How to quote materials accurately and avoid underbidding as a landscaping contractor?

Usually on a job with materials I will do 2.5x mark up on materials, plus labor. 

Here's an example for 5 yards of mulch:

Mulch:

$150 x 2.5 = 375

Labor:

Labor rate x 2.5 hours (assumed 2 yards and hour)

Then I add a delivery fee if i have it delivered. Usually don't mark this up, just pass it to the customer.

If you're not already tracking things I would start. At least track billable hours and revenue to get an idea of break even then add whatever percentage profit margin you want and that'll give you your labor rate.