When spraying a plant with neem oil—either as a bug treatment or preventative measure—it’s important to coat both the tops and bottoms of leaves, since bugs love to hand out on the underside of foliage. You should also lightly coat the stems and soil, just in case any critters have made their way to those areas.

Only use neem oil on healthy plants. If your greenery is yellowing, browning, droopy, or otherwise looks off, it might be dealing with water, sunlight, or nutrient imbalance and the neem oil could make the problem worse.

Most plant varieties can handle the occasional neem oil spray but use it sparingly on greenery that has fuzzy leaves (some succulents, cacti, calathea, etc.). This texture prefers drier conditions and any excess moisture can throw it off.

Finally, don’t apply neem oil to plants that are sitting in a bright window. “As with any horticultural oil, it’s best not to apply to plants when they are exposed to direct sun rays to avoid leaf scorch,” Halleck says. Before spraying, move your plant into a darker room like the bathroom for application and wait 2-3 days before putting it back in a sunnier spot. This should leave enough time for the neem oil to break down.

https://www.mindbodygreen.com/articles/neem-oil-for-houseplants