Milk cartons can be used when you want or need to make a recycled tree guard. These are especially useful to make when you have hundreds or thousands of seedlings to plant when involved in revegetation.
You can see here a photo of inside a plant guard made from an old milk carton.
Using milk cartons can be a problem though for the following reasons.
1. The diameter of the milk carton is quite small and it can be difficult gathering all of the small branches of the seedling and placing them inside the carton without damaging the plant.
2. The cartons are shorter than commercially available ones. This means the plant will be overflowing the carton sooner than might be advised. This can also cause the plant to grow in a more 'spindly' way.
3. The sides of the carton are quite thick and this reduces the ability for natural light to penetrate the bottom of the carton.
Here the local Landcare group has planted thousands of young seedlings into old milk cartons along creek beds, valleys, hills and dales.