supposed to be garden design, installation and maintainance.

Do you want to do this because you have experience, insight, and capability such that it will make you a competitive force in your landscape market or because it is so easy to do and you can't find a job? You worded it, so that it sounds like the latter.

i'm not trying to do this just because i can't get a job. the only reason i've been trying to find a job in the green industry is because i wanted a little more professional experience than i have. i do have experience, i'm 4 classes from away from a degree in ornamental horticulture, which i plan on finishing in the off season. i've been gardening seriously for ten years and have designed gardens for several people, but it was done as favors for the individuals. i've been told that i have insights into design principles, both interior and garden, i'd like to think thats true. as for the market, i don't know as there's anyone else in my area that is in this line of work. i worded it that way, because i can't afford to do what i really want to do and open my own nursery. i thought this would be a good way to break into the industry. it's a lot of hard work, so it should be fairly clear that i'm not in it because it's easy.

Pretend you're a client and look for those services. Drive around and see what trucks are in the area. I don't know how many second home owners fuss with containers- maybe they would if someone was coming around to water them. See what the local nurseries have to say.
Then figure out your start-up costs- licensing, insurance (esp. for installations), equipment and marketing (logo, cards, stationary, ads, fliers, etc.). Figure 25-30% of income goes to the government (income and SSI) and health insurance.
If it looks like there's money for food and shelter after that- go for it. You have an interest and the desire, and have started preparing for this already. Now may just be the time for you to do it! (also consider a part-time job at said local nursery, especially if they don't offer design services)

Good.

You'll have to fight the usual problem that most have when starting out. That is the battle to stay focused against the battle to stay busy. You'll be tempted to diversify in order to get more work while specializing reduces overhead and keeps you efficient.

Maintenance is a great way to get busy quickly and get introduced to a lot of people. It is a good way to build a captive market (in other words, you'll be the first they'll call when they want something more). The difficulty is that it can suck up most of your time during a week which limits your ability to take on bigger projects because you can't get them done in a timely manner.

In the end, it is more profitable to be able to sell marked up product along with your hours, so installation is more profitable. It is harder to make a years income selling only seasonal hours of only yourself.

I might suggest that a part time job unrelated to the green industry might work well because you will not have to worry about having competing interests with your employer.

Always look to provide services that are not generic enough to be of little value. Although your time is valuable to you, the price a consumer will put on it is related to how much they want it done and how much they want it done by somebody else. Be valuable and you will be valued.