1. Not Managing pH
When you start cultivating cannabis for the first time, there are a lot of acronyms, formulas, and regimens it seems you need to remember. After hearing PPM, PPK, pH, Lucas Formula, 18/6, 24/0, etc., most people want to ignore the jargon and try just watering a plant.
The jargon can definitely be overwhelming, and I would agree that a lot of it can actually be ignored...except pH.
Managing pH is one of the more technical details that pretty much ALL growers have to deal with. Poor pH makes for sickly plants that struggle to make small buds, and bad pH makes dead cannabis plants.
How to Avoid:
To your plants, pH is the thing that determines which nutrients it can uptake. When pH is in the correct range, your plants can uptake any nutrients they need!
Get yourself a pH Testing kit and make sure you're feeding your plants with water in the correct range:
Hydroponics: 5.5 - 6.5
Soil: 6.0 - 7.0
Generally, most nutrient deficiencies are actually pH problems that cause the plant to be unable to uptake a certain nutrient. Remember this and you'll keep your plants looking beautiful.
2. Using Too Much Nutrients
This is a mistake that's very easy to make, and the majority of growers will do this at some point.
Here's a large part of the problem: Most nutrient companies include a feeding schedule with their nutrient systems, and most of the feeding schedules recommend nutrient dosages that are way too high!
Overfeeding leads to a well known affliction called nutrient burn (commonly referred to as "nute burn"). Nutrient burn won't kill your plants in most cases, but they will leave an eye-catching reminder of their presence for the remainder of the grow.
How to Avoid:
Use the same feeding schedule that comes with your nutrient system, but only use one quarter (1/4) of the recommended dosage.
Example: If the feeding schedule calls for 2tsp of nutrient solution per gallon of water. Only use 1/2tsp (2 * 1/4 = 1/2).
If your plant begins to exhibit signs of nutrient deficiency and the pH is in the correct range, slowly move up to 1/2 or more as needed. Personally, it's very rare that I feed my plants nutrients at more than half (1/2) strength.
3. Overwatering (watering too often)
Many of us were brought up thinking that plants needs sunshine and water every day. This combined with a new growers thirst to provide for their plants causes many people to water their plants more than it's needed.