growing horseradish

Question

Plant was looking really healthy but in the last couple weeks the outer leaves have started to dry out after a couple heavy rains.

Answer 1

Brown leaves are usually symptoms of under- or overwatering, and it sounds like the recent bout of rain that you've had could be the culprit here -- but there are other possibilities. This article outlines the common causes for leaf browning in vegetable plants and some remedies:

https://www.gardeningknowhow.com/edible/vegetables/vgen/brown-leaves-on-vegetables.htm