If you go through some of the threads in the Visa section you will get an idea regarding the Visa status for work.

Generally, the answer will be that it will be hard. You are potentially competing for work with Japanese and other Foreigners alike. They would rather hire a Japanese for the position than a foreigner, posessing a skill maybe in your related field or luckily finding a company and job that will sponsor you will increase your chances. Plus your level of Japanese will have to be very high.

The reason why teaching English is popular and always in supply for foreigners is:
- It's a job that Japanese want english speakers. You get better learning efficiency through a native speaker who is teacher than someone who isn't a native speaker
- You do not require a specific degree to do it, any type of degree is generally accepted (level and years taken pending)
- You do not need any particular level of Japanese to get the job. Because the job itself does not entail the use of Japanese Language (you are teaching English after all)

Realistically, outside of teaching you require a high level of Japanese. The Visa's to get the other jobs are specific and you require your specific degree to get the job (depending on the Employers requirements) to then get your Visa.

I would not be entirely sure what Visa Horticulture would fit into anyway, possibly Artist? Or maybe Skilled Labour.

Basically, bottom line is that it will be hard. The Market for jobs in Japan for Foreigners lay greater in area's such as Teaching, Languages & Business. Of course there are other jobs outside of these, but amongst the mix they are very few and far between (thus making the competition greater).

If you want to get into Horticulture but also may think of comming to Japan. I would study your degree but in the meantime start learning Japanese and build up your proficiency in it.