Yes, the store that brings you cases of 891 cookies and 56 chicken drumsticks actually is a garden resource at this time of the year. During the month of March, Costco has a wonderful bulk landscape material selection. The 2 products that Costco carries right now that you should run and buy are weed block and Preen.
Commercial grade weed block (see photo) is an important material that is laid on bare soil to control weeds. You then cut holes in the fabric, dig a hole and plant the plant. Weed block can only be used on flat areas. The gray weed block that Costco has right now, sells for about $35 for an 880 sq ft roll. That is about half the price of Home Depot's heavy duty weed block.
The second product at Costco that you should buy is Preen. Preen is a pre-emergent herbicide that stops all weed seeds from germinating. It will not kill any existing plants (including weeds that have already sprung up). So it is safe to use around roses, shrubs, perennials and trees. Preen should not be used around fruit trees or vegetable gardens, since the herbicide can slowly work its way into the plant. Preen is a good long term method for controlling weeds.
In addition to those items, Costco has carries useful things like soil, bulb and plants. So run to Costco and save some money on landscape items before March ends.
For more information, here is their web page.
Photo by Doug Kalal, taken at Mission Valley Costco.