Seedlings do not come up

It may simply be too soon. Check that the soil is not dust-dry and that temperature matches the crop (cool-season seed in hot soil often sits; warm-season seed in cold soil often rots). See Gardening Through the Different Seasons and the cheat sheet.

Seedlings collapse at the soil line (damping-off)

A fungal or water-mold problem of wet, still, crowded trays — or young plants outdoors with rotting roots, cutworms, or root maggots. Improve drainage, use clean trays and fresh mix, sow less thickly, add air with a small fan. Keep rotting plant matter out of the bed. A dusting of cinnamon, a cooled chamomile tea drench, or a dilute hydrogen peroxide mist are kitchen-level helps. Do not reach for a synthetic fungicide in an organic garden. Stop watering for a day if the mix is soggy. Check the stem base for grubs.

Plants wilt

Too much water or too little. Roots may be rotting, or root-knot nematodes may be feeding. Water deeply and consistently. If the soil is soggy, stop and let it dry. Rotate crops and choose resistant varieties when you can. See Watering.

Plants are spindly and weak

Usually not enough light, or crowding, or too much nitrogen, or too much water. Indoor seedlings that stretch need a stronger lamp, closer. Outdoors, thin, and give fruiting crops 6–8 hours of sun. Wondering where you should plant a garden? Start with the sunniest patch you own — shade makes spindly plants.

Leaves pale, light green, slow growth

Often nitrogen shortage, sometimes iron in high-pH soil, sometimes not enough sun or warmth, sometimes overwatering. Compost and a fish-emulsion side-dress help a true nitrogen shortage. More light helps a shade problem. Do not guess — look at the whole plant and the soil.

Lots of leaves, no fruit

On tomatoes, squash, and cucumbers this is often too much nitrogen, too little sun, or missing pollinators. If plants are lush but not setting fruit, stop adding nitrogen. Switch to a bloom/fruit approach (compost plus a fertilizer higher in phosphorus and potassium, such as bone meal and kelp), keep soil evenly moist, and hand-pollinate. Temperatures that are too cold or too hot also stall fruit set. Compacted soil does not help — loosen it.

Blossom-end rot

See Tomatoes. Even water, mulch, and a moderate nitrogen diet fix more cases than a bag of lime.