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Seedlings · 8 minute read · Updated October 20, 2026

What is wrong with my cannabis seedling?

A seedling has almost no reserves and a root system smaller than your fingertip. That makes it fragile, but it also makes it honest: it shows you the problem within days.

Quick diagnosis table

Why is my seedling tall and stretchy?

The common belief is that a tall seedling is a vigorous one. It is the opposite. North Carolina State Extension explains that too little light causes long internodes and a spindly stem, called stretching or etiolation. Illinois Extension calls insufficient light the most common cause of leggy seedlings.

• Light too far or too weak. A seedling reaching upward is searching for light. For fluorescent tubes, Illinois Extension says they are typically kept 2 to 4 inches (5 to 10 cm) above the tops of seedlings, and notes that tubes lose brightness as they age. LEDs are far stronger, so do not copy that distance: follow your light maker's seedling setting and see LED light height by stage.
• A window is not enough. University of Minnesota Extension says window light is not enough for seedlings and recommends a grow light.
• Crowding. Seedlings packed together compete for light and stretch.

Fix: raise the light level gradually so you do not scorch the leaves, and support a floppy stem with a small stake. A stretched stem will not shrink, but new growth above it will be tighter once the light is right.

What is damping off, and can a seedling survive it?

Damping off is a disease of seedlings caused by soil fungi and water molds. University of Minnesota Extension names Rhizoctonia, Fusarium and Pythium as the most common causes. North Carolina State Extension reports Pythium causing damping off in hemp, which is the same species as cannabis.

Signs: seeds that never come up, or a seedling whose stem turns water-soaked and thin, almost thread-like, at the soil line until it falls over. In humid conditions you may see fluffy white growth.

Can it be saved? Usually not. Minnesota Extension says seedlings infected by damping off rarely survive to produce a vigorous plant. Remove the affected seedling and protect the rest.

What favors it: cool, wet conditions. Minnesota adds low light, overwatering, cool soil and high salts from overfertilizing. NC State notes that hemp is sensitive to excess water.

Prevention, from the extension pages:

• Use new potting mix. Do not reuse old mix or use garden soil or compost for starting seeds.
• Clean used pots and trays. Minnesota recommends soaking them in a 10 percent household bleach solution for 30 minutes. Rinse them well afterward, work with gloves in a ventilated spot, and never mix bleach with ammonia, vinegar or other cleaners.
• Keep the mix moist, not soggy, in pots with drainage holes.
• Keep the root zone warm: Minnesota suggests warming the trays from below to hold soil at 70 to 75 F (21 to 24 C), and watering with water at 68 to 77 F (20 to 25 C) rather than cold water. If you use a heat mat, use one made for seed starting and follow its maker's instructions.

NC State notes there are currently no effective, legal chemical treatments for Pythium in hemp, so prevention is the whole plan. If you ever use any product, use only one labeled for the use and follow the label.

Am I overwatering my seedling?

Probably, if you water every day. University of Minnesota Extension notes that daily watering is often more water than seedlings need. When the root zone runs out of air, the plant makes ethylene gas and the leaves curl downward, a response called epinasty. It looks like thirst, so many growers water again, which makes it worse. Illinois Extension describes the same trap.

When did you last lift the pot before watering? A small pot of wet mix is heavy; a dry one is light. Water when it feels light, give a small amount near the stem, and let it dry back before the next watering.

More detail, including how to tell overwatering from underwatering, is in overwatered cannabis and how often to water cannabis.

Why are my seedling's first leaves turning yellow?

The first round leaves are cotyledons, the seed leaves. North Carolina State Extension describes them as energy storage structures for germination and for getting the first true leaves established. Once the true leaves are working, the cotyledons have done their job, and slow yellowing of those two round leaves is usually nothing to worry about.

It matters more when the true leaves, the serrated ones, turn pale. University of Minnesota Extension notes that slow growth with yellow leaves in seedlings likely means low nitrogen, which happens when a seed starting mix carries little or no fertilizer. Check temperature and watering first, since cold, soggy roots also slow growth. Our nitrogen deficiency page shows the pattern on older plants.

Can seedlings get nutrient burn?

Yes, easily. University of New Hampshire Extension warns that young seedlings are easily damaged by too much fertilizer. Minnesota Extension adds that some potting mixes already hold very high salt levels, which can stop germination, and that heavy fertilizer rates raise salts further.

• Signs: brown, crispy tips on the leaves.
• Fix: stop feeding and water plain until the new growth looks clean.
• Prevention: University of Minnesota Extension suggests waiting until several true leaves have developed, then using a quarter strength soluble fertilizer, and notes that many potting mixes already contain slow-release fertilizer. Read the bag before you add anything.

More on spotting it in nutrient burn.

Why is my seedling drooping?

Drooping has two opposite causes, and the pot tells you which.

A seedling that droops right after transplanting or a big change in light is usually adjusting. Give it a day before you change anything else. If it droops and the stem is thin and dark at the soil line, go back to the damping off section.

Why is my seedling growing so slowly?

Slow growth is rarely one dramatic problem. It is usually a quiet one that runs every day.

• Cold roots. Illinois Extension says most seeds germinate best with the growing mix around 70 to 75 F (21 to 24 C), and that cooler conditions slow germination and raise damping off risk. A cold floor or a draft can keep a pot well below the room temperature.
• Too wet. Waterlogged roots cannot take up much, so the plant stalls.
• Too dim. Weak light means less growth, even if the plant has not stretched yet.
• Not enough food. If the mix has no fertilizer and the true leaves are pale, a light feed helps once several true leaves are out.

Change one thing at a time and give it about a week. Note the date and what you changed. A grow journal makes it easy to see which change actually worked, and our germination guide covers the steps before this stage.

Questions growers ask

Read next

• How to germinate cannabis seeds
• How often should you water cannabis?
• How high should an LED grow light be at each stage?
• What size pot does a cannabis plant need?

Education only, for adults 21+ where home cultivation is legal. Updated October 20, 2026.