Common Plant Problems in Cannabis Cultivation

Both beginners and experienced growers can encounter a variety of issues when cultivating cannabis. This article provides examples of how to identify plant ailments and offers solutions to common problems. The information is based on Jorge Cervantes’ tables and diagnostic guides.

Diagnosing Nutrient Deficiencies and Excesses

Use the following step-by-step algorithm to identify and address nutrient-related issues in your plants. Start with the first point and proceed as directed. Carefully diagnose the problem before taking any action.

1. If the problem appears only at the bottom or middle of the plant: Go to step 2.
   If the problem appears at the top or on the tips: Go to step 10.
   If the problem is uniform throughout the plant: Go to step 6.
2. Leaves are uniformly yellow or light green; leaves wilt and fall off; growth is slow. Leaf edges do not curl upwards.
   → Nitrogen (N) deficiency.
   If not, go to step 3.
3. Leaf edges are turned or curled upwards, and tips may twist. Leaves yellow (and may turn brown), but veins remain green.
   → Magnesium (Mg) deficiency.
   If not, go to step 4.
4. Leaves turn brown or yellow. Yellow, brown, or necrotic (dead) spots, especially on the edges, which may also curl. The plant may be too tall.
   → Potassium (K) deficiency.
   If not, continue reading.
5. Leaves are dark green or red/purple. Stems and petioles may turn purple or red. Leaves may yellow or curl downwards and fall off easily. Growth may be slow and leaves small.
   → Phosphorus deficiency.
   If not, go to step 6.
6. Leaf tips are yellow, brown, or dead. The plant otherwise looks healthy and green. Stems may be soft.
   → Overfeeding (especially nitrogen), overwatering, root damage, or poor soil aeration (add more sand or perlite). Sometimes, it may be a deficiency of N, P, or K.
   If not, go to step 7.
7. Leaves curl down like a ram’s horn and are dark green, gray, brown, or golden.
   → Overfeeding (too much nitrogen).
   If not, go to step 8.
8. The plant looks wilted even if the soil is moist.
   → Overfeeding, overly wet soil, root damage, disease, or (rarely) copper deficiency.
   If not, go to step 9.
9. Plants do not flower even after receiving 12 hours of darkness for two weeks.
   → Night period is not completely dark, too much nitrogen, or excessive pruning/cloning.
   If not, go to step 10.
10. Leaves are yellow or white, but veins remain mostly green.
    → Iron (Fe) deficiency.
    If not, go to step 11.
11. Leaves are light green or yellow at the base, while edges remain green. Necrotic spots may appear between veins. Leaves are not curled.
    → Manganese (Mn) deficiency.
    If not, go to step 12.
12. Leaves are curled, otherwise similar to step 11.
    → Zinc (Zn) deficiency.
    If not, go to step 13.
13. Leaves are curled, turning brown or dying.
    → Lighting is too close to the plant. Rarely, this may be a calcium (Ca) or boron (B) deficiency.
    If not, your plant may simply be weak.

Key Nutrients and Their Management

• Nitrogen (N): Plants need a lot of nitrogen during vegetative growth, but it’s easy to overfeed. If you add too much, flush the soil with plain water. Soluble nitrogen (usually nitrate) is quickly absorbed, while insoluble forms (like urea) must be broken down by soil microbes first. Avoid excess ammonium nitrogen, which can interfere with other nutrients. Too much nitrogen delays flowering. Plants should experience a nitrogen deficiency at the end of flowering for better aroma and flavor.
• Potassium (K): Too much sodium (Na) can displace potassium, causing a deficiency. Sources of high sodium include baking soda (“pH-up”), excessive manure, and water softeners (which should not be used). If sodium is the issue, flush the soil. Potassium can also be blocked by excess calcium or ammonium nitrogen, or by cold weather.
• Phosphorus (P): Some deficiency during flowering is normal, but severe deficiency should be avoided. Red petioles and stems are often genetic and not necessarily a sign of phosphorus deficiency. Excess phosphorus can cause iron deficiency.
• Magnesium (Mg): Magnesium deficiency is common because cannabis uses a lot and many fertilizers lack enough. Fix it by adding one tablespoon of Epsom salt (magnesium sulfate) per 1.2 gallons (4.5 liters) of water (dissolve in warm water first), or one teaspoon of foliar feed per 1 quart (1.1 liters) of water. When preparing soil, use two teaspoons of dolomite lime per 1.2 gallons (4.5 liters) of soil. Magnesium can be blocked by excess calcium, chlorine, or ammonium nitrogen. Don’t overfeed magnesium, as it can block other nutrients.
• Iron (Fe): Iron is unavailable to plants when water or soil pH is too high. If deficient, lower pH to about 6.5 (hydroponics: 5.7) and check that you’re not adding too much phosphorus, which can block iron. Use chelated iron (“iron EDTA”) for best absorption. Too much iron without enough phosphorus can cause phosphorus deficiency.
• Manganese (Mn): Blocked by high pH and excess iron. Use chelated manganese.
• Zinc (Zn): Also blocked by high pH. Deficiencies of Zn, Fe, and Mn often occur together, usually due to high pH. Don’t add too many micronutrients—lower the pH if it’s high to make nutrients available. Use foliar feeding if the plant looks very bad. Use chelated zinc.

Water Quality and Testing

• Check your water: Rusty faucets and showers indicate “hard” water, usually due to excess minerals. Tap water with TDS (total dissolved solids) over 200 ppm is considered hard and should be tested, especially if your plants have chronic problems. Ask your water utility for a water analysis, which should include pH, TDS, and mineral levels. Standard water filters won’t reduce high TDS, but expensive distillers and deionizers will. A digital TDS meter (about $40+) is a valuable tool for monitoring nutrient levels.

General Feeding Tips

• Potted plants are adaptable, but the main rule is to use more nitrogen and less phosphorus during vegetative growth, and the opposite during flowering. For vegetative growth, aim for an N:P:K ratio of about 10:7:8 (or 20:14:16), and for flowering, 4:8:8. Check pH after adding nutrients. If using a reservoir, let the solution circulate and change it every two weeks.
• Recommended TDS levels:
  • Seedlings: 50–150 ppm
  • Unrooted clones: 100–350 ppm
  • Small plants: 400–800 ppm
  • Large plants: 900–1800 ppm
  • Last week of flowering: stop watering (flush)

  These are guidelines; many factors can affect the actual needs. Some nutrients are “invisible” to TDS meters, especially organics, so use TDS readings as a rough estimate. When in doubt, follow tomato feeding recommendations and grow a few tomatoes or radishes nearby for comparison.

pH Management

• Water pH after adding nutrients should be about 5.9–6.5 (hydroponics: 5.5–6.1). Micronutrients (Fe, Zn, Mn, Cu) are blocked at high pH (alkaline, above 7.0), while macronutrients (N, P, K, Mg) are less available in acidic soil or water (below 5.0). Tap water is often too alkaline. Peat-heavy or organic soils can become too acidic and benefit from dolomite lime. Soil pH meters vary in accuracy; generally, the more you pay, the better. For water, aquarium pH test kits are cheap but not very accurate. Invest in a digital pH meter ($40–80), preferably waterproof.

Environmental Factors

• Cold: Cold weather (below 50°F/10°C) can block phosphorus uptake. Some strains, like equatorial sativas, are sensitive to cold. Keeping roots warm helps plants tolerate lower temperatures.
• Heat: If lights are too close, tops may curl, dry, and look burned, mimicking nutrient problems. Your hand should not feel hot after a minute above the plant tops. Raise the light and/or aim a fan at hot spots. Ideal temperature is up to 85°F/29°C, or 91°F/33°C with added CO₂.
• Humidity: Thin, wrinkled leaves may be due to low humidity. 40–80% is usually sufficient.
• Mold and Fungus: Dark, spotted areas on leaves and buds may be mold. Lower humidity and increase ventilation if you have mold issues. Remove all dead leaves and keep your grow area clean.
• Insects: White spots on leaf tips may indicate mites on the underside of leaves.
• Sprays: Foliar sprays can cause a “magnifying glass” effect under bright light, leading to white, yellow, or burned spots that can be mistaken for nutrient problems. Some sprays also cause chemical reactions.
• Insufficient Lighting: Tall, stretched plants are usually due to poor lighting. Do not use regular incandescent or halogen bulbs for cannabis. Invest in fluorescent lights (good) or HID lights (better) for the high-intensity light cannabis needs. Even better, grow under sunlight.
• Clones: Yellowing leaves on unrooted clones may be from too much light or an unstable stem. Isolate them from CO₂ until they root. Too much fertilizer can dry out or wilt clones—plain tap water is enough.