Seeds Not Germinating: Old Seed, Depth, Warmth or Damping Off

Seeds not germinating: find the cause

Start with the age of the seed, then how it was sown, then where the tray sits.

Where did the seed come from?

Saved or leftover seed from earlier years

Deal with it soon

The seed may be past its best

Illinois Extension says seeds lose viability over time. Some last about a year and others four or five years if they are stored properly.

What to do

1. Test a sample: lay at least 10 seeds on a damp paper towel, seal it in a plastic bag, keep it above 70 degrees F and check after 7 to 10 days.
2. If 70 to 90 percent sprout, sow as normal. If fewer than 70 percent sprout, buy new seed.
3. Store leftover seed somewhere cool and dry.

Fresh seed from this year

What did you sow, and how?

Lettuce, dill, snapdragons or coleus, covered with soil

Deal with it soon

These seeds need light to germinate

Illinois Extension says lettuce, dill, snapdragons and coleus need light to germinate, so burying them stops them.

What to do

1. Re-sow on the surface of the growing mix and press the seed in gently without covering it.
2. Check the seed packet for depth instructions for anything else you sow.

Native trees or wildflowers

Keep an eye on it

Seed that needs a dormancy treatment

Illinois Extension says some seeds, especially many native trees and flowers, need cold exposure, scarification or soaking before they will germinate.

What to do

1. Look up the species on your state Extension website or the seed seller's page to see which treatment it needs.
2. Give the seed that treatment and sow again.

An ordinary garden vegetable, sown at the depth on the packet

What are the tray's conditions?

A cool room, or the tray is not covered

Deal with it soon

Too cool, or drying out

Illinois Extension says seeds need uniform moisture and warmth, and recommends mix around 70 to 75 degrees F. Auburn's chart lists 60 degrees F as the lowest soil temperature for pepper and 50 degrees F for tomato, so a cool room can hold the warm crops back.

What to do

1. Cover the tray with a dome or plastic until seedlings emerge, then take the cover off.
2. In a cool room, use a seed-starting heat mat to bring the mix to around 70 to 75 degrees F.
3. Check the crop's minimum on the soil temperature chart.

• Soil temperature for planting

The mix stays soggy, with gnats or green algae on top

Deal with it soon

The mix is too wet

Illinois Extension says waterlogged mix often lacks the oxygen roots need, which leads to root dieback, and that fungus gnats and algae growing on the surface can mean it is too wet.

What to do

1. Reduce watering and let the mix dry out, then keep it moist but not wet.
2. Make sure the pots drain.

Seedlings came up and then fell over at the soil line

Act now

Damping off

Illinois Extension describes damping off as soil-borne fungi that usually strike seeds started in cool, wet soil, collapsing seedlings at the base of the stem. Nothing can be done once plants are infected, so prevention is the only cure.

What to do

1. Discard the infected seedlings and the mix they were in.
2. Re-sow in new, well-draining, sterile potting mix, not garden soil or compost.
3. Sterilize reused pots and trays by soaking them in a 10 percent bleach solution for 30 minutes. Follow the bleach label's safety directions.
4. Plant at the right depth, start seeds at 70 to 75 degrees F, and avoid overwatering.

In short: Check the age of the seed first, then whether the species needs light or a dormancy treatment, then the tray's warmth and moisture. A paper-towel test tells you whether the seed is any good.

Work from the seed outward

When a tray stays bare, look in turn at the seed, the way it was sown, the temperature and the moisture. The tree above takes them in that order because the cheapest checks come first. University of Illinois Extension, whose seed-starting post supplies most of the logic here, begins with the seed itself.

Seeds lose viability over time. Illinois says some last about a year and others 4 or 5 years when stored properly, so leftover packets from several seasons ago are the first suspect. The fix is a simple germination test. Lay at least 10 seeds on a damp paper towel, seal the towel in a plastic bag, keep it above 70°F and check after 7 to 10 days. Illinois says seed that sprouts at 70 to 90 percent should be fine, and below 70 percent you should buy new.

The sowing

Some seeds need light. Illinois names lettuce, dill, snapdragons and coleus. Covering them stops them, so you sprinkle the seed on the surface and press it in gently. Others, especially many native trees and flowers, need a dormancy treatment such as cold exposure, scarification or soaking before they will sprout, which is why a packet of wildflower seed sown like a bean will do nothing.

Warmth and moisture

Illinois recommends keeping the growing mix at 70 to 75°F, covering the tray with a dome or plastic until the seedlings emerge, and using a heat mat in a cool room. The Auburn chart behind the soil temperature page shows why: pepper has a floor of 60°F and tomato a floor of 50°F, so a tray in a cool room can sit idle.

The opposite error is too much water. Waterlogged mix runs short of oxygen, and roots die back. Fungus gnats and algae on the surface are signs it is too wet, and Illinois's advice then is to cut back and let the mix dry out. Its standing rule is to keep the mix moist but not wet, and it points out that tiny seedlings, with little root, take up far less water than people expect. And if seedlings come up and then topple at the soil line, that is damping off, a set of soil fungi that favour cool, wet starts. Illinois says nothing can be done once plants are infected, so the cure is prevention: new sterile mix, clean pots, the right depth, warmth and light watering.

Worked example: a flat of peppers that never came up

You sowed peppers in a cool back room and covered the flat loosely. Start the tree with fresh seed, then answer the sowing question with the ordinary vegetable answer, then pick the cool-room answer. The outcome says the tray is too cool or drying out. It quotes the pepper floor from the Auburn table, which is 60°F, and tells you to cover the flat until emergence and warm it with a mat. A tomato flat in the same room has a lower floor, 50°F, so tomatoes may have sprouted while peppers sat idle.

Next steps

Once seedlings are up, strong light is the next need, covered in the leggy seedlings fix. To plan how long to grow each crop indoors, see the seed starting schedule. Sterilizing reused pots, the post says, takes a soak in a 10 percent bleach solution for 30 minutes; follow the safe-handling directions on the bleach label.