Why Are My Seedlings Not Growing?

When seedlings stop growing, the cause is almost always environmental, not bad seed. A seedling that germinated fine but then sits at the same height for a week or two is telling you that one of its basic needs has fallen out of range: the soil is too cold, the light is too weak, the mix is too wet, or it has run out of room or food. The good news is that stalled seedlings rarely die quickly, so you usually have time to diagnose the problem and correct it before you lose the tray. Work through the causes below in order, because the early ones (temperature and light) account for the large majority of stalled trays I see. Each section tells you how to confirm the cause and exactly what to change.

Part of our Vegetable Gardening & Raised Beds guide →

In this guide

1. Cause 1: Soil and air temperature are too low
2. Cause 2: Not enough light (leggy, pale, leaning seedlings)
3. Cause 3: Overwatering, waterlogged mix, and damping-off
4. Cause 4: Nutrient deficiency after the true leaves appear
5. Cause 5: A poor, compacted, or inert seed-starting mix
6. Cause 6: Root-bound seedlings in cells that are too small
7. Cause 7: Cold drafts and temperature swings
8. How to prevent stalled seedlings next time
9. Frequently Asked Questions

Cause 1: Soil and air temperature are too low

Temperature drives the speed of nearly every process in a young plant. Seeds may germinate within their viable range and then crawl to a near-standstill if the room cools off afterward, which is exactly what happens when people move a tray off the heat mat the moment sprouts appear. Warm-season crops like tomatoes, peppers, and eggplant want soil at 70-85°F to grow vigorously, and they sulk below 60°F. Peppers are the most temperature-sensitive of the common starts and will essentially pause at room temperatures many people consider comfortable. Cool-season crops such as lettuce, brassicas, and onions tolerate 55-65°F, but even they slow noticeably below 50°F.

How to diagnose it

Push an inexpensive soil thermometer into the root zone of a cell, not the air, and read it in the early morning when the room is coldest. Air temperature near a windowsill can be 10-15°F colder than the center of the room at night, and the soil in a shallow cell tracks the air closely. If your root-zone reading sits below 60°F for warm-season crops or the plants are growing only when the afternoon sun warms them, temperature is your bottleneck. Pale color plus slow growth without legginess also points toward cold rather than low light.

How to fix it

Put the tray back on a seedling heat mat and leave it there through the early growth stage, not just for germination. A mat typically holds the mix 10-20°F above ambient, which is enough to bring a 55°F windowsill into the productive range. Move trays off cold windowsills at night, raise them off cold floors and concrete, and aim for a daytime air temperature of 65-75°F. If you use a heat mat, pair it with a thermostat probe set to about 75°F so you never cook the roots, and confirm with the soil thermometer rather than trusting the dial.

Cause 2: Not enough light (leggy, pale, leaning seedlings)

Insufficient light is the second great stall, and it usually masquerades as growth. Seedlings reaching for a weak source stretch upward, producing long, thin stems with widely spaced leaves and a pale green color. That stretch is not healthy progress; it is the plant spending stored energy to chase light it cannot reach. A south-facing window in winter delivers far less usable light than people assume, especially through double glazing and on overcast days, so windowsill seedlings often grow tall and floppy while making almost no real leaf area.

How to diagnose it

Look at the stem-to-leaf ratio. If the distance between the seed leaves and the first true leaves is more than the seedling is wide, and the plant leans toward the window, it is starved for light. Seedlings that flop over or cannot hold themselves up are a classic sign. Pale, lime-green leaves combined with stretching confirm it, as opposed to the compact pale look of cold stress.

How to fix it

Move the seedlings under a dedicated grow light. Keep the fixture 2-4 inches above the leaf tops for standard fluorescent or LED shop lights and raise it as the plants grow to maintain that gap. Run the lights 14-16 hours a day on a timer; less than 12 hours is rarely enough indoors. If you are relying on a window alone, it almost always falls short in late winter and early spring, which is precisely the season most people start seeds. Rotate window-grown trays a quarter turn daily to keep them from leaning, but understand that rotation only manages the symptom. For seedlings already leggy, you can bury the stretched stem deeper when you pot up tomatoes (they root along buried stems), but prevention with proper light is the real fix. If your problem looks more like collapse than slow growth, see the related guide on why seedlings wilt.

Cause 3: Overwatering, waterlogged mix, and damping-off

Roots need oxygen as much as they need water, and a mix that stays saturated suffocates them. Waterlogged seedlings stall, yellow from the bottom up, and become prime targets for damping-off, a fungal complex that rots stems at the soil line and topples seedlings overnight. Overwatering kills far more indoor seedlings than drought because trays sit in still, humid air where the mix dries slowly, and many growers water on a schedule rather than by need.

How to diagnose it

Lift the tray. A waterlogged tray is noticeably heavy, and the surface may show green algae or white mold. Press a finger into the mix; if it feels cold and sodden a day or two after watering, drainage or watering frequency is the issue. Seedlings that pinch and fall over at the soil line, with a thread-thin darkened stem, have damping-off, which is the terminal stage of a too-wet, too-stagnant environment.

How to fix it

Water only when the top half-inch of mix feels dry, and water from the bottom by setting trays in a shallow reservoir for 10-15 minutes, then removing them once the surface darkens. Always use containers with drainage holes and empty any standing water from the catch tray. Improve airflow with a small fan running on low across the seedlings for a few hours a day, which dries surfaces and dramatically reduces damping-off. Remove humidity domes as soon as most seeds have sprouted; leaving them on traps the moisture that fungi thrive in. If damping-off has started, there is no cure for affected seedlings, so remove them, let the tray dry out, and improve airflow to save the rest.

Cause 4: Nutrient deficiency after the true leaves appear

A seed carries enough stored energy to push out its first pair of seed leaves, called cotyledons, without any outside food. Most commercial seed-starting mixes are deliberately low in nutrients or nearly sterile so they drain well and stay disease-free. That is fine for germination, but once the first set of true leaves opens, the seedling has exhausted its packed lunch and needs feeding. Seedlings left in plain seed mix for two or three weeks past the true-leaf stage simply stop growing and turn pale or purplish.

How to diagnose it

Check whether the seedling has its true leaves (the second and later set, which look like miniature versions of the adult plant, not the simple rounded cotyledons). If true leaves are present and the plant has stalled with uniformly pale or yellowing older leaves, and you have ruled out cold and overwatering, it is hungry. A reddish or purple tint on stems and leaf undersides often signals phosphorus being locked up, which is itself frequently a cold-soil symptom, so confirm temperature first.

How to fix it

Begin feeding with a balanced water-soluble fertilizer once the first true leaves are fully open. Mix it weak: a quarter to half the label strength for the first feedings, applied about once a week. Young roots burn easily, so it is far safer to feed dilute and often than strong and rarely. A liquid fish or kelp blend at quarter strength works well for organic growers. Increase toward half strength as the plants gain size and develop several true-leaf pairs.

Cause 5: A poor, compacted, or inert seed-starting mix

The growing medium itself can stall seedlings two ways. Heavy garden soil or dense potting mix compacts in small cells, cutting off the air roots need and holding water far too long, which feeds straight back into the overwatering problem. At the other extreme, a very fine, peat-heavy mix that has been allowed to dry out completely can turn hydrophobic, shedding water off the surface so the root zone stays dry even though you are watering. Both situations leave roots unable to do their job.

How to diagnose it

Squeeze a handful of moist mix. A good seed-starting medium is light and fluffy and crumbles apart; if it forms a dense, sticky ball that holds its shape, it is too heavy for cells. If water beads on the surface and runs off the edges instead of soaking in, the mix has gone hydrophobic. Pull a struggling seedling gently and look at the roots: healthy roots are white and branching, while roots in a bad mix are sparse, brown, or barely developed.

How to fix it

Use a purpose-made seed-starting mix, or blend your own from roughly equal parts coir or peat, perlite, and a little fine compost or worm castings to add gentle nutrition. Never fill cells with straight garden soil, which compacts and can carry disease. To rewet a hydrophobic mix, water slowly from the bottom and let it wick up over 15-20 minutes, or add a drop of mild soap to the water to break the surface tension once, then keep the medium from ever drying out fully again.

Cause 6: Root-bound seedlings in cells that are too small

A seedling can outgrow its container long before you plan to plant it out. In small plug trays and 6-cell packs, fast growers like tomatoes, squash, and brassicas fill the available volume with roots in a few weeks. Once the roots circle the cell and run out of fresh mix, top growth slows or stops even though light, water, and temperature are all correct. This is a common stall in late winter when planting weather is still six to eight weeks away.

How to diagnose it

Slide a seedling out of its cell and inspect the root ball. If it holds the shape of the cell, shows a dense white mat of roots circling the outside, or roots are escaping the drainage holes, the plant is root-bound. Stalled growth combined with mix that dries out very fast (because roots have replaced most of the medium) is another giveaway.

How to fix it

Pot up into a container one or two sizes larger, typically from a small cell into a 3- to 4-inch pot. Gently tease apart any tightly circling roots before replanting so they grow outward into the new mix rather than continuing to spiral. Bury tomatoes deeper than they sat before, up to the seed leaves, since their stems will root along the buried section and build a stronger plant. Water in after potting up and return to your feeding schedule. Deciding when to move plants outdoors instead is its own question; the comparison in our guide on direct sowing versus transplanting helps you plan cell size and timing from the start.

Cause 7: Cold drafts and temperature swings

Even when the average temperature looks fine, repeated swings stress seedlings and stall their growth. A windowsill that bakes to 75°F in afternoon sun and drops to 50°F against the cold glass overnight gives seedlings no stable conditions to settle into. Drafts from a frequently opened door, a leaky window, or a heating vent blowing directly on the tray cause the same problem. Plants spend energy coping with the swings instead of growing.

How to diagnose it

Place a min/max thermometer next to the tray for a day and check the overnight low against the afternoon high. A swing of more than 15-20°F, or a nighttime reading dipping under 55°F for warm crops, indicates draft or window stress. Seedlings right at the glass that lag behind identical trays a few feet into the room confirm a cold-window effect.

How to fix it

Move trays a foot or two off cold glass and away from doors and direct vent airflow. Pull seedlings back from the window at night, or set a piece of cardboard between the tray and the glass after dark to buffer the cold. A heat mat with a thermostat smooths out the swings by holding the root zone steady regardless of room fluctuations. Aim for a stable 65-75°F day and no lower than 60°F night for warm-season crops.

How to prevent stalled seedlings next time

Most of these causes are easier to prevent than to reverse, and a few habits keep trays growing steadily from sprout to transplant.

• Hold the root zone at 65-75°F with a thermostat-controlled heat mat through the early growth stage, not just germination.
• Provide real light from day one by running a grow light 14-16 hours daily, kept 2-4 inches above the leaves, instead of relying on a winter windowsill.
• Water by weight and feel, bottom-watering only when the surface dries, and run a gentle fan a few hours a day to keep foliage dry and prevent damping-off.
• Start feeding at the first true leaves with quarter- to half-strength liquid fertilizer once a week, since seed mixes hold little nutrition.
• Use a light, fluffy seed-starting mix and pot up before roots circle the cell, sizing your initial cells to your transplant date.
• Keep conditions stable, away from cold glass, drafts, and blasting vents, so plants spend energy growing rather than coping.

Work through warmth, light, and water first whenever a tray stalls, because those three account for most cases. For broader help choosing and raising the right starts, browse our Plants & Flowers hub.