Cardboard, we see it every day.  It is made of processed paper and broken-down wood fibers bound back together again, and as such, can be sturdy for awhile, but decompose easily within a short period of time.  That is a quality that gardeners can take advantage of.  So before you throw that box your big screen TV came in, or those old moving boxes, or even your morning cereal boxes, check out these 8 great ways to reuse cardboard in your garden.

1. WEED BARRIER -  Break down a corrugated packing box and lay it on unwanted grass or weeds to choke them out.  Weigh it down with a brick or cover it with a layer of bark mulch for a clean and neat look.  Keep the cardboard at least 4 inches away from the base of plants to allow water and air flow as the cardboard decomposes.
2. POTS FOR SEEDLINGS - Fill the little single- serving cereal boxes with soil and pot up your spring seedlings.  They’ll tear off easily for planting, and the wax paper inside will help hold moisture. Cardboard for food packaging have safety standards to adhere to, so the inks and glues should be OK for the compost pile when you are finished.
3. CAR LINERS - Keep an old copy paper box or lid or a beverage case flat in the back of the car to use whenever you need to keep spilled dirt from messing up the carpet after a trip to the garden center.  Waxed boxes are especially helpful for those leaky pots too.
4. KNEELING PAD - Cut or fold a length of cardboard into several layers to protect your knees while gardening.  Waxed cardboard will stay dryer longer on damp earth, and when it gets worn out just toss it into the recycle bin.
5. FROST PROTECTION - Cut apart the top and bottom of a tall corrugated box and slip it over roses or other young plants that need extra winter protection.  Lay the bottom flaps out flat on the ground and anchor them with landscape pavers or landscape pins.  Fill the boxes with leaves, pine needles, straw or other organic insulating materials and fold the top flaps together for the winter.  On warm winter days, open the top flaps of the box for air circulation. It’s only good for one season, so in spring, compost the box and it’s contents after it is warm enough to remove.
6. TARP SUBSTITUTE - Pile weeds, trimmings, or dead stems on that big-screen TV box that you have opened and layed out flat on the ground. It makes for easy transport to the compost bin as it will slide easily over the grass or concrete patio.  Also use a smaller sheet of cardboard as a place to pile the soil as you dig holes to plant flowers and shrubs. When finished, tip the cardboard up and backfill your hole.  It will keep soil off the grass or patio for a much cleaner post-planting look.
7. RAISED BED BOX - Grow potatoes or other root vegetables in a large corrugated box filled with compost or potting mix. Use any packing box at least 18 inches deep with the bottom intact, and fold the top flaps into the box for stability. (Sometimes a smaller box inside the big box will help guarantee the box’s survival for long season plantings).  At the end of the growing season, just tear the box open, pull out the potatoes and compost the rest.
8. COMPOST IT- Remove staples, tape or patches of glue holding the cardboard together and tear or shred it into small pieces to toss on the compost pile for “brown material.”  There isn’t much nutrient value, but it’s good dry bulk for the composting process.  Corrugated brown cardboard is one of the least processed, usually made with cornstarch-based glues, so it’s a good type to compost.  In contrast, white paperboard is often bleached, so use it in small amounts in the garden.  Many of the printing inks used on boxes are soy based, so look for the soy ink symbol to know for sure.  These can be safely composted, but peel off glossy or colored labels or Duck Tape as they may have chemical additives.
9. MILK CARTONS - Absolutely one of the best containers for blueberry pickers.  Cut the spout off of the top of a 1 quart milk carton and poke a couple of holes on either side of the top 1/3 of the carton. Thread twine thru the holes making it just long enough for  the carton to fall to a comfortable level where you can do two fisted blueberry picking.
10. STORAGE BINS - Cardboard boxes serve as great containers for garden tools.  Keep small shovels, or clippers and  other hand tools organized in various sized boxes.  Label the outside of these boxes so you can easily read it’s contents when they are stored on shelves in your tool shed.

Cheers … Deb (December 2016 Horticultural Newsletter)

References: 
• “Creative Uses for Corrugated Cardboard” Boxsmart.com
• UNH Cooperative Extension Gardens & Landscapes Resource Bulletin
• Personal Experience and Common Sense