Open the top and remove the plastic spout. Rinse and use the water for your houseplants.
Some of these milk cartons have been standing around a few seedling trees and shrubs in my yard over two years. Some are used full height and some are halved. My ridge area plantings are challenged by rabbits/other wildlife and wind.

Using a serrated bread knife, cut the carton base on three sides, slip the carton over the plant, with the 'bottom flap' towards the outside. I position the flap to the south west and place a stone on top. Sometimes, I scoop a few cups of soil around the base as well. If you have dogs running about, you might use two thin stakes at opposite corners on the inside of the carton for additional stability.

In my vegetable garden, I use a whole carton for each tomato, pepper, and eggplant. They remain the entire season. Usually, the cabbages get a half carton.

After a heavy rainstorm, it is easy to notice your early plantings with these cartons standing watch.

After harvest, other clean cartons are wonderful freezer storage containers, again used whole or halved. I place 1-2 cup portions of veggies and fruits in ziplock bags and stack them in the cartons. When used whole, just fold down the top, label and rubberband. For a half size, do not cut in half, but cut off 1/3 of the top. then slit the sides about 2" and then fold over to cover.

Cartons for outdoor use, can be flattened for storage, whereas, the 'freezer' cartons are stored upside down in bags on the shelf above the freezer.
Have heard of some folks using these cartons to make mason bee nests. Also heard of some using cartons for blanching celery.

Happy re-purposing!

My mother used to save her milk cartons for seed starting. She would cut off one long side, fill the box with potting soil--homemade, which she baked in the oven--and plant seeds for indoor starting. Or, you could cut off the top, punch holes in the bottom, and use them for small pots. I sometimes transplant seedlings into pots to grow bigger before they go into the garden.

These are great tips!!!!
Thanks!