Horseradish is at its best in fall and winter. Like so many other root vegetables, however, it stores well and is often available in decent shape well into spring. Jerusalem artichokes/sunchokes are brown nubs, that look a bit like small pieces of fresh ginger.
Using Ms. or Mrs. in a cover letter depends on whether or not you know the marital status of the woman. If you are ever uncertain, it is always safe to use Ms. because it does not make an assumption about her marital status.
Your cover letter should include an expression of your interest in knowing more about the job. At this stage in the process, you want an interview. Your cover letter shouldn't state that you want the job, because in many cases, that's premature and gives your cover letter a tone of desperation, which is counterproductive. You won't know whether you want the job until you have an interview.