Cannabis Growing Problems: How to Diagnose and Fix Them

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Cannabis plant showing common growing problems

Cannabis growing problems often come down to water, feed, pH, heat, light or pests. University of California ANR (UC ANR) lists too much or too little water, feeding problems often tied to soil pH, and too much heat or sunlight among the common nonliving causes of plant trouble. The pattern of damage then points to which one you have.

OSU Extension in Oregon teaches that damage spread evenly across a whole planting usually has a nonliving cause. Damage in a random pattern that keeps spreading points instead to a pest or disease. Kind Seed Co starts with that method, then works through each problem area in turn. Once you know the cause, our guide on bringing a struggling cannabis plant back covers the next steps.

How to Diagnose Problems Growing Cannabis

Work through the same checks in the same order each time. NC State Extension and MU Extension in Missouri both describe a step by step inspection rather than a guess from one leaf.

  1. Check history and recent changes. NC State asks what was applied recently, in what amounts, and what the weather or room conditions were.
  2. Look at the pattern across plants. Oregon State links an even pattern to nonliving causes and a random, spreading one to pests or disease.
  3. Check old and new leaves. Missouri Extension asks whether damage shows on old or young leaves, or only on one side of the plant.
  4. Check the roots and the mix. Missouri says to examine every part, roots included, because the spot where damage starts is often not where it shows.
  5. Look for signs of pests or pathogens. NC State lists cast skins, droppings, silken threads and holes, plus mold growth such as powdery mildew.
  6. Make one change, then wait. A single change lets you see what worked.
  7. Watch new growth. Old leaves rarely recover, so judge the fix by the leaves that come next.
ClueNonliving cause more likelyPest or disease more likely
Plants affectedUnrelated plantsOne type or close relatives
PatternRegular or evenRandom or irregular
SpeedSudden onsetSlow, worse over time

The table follows the UC ANR comparison of nonliving disorders and pest or disease damage.

Feeding and pH Problems

Feeding trouble often starts with pH rather than the feed itself. MSU Extension calls pH 5.5 to 6.5 the generally accepted range for plant growth and for access to a broad set of nutrients, and notes that availability of some nutrients drops outside it.

MSU adds that too little of a nutrient is more common than too much, and that leaf tissue and soil tests can rule nutrients in or out. Check your water and runoff pH before you change the feed. Needs shift with the stage. USU Extension says plants need more nitrogen in vegetative growth, and that too much brings extra foliage and weak stems. Our soil feeding schedule lays out the mix week by week. For leaf by leaf detail on each nutrient, see our leaf symptom guide.

Watering Problems

Too much and too little water can look alike. MSU Extension warns that overwatering causes many of the same problems as too little water, and per NC State, overwatering mimics a plant set too deep.

Check the mix before you water. Lift the pot, since a light pot is dry and a heavy one is still wet. Or push a finger about 2 inches into the mix. OSU Extension says to water when it feels dry at your fingertip and to wait while it feels moist. MSU says roots short on water lose their white tips and turn dry and shrivelled. A separate page on overwatering cannabis goes deeper into the wet side.

Root Zone and Drainage Problems

Roots need air as well as water, so a mix that stays soggy hurts a plant from below. MSU Extension explains that flooded roots run short of oxygen and build up toxic compounds.

In a wet, poorly drained mix, MSU says roots lose their white tips and the dark outer layer slips away to show gray, water soaked tissue. Our Pythium root rot guide goes further.

For a drainage check, water slowly until it drips from the holes, as OSU Extension advises for containers, then use the finger check before the next watering. NC State Extension calls drainage holes necessary so roots do not sit in water and rot. Outdoors, dark pots in full summer sun can burn fragile roots, per NC State, so shade them or move them.

Temperature Problems

High heat mainly drives water loss. MSU Extension says the main way heat harms a plant is by raising water loss, and high temperatures can scorch leaves, per NC State.

Indoors, measure at the canopy, not on the wall. Raise the light or add air movement if the tops run hot. Outdoors, shade cloth and steady watering help through hot spells.

Room Climate Problems

Swings in heat, cold and humidity cause trouble, and the humidity plants want falls as they grow. A Simon Fraser University review gives relative humidity above 90 percent for clones and seedlings, 65 to 75 percent during vegetative growth and 50 to 70 percent in flower.

For damp air late in flower, the review says fans near the plants before harvest lower the humidity around them, and UMass Extension lists moving air and venting moisture. In a cold room, UMN Extension notes indoor potting mix can run up to 5°F below the air. Our guide to environmental stress in cannabis plants takes the longer view.

Light Problems

Too much light shows first on the leaves nearest the lamp. MSU Extension notes that leaves moved from shade into strong sun can take on a sunburned look.

Look at the top leaves under the fixture. Bleached or crisp tops with lower leaves still green point to light, not feed. Raise the fixture or lower its output, then watch the new tops. Plants moved outdoors after a stretch under lamps need a few days in partial shade first.

UF/IFAS Extension says flowering starts once nights reach at least 10 to 11 hours, so keep nights short until flowering. A University of Guelph team notes that most indoor growers flower photoperiod plants on equal 12 hour light and dark periods, and that flowering responds strongly to even faint stray light or a break in the night. Check for leaks while the lamps are off. Our page on how the light cycle works gives the schedules.

Fan and Airflow Problems

A fan pointed straight at plants can damage the leaves in its path. NC State says high winds can make a plant lose water and break stems.

Leaves near one fan that curl or crisp while the rest look fine are a clue. Aim fans across the canopy rather than at it, and use a setting that moves leaves gently. Small nicks to leaves and branches tend to be minor, much as UMN Extension says established ornamentals usually tolerate leaf feeding by caterpillars. Severe feeding for one to three years in a row may slow growth.

Too Much Feed

Cannabis leaf with burnt tips from too much feed

Too much fertilizer leaves burnt tips and edges, often on many plants at once. MSU Extension notes that high levels of soluble salts can cause toxicity problems.

MSU suggests learning to check pH and electrical conductivity with water quality meters. Stick to the product label. Measure runoff before you feed again. Leaching the mix with plain water lowers the salt level, and MSU ranks it first among fixes for high soluble salts.

Feed in the Mix but Not Taken Up

Sometimes the feed is there and the plant still cannot use it. MSU Extension ties this to pH, since nutrient availability drops when pH moves outside the range it gives.

The plant looks underfed even though you feed on schedule. Test the runoff pH first, then adjust the feed or water pH before adding anything new.

Pest and Disease Signs

Underside of a cannabis leaf checked for pests

Living causes leave evidence. NC State Extension lists cast skins, droppings, silken threads, holes and slime trails, among visible signs of pests, and powdery mildew and sooty mold among signs of pathogens.

Missouri Extension suggests holding leaves up to the light to look for webbing and mites. NC State also warns that spider mites can bleach the area between veins while the veins stay green, which can look like a feeding issue. Check under leaves with a loupe. For prevention, read our cannabis pest prevention guide.

Troubleshooting

When the first fix fails, go back to the method rather than adding more fixes. These cases trip up growers most.

Two problems show at once. UC ANR notes that more than one cause can affect plants together. Fix the root zone first, then reassess.

A fix made things worse. Undo the last change. Then go back to step 1 and check history and pattern again.

Only one plant is affected. Oregon State links a random pattern to living causes. Check that plant closely for pests and look at its roots.

The problem returns every grow. Look at what stays the same between runs: the water source, the mix, the room and the feed.

Pest damage looks like a feed problem. NC State notes spider mites can leave bleached patches on leaves. Check leaf undersides with a loupe before you change the feed.

Stems were bent on purpose. Low stress training bends stems by design, which is not one of the problems on this page.

Male flowers appear on a female plant. OSU Extension says sex expression can respond to stress in the surroundings, and that at any time, plants with female genetics can form male flowers. See our page on hermaphrodite cannabis.

Before Your Next Grow

Begin with fresh seeds and a clean room. Browse our feminized cannabis seeds.

Frequently Asked Questions

What are the most common cannabis growing problems?

Many trace back to water, feed, pH, heat, light or pests. UC ANR lists too much or too little water, feeding problems often tied to pH, and too much heat or sunlight among common nonliving causes. Read the pattern of damage first, then check the roots.

How do you tell what is behind your cannabis growing problems?

Work through a set order. NC State Extension asks what changed recently and what was applied. Missouri Extension says to examine every part, roots included, and to note old or young leaves. Look for signs of pests, then change one thing and watch the new growth.

What does the spread of damage across your cannabis plants tell you?

OSU Extension in Oregon teaches that damage spread evenly across a whole planting usually has a nonliving cause such as water, heat or feed. Damage in a random pattern that keeps spreading points to a pest or disease instead. Its guide also warns against overanalyzing the split between even and random.

What should you check first when a cannabis grow problem shows up?

Start with recent changes. NC State Extension asks what was applied recently, in what amounts, and what conditions the plants faced. Then check the pattern across plants, the roots and the mix, and leaf undersides for pests before you change the feed or the watering.

Why do different cannabis grow problems look alike?

UC ANR notes that different nonliving causes can produce the same damage, and more than one cause can affect a plant together. NC State also says overwatering looks the same as planting too deep. A set inspection order beats a guess from one leaf for that reason.

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