Let's crunch some numbers.
The largest tree planting project that I know of is the Civilian Conservation Core which planted about 3 billion trees in the US over about a decade (source).
Let's say that a 40-year-old tree is sequestering about 1 ton of CO2 (source, and yes, I realize this will vary a lot based on species and location, but we need to start somewhere).
So, let's say that we magically plant 3 billion trees tomorrow. That will sequester 3e9 trees*2e3 lbs/tree*4.54e-13 lbs/gigatonne / 40 years = 0.068 gigatonnes/year of CO2 sequestered. (Note that ton and tonne are different.) In comparison, the US produces about 1.4 gigatonnes a year (source).
I'm not saying that sequestering CO2 in rock is a better scheme, but planting a few *billion* trees won't solve our problem.
(PS. someone check my math. It's easy to screw these calculations up.)