I don't know why your question intrigues me so much ... maybe it's the mystique because you didn't say exactly what you do! I'm assuming landscaping (design or maintenance), or tree planting, or working in a garden centre, or ...
Anyway, your question wasn't about what you do in the summer months, but about what you could do over the winter. So, a few thoughts off the top of my head:
1. I know someone who is a zookeeper, but not enough seniority to work full years yet. She works retail for the rest of the year, at the same place for the last few years so they are happy to hire on summer staff when lots of young people are available, and take her back on when they go back to school.
2. Landscapers often spend the off-season doing snow removal - depending on climate in their area, of course. If you're up for self-employment, and in a cold place, this could work for you. Could require investment in truck/plow, or you could do it on a smaller scale (i.e. residential) with a snow blower and a little trailer to move it from one street to the next. I used to pay a reliable neighbour kid $20, more or less depending on level of snowfall, and he had more clients than he could manage on heavy snow days, when his parents would have to help him out.
3. Some areas have flexible standards for supply teachers - if science background is in demand in your area this could work for you.
4. Again, in terms of self-employment, office cleaning and office plant care for large corporations could be a possibility - this would be year round, of course, but you could then turn around and contract it out to a student for the summer months.
5. Put together a variety of jobs at different times of the year. My sister-in-law spends a couple of months before Christmas working at a place that makes up gift baskets. She hates it, but she makes enough during that time to fund her art supplies and spending money for the rest of the year. There is also seasonal work in retail, working for elections or the census periodically. You could take a tax preparation course and spend the last few months before the tax deadline working for H&R Block or hanging out your own shingle. Or find a financial planner (many, even if working through a big institution are self-employed) and get them to take you on for their busy season to help with setting up appointments and processing paperwork - here in Canada Jan/Feb is known as RRSP season, because contributions made during those months can be claimed on the previous year's taxes.
6. School bus driver or crossing guard - this is Sept to June, obviously, so might not work, but it's also not full days so don't know if it might fit in with your other work.
7. Finally, what are your hobbies and interests? Are you a photographer? A cake baker? Do you sew or write? Many different hobbies can be turned into small scale employment from home. Could your primary field be blogged about, and monetized in some way?
Do let us know what you end up exploring, and welcome to the forums, officially!