To succeed in a garden business you need to avoid the pitfalls that cause most businesses to fail; and it really isn't all that difficult to do this, provided to take time to plan and execute a sensible approach when starting out.
Common Reasons why businesses fail are:
Poor Cash Flow (Often caused by charging too little; or not properly managing money when it does come in.
Bad Attitude (Successful businesses make the customer number 1. Failing businesses often treat the customer as if he is low on their list of priorities).
Poor Service (Gardeners who can distinguish weeds from plants, know how to prune without causing damage and can successfully identify pests are remarkably few and far between. Even many "qualified" gardeners often attempt tasks beyond their skill level. Making mistakes will loose you customers and mean no new customers through referral)
Poor Marketing (Some gardeners can do the job, but never succeed because no one is aware)
What about a Franchise?
Buying a franchise (eg. gardening or lawn mowing) can solve some of the common reasons for failure, but alone it is still no guarantee of success. Franchise Businesses are rarely as hassle free as much of a guarantee as what you expect.
References
John Mason's book "Starting a Garden or Landscape Business" is one of the few books on this subject and should be essential reading for anyone going into a garden business for the first time. Click to purchase a copy
What about Studying?
No matter what sort of garden business you go into, you will need to have two things:
A knowledge of Plants and an ability to look after them
Skills in Business Management (eg. basic bookkeeping, understanding law etc).
If you are only going to do one course the following are your best three options. You should choose from these on the basis of the amount of time you are able to devote to study:
Horticulture I -only 100 hours; a very solid foundation; but alone only part of a certificate
RHS Certificate II in Horticulture -to 150 hours, better; gives a UK accredited certificate
Certificate in Horticultre VHT002 -700 hours, bigger is better, gives IARC accredited certificate