separate your seeds into a few separate baggies. That way if something goes wrong in one, you won’t be left empty-handed!
Keeping a consistent moisture level is also critical during the germination period. Setting the seed pots in a tray and watering from underneath is recommended, as it keeps the seeds from drowning. In addition, the new seedlings will be delicate, and can be damaged by watering from above. Bottom heat is indeed recommended, in the form of a propagation mat.
Homeguides recommends this step-by-step germination process:
- Prepare growing containers before removing the lilac seeds from storage.
- Fill 4-inch pots with a mixture of equal parts seed compost, horticultural grit and perlite. (A packaged sterile potting mix such as Pro-mix will also work.)
- Sow one lilac seed in each container at a 1/4-inch depth.
- Drizzle water onto the mix until it feels very moist at a depth of 1 inch. Do not let the lilac seeds dry out during the germination process.
- Place the containers indoors near a bright window. Warm the pots with a propagation mat set to 70 degrees F.
- Watch for the first sprouts in approximately one month.
- Leave the propagation mat in place for two more weeks, then remove it.
- Transplant the lilac seedlings into larger containers once roots appear near the drainage holes at the bottom of their original containers.
- Grow the lilacs in a cold frame until spring, then move them to a lightly shaded spot outdoors after the last frost.
Good air circulation helps during the germination period. Give your seedlings some space between them, and keep them in a well-ventilated room.
Finally, germination time varies greatly, so be patient if it takes longer for the seedlings to appear, and if you have a few seed pots that haven’t sprouted, don’t throw them away unless they have obvious mold. Put them out with the others, often you'll be pleasantly surprised to see them come up a few weeks or even months later!