Many Americans grow plants for their gardens from seed.  Many others
buy established plants for their gardens from local greenhouses or
stores like Walmart or Home Depot.  I'm specifically interested in
what percentage of gardeners grow from seed versus buying plants from
a nursery or store.

I don't know if you are going to find an answer as to how many people
do one or the other. I suspect everyone who gardens does both.

Buying seeds versus buying plants has more to do with the plant than
the person. At the risk of generalizing, nobody but the diehard
gardener buys tomato seeds. They buy plants. And nobody buys pumpkin
plants, they buy seeds. This holds true for decorative plants as well.

Hybrid plants can't be started from seeds. They are sterile, so only
plants are available to gardeners. Some seeds have to be germinated
under very careful conditions not available to the average gardener,
so these are sold as plants.

Other plants germinate quickly and easily, and are only sold as seeds.
Spring annual flowers are sold as plants because people want instant
color for their gardens after the perennials (usually bulbs) die back.
 They don’t want to wait all summer for the seeds to sprout and
mature.

This brings us to growing season which also has a lot to do with why
some plants are available as seed or plant. A long growing season may
require plants to be started in a greenhouse well before the average
gardener is able to break ground outdoors, so people count on the
greenhouse to get the seedlings started and then purchase them as
plants once the garden (and weather) are ready.

And then there are bulbs, tubers, cuttings  ….. 

So, what I’m getting at here is that I would bet, unless a person has
a very specialized garden, 100% of gardeners purchase both seeds and
plants for their gardens.

Maybe, if you could tell us what you are trying to determine by asking
your question, we could help you rephrase it to get a more meaningful
answer?

Let us know how we can help…

-K~

Yes (almost) everyone that gardens plants zuchini seeds versus plants,
but not everyone, as the greenhouses around me are now selling corn,
zuchini and others as plants as well as seeds these days.

My question was more to this, of people that are growing, say,
tomatoes, how many are starting the tomatoes ahead of time from seed
versus waiting til later and buying an already started tomato plant
from a garden shop?

I'm not sure that this is known info either, but that might clarify
the question.