It is recommended to plant lavender from cut branches instead of seeds. Why?
Here is the answer:
Starting Lavender from seeds sounds like a great inexpensive way to get all
the lavender you desire but it can have some major drawbacks.
The first obstacle is finding the seeds. Even though Spanish, Yellow, and
other species Lavenders can be started from seeds, it is usually only the
Lavandula angustifolias--Hidcote, Vera, and Munstead-- that are available as
seeds.
The second drawback is what we call 'low and slow' germination. Lavender
seeds have a short shelf life, and therefore the germination rate (how many
seeds out of 100 come up) is usually pretty low. They can also take a long
time to sprout (two weeks or more) and this invites fungus to the seed tray,
often causing the seed to rot before it can sprout. Seeds benefit from
light, so cover lightly when sowing. The germination temperature should be
around 70 degrees and spring seeding is more successful than fall seeding.
Those seeds that do sprout will take one to three months before they have
enough roots and top growth to allow successful transplanting. Adding
fertilizer to the sterile medium used in the seed tray can help the little
plants get off to a better start, but it can also invite fungus in cool,
humid situations.
The third disadvantage is the time it takes for the seedlings to get to a
good size. After they are transplanted into small pots, the plants will be
about three inches tall and have a single stem. It will take another three
months or more to make a plant substantial enough to transplant to a larger
pot or to the garden.
The fourth inconvenience is the difference factor. Because little care has
been taken over the years to insure that the seeds have not crossed with
each other, the plants will be varying shades of color. They might also vary
some in height and width. This was the surprise our customer had. The
perfect hedge of Hidcote Lavender she had dreamed about and worked so hard
to grow the plants for turned out to be more like a cottage garden: still
beautiful, but irregular in form and color.
And, lastly, the most popular Lavenders (the Lavandula x intermedias;
sometimes called Lavandins), either do not make seeds or the seeds are
sterile, so you will never see a seed packet of these.