Yes, a plant can recover from a deficiency of iron if it is detected and corrected early. The best way to treat a deficiency of iron is to adjust the pH of the medium, as this affects the availability of iron for the plant. The optimal pH range for cannabis is 6.0 to 7.0 for soil and 5.5 to 6.5 for hydroponics or coco coir. If the pH is too high or too low, it can lock out iron and other nutrients from the plant. You can use pH meters, pH strips, or pH drops to measure and adjust the pH of your medium. You can also use products such as elemental sulfur, vinegar, or lemon juice to lower the pH, or lime, baking soda, or wood ash to raise the pH. In addition to adjusting the pH, you can also apply an iron supplement to your cannabis plants, either as a foliar spray or a root drench. Follow the instructions on the product label and avoid overfeeding your plants, as this can cause a nutrient burn or other imbalances. You should see an improvement in your plants within a few days or weeks after applying an iron supplement.