Well, I applaud you first of all for wanting to open an organic center. Depending on where you live will determine the marketability. I would advise to start with the center and offer design/installation as well. That way you will cover both areas of interest and generate leads on both ends.

There are probably lots of designers/installers around you, but not many if any organic supply centers. You can do wholesale to public as well, and if you do it can be operated as a farming business as long as you are growing things yourself even if from plugs (if you can find a supplier of organic plugs) to save on taxes.

Be advised…..There is a lot of back breaking manual work involved in landscaping. Heavy lifting, trench digging on super hot summer days,

Any business is competitive. If you have no experience in retail or landscape design or education and training, you will be eaten alive by those already successful at these things. Get some education or experience in the field, then try to start a business but understand that even when well prepared, new businesses fail at very high rates.