How important is showing an upward trend in grades?

So, my first two years of high school were... not great, grade-wise. But I've been doing a lot better lately! Do colleges care a lot about seeing improvement over the high school years, or will my rough start be a big issue?

Absolutely, colleges do take note of an upward trend in grades throughout high school. It's understood that students grow and mature at different rates. So, a rough start can certainly be mitigated by marked improvement in your later years, as it demonstrates resilience and an increased dedication to your studies. Plus, your performance junior and senior year is more indicative of the kind of student you'll be when you arrive on campus.

Consider highlighting this growth in your application, perhaps, in your essays, or by requesting that your counselor or teachers highlight it in their recommendation letters, so that admissions officers get full context for your academic improvement. Best of all would be if you can maintain that positive momentum all the way through the midpoint of your senior year (as colleges receive your midyear academic report). Good luck!