If I purchase plants wholesale but pay sales tax on that amount, then
I place the plants in a customer's place of business, and maintain
those plants for a fee do I owe any additonal tax?  ( I still own the
plants, and when I am done with the maintenance contract with my
customer, the plants come back to me.)

As a seller, you must collect, report, and pay district sales tax
(called a transaction tax) or use tax on your taxable sales and leases
if you:

- Have a business location or are engaged in business within the district; 

(a)- Lease, store, or consume tangible personal property in the district; or 

- Sell or lease vehicles, undocumented vessels, or aircraft that will
be registered in the district.

You are engaged in business in a district if you are a retailer who:

- Maintains, occupies, or uses any type of office, sales room, warehouse,or
other place of business in the district, even if it is used
temporarily, indirectly, or through an agent; or

- Has any kind of representative operating in the district for the
purpose of making sales, making deliveries, or taking orders; or

(b)-Receives rental income from leases or tangible personal property
located in the district.

Point a and point b seem to be the most relevant to your situation
(plants are tangible property)

you can read up on it on these pages 

http://www.boe.ca.gov/sutax/faqpurch.htm

search key used : california sales tax


Note : I am NOT a google reseacher and I am NOT in any way or form a Tax advisor

I'm not a tax professional.

I do not think you should be paying a sales or use tax based on the
wholesale amount when you purchase it.

You should be paying a sales tax on the retail amount when you sell it.

I don't think you owe a sales/use tax on your maintenance income. 
Sales/use tax pertains to tangigle property.

I would just call the California State Board of Equalization for
verification, which they will happily answer for free.