Borax is sodium tetraborate (a salt). It supplies boron, which is an essential plant nutrient in teeny tiny amounts but toxic to plants in larger amounts. Some plants are more sensitive to the toxicity than others and those are the ones that are affected by it. If you keep on applying it it could build up in the soil and cause other plants not to be able to grow in that spot. So if one uses it, it is recommended not to use it other than occasionally, don't apply too much, and also to make sure your soil isn't already high in boron.
