I drove to Joshua Tree and hiked there one November. The landscape and vegetation vary based on the altitude. Generally, the high desert portion is in the north, and that was the part I loved (and which has most of the joshua trees). The temperatures were near-perfect in daytime november. One thing I recall is that it's the ONLY national park I've been to where I saw more people on the hiking trails than I saw driving on the roads, not that I saw that many people, period.