The diagnostic order
Step one: check pH. In soil, target 6.0 to 6.8. In coco or hydro, target 5.5 to 6.5. If the pH of your input water or runoff is outside that band, fix it before adjusting anything else.
Step two: check runoff PPM/EC. If the runoff is much higher than your input, you have salt buildup. Flush with 2 to 3 times the pot volume of plain pH-adjusted water.
Step three: review your feeding chart. Are you actually giving the nutrients the plant needs at this stage? Most beginners over-feed.
Step four: check for pests. Lift leaves. Look at the underside. Inspect the soil surface.
Only after these four checks should you consider adding more nutrients.
Heads upIf you only remember one thing from this guide, remember pH first. 80% of what looks like a nutrient deficiency is actually pH locking the roots out of nutrients that are already there. Adding more nutrients on a bad-pH plant makes things worse.
Eight problems you will actually hit
- Nutrient burn (brown crispy leaf tips). You are feeding too strong. Cut feeding strength in half.
- Nitrogen deficiency (older leaves yellowing from the bottom up). Increase nitrogen, but check pH first.
- Calcium/magnesium deficiency in coco (rust spots, brown patches, leaf curl). Add Cal-Mag at 5 ml per gallon from week one.
- Light burn (top leaves yellowing or bleaching white, edges curling up like a taco). Raise your light.
- Heat stress (leaves taco-ing upward, drooping at midday). Lower temperature, improve airflow.
- Overwatering (drooping leaves on a wet pot, slowed growth). Wait longer between waterings. Lift the pot. If it is heavy, do not water.
- Fungus gnats (tiny flies hovering over soil). Top-dress with diatomaceous earth or perlite. Let soil dry out more between waterings. Yellow sticky traps.
- Spider mites (fine webbing, tiny moving dots, stippled leaves). Hit fast with a Bt or neem oil spray every 3 days for 2 weeks. Spider mites are the worst pest because they reproduce in 5 days.
The pests that end grows
Spider mites and powdery mildew are the two that turn into full-grow disasters when missed early. Spider mites: tiny, hard to see, leave fine webbing and stippled leaf damage. Powdery mildew: looks like white flour dusted on leaves, often appears on lower fan leaves first in high humidity.
Bud rot (Botrytis) shows up late in flower, especially on dense buds in high humidity. There is no cure once it spreads. Remove the affected bud and surrounding tissue, lower humidity, increase airflow, and hope it stops. Most growers lose at least one cola to bud rot in their first few outdoor or low-airflow grows.
When to give up on a plant
Most problems are recoverable. Two are not. Hermies (a plant that develops both male and female flowers, often from stress) will pollinate the rest of your grow with seeds. Cull or isolate immediately. Active bud rot in late flower can spread through a tent in days. Harvest the unaffected bud early if you cannot stop the spread.