Unless you have a very large yard, measured in acres of land, it is unlikely that you could offset your carbon footprint by planting trees. However, trees do absorb CO2, so planting trees in any size yard can help offset your CO2 budget. The trees’ absorption of CO2, added to the benefit of increasing your energy efficiency through providing effective shade around your home, are very positive ways to offset your carbon footprint.
Check out this neat carbon footprint calculator put together by the U.S. Environmental Protection Agency to calculate your carbon impact at home!
http://www.epa.gov/climatechange/ghgemissions/ind-calculator.html