The first thing that comes to mind when I hear the word stubble is facial hair. As with many things in the scriptures, this word has an entirely different meaning.
In agriculture, stubble what is left in the field after the harvest. It is the short, hard, stalky part that is left behind after the crop has been cut close to the ground. Apparently, it is easily burned. The gospel symbolism here runs deep. Let's make sure we are gathered with the harvest, not left behind as stubble to be burned.
Other prophets have similarly written:
Isaiah 5:24 - Therefore as the fire devoureth the stubble, and the flame consumeth the chaff, their root shall be as rottenness, and their blossom shall go up as dust: because they have cast away the law of the of hosts, and despised the word of the Holy One of Israel.
Malachi 4:1 - For, behold, the day cometh, that shall burn as an oven; and all the proud, yeah, and all that do wickedly, shall be stubble: and the day that cometh shall burn them up, saith the Lord of hosts, that it shill leave them neither root nor branch.