I know too many authors who failed to deliver and were never able to land another contract.
For the next few months, your editor will probably leave you alone to let you write. If it's a 2014 pub date, then you've got lots of time before the sales dept will go out with your title in the catalogue. The publishing house probably isn't even thinking about sales and marketing yet, and the cover will not be designed until 2013, after your photos come in. (Those photos may be important to the cover design.)
For a 250 page manuscript that's not due till June, you've got a pretty relaxed writing goal. But don't procrastinate, and be sure to get a few pages in every day. (When I'm working on my first drafts, I try to stick to a goal of about four pages a day -- that gives me a finished first draft of 100,000 words in about 6 months.) Since yours is a nonfiction handbook, a chapter outline will help you organize. I'm sure you know your subject matter inside and out, but if your audience is non-gardeners, make your writing absolutely clear and simple but punchy.
Oh, and btw, I grow tomatoes. So I may be buying that book!
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