Rainwear & packaging made of plant-based plastics 

 The sheets are normally supplied in agricultural contexts. They have a  role   in accelerating crop growth and in protecting plants from bad weather.   They are made of different types of cereals, like corn.  

The material bears the OK compost INDUSTRIAL label which guarantees to be biodegradable in an industrial composting plant. It is also certified compostable in soil. This applies to all components, inks and additives. In the agricultural sector, the foil is often intentionally left to decompose in the soil after its use.
Weather Underground products combine compostability & water-repellance.

 No, it will not.  

The cape can be used actively for 2 or 15 seasons, depending on how gently it is treated. 
The material thickness determines the cape´s durability x degration time frame. The capes aree made 5 times thicker than generic packaging film. In order to decompose in a controlled time frame, the material should not be thicker than this.

6 months in soil, 12 weeks in industrial compost.

The material supplier indicates that it does not.