You prepared the soil and planted the seeds; now it’s their turn. Pop up little sprouts! But sometimes they don’t. What happened? 

Problems with outdoor seed germination can be frustrating to identify because so few of the conditions are under our control. Direct seeding outdoors is simple, but if the basics of seed germination aren’t met, it can lead to poor success rates and trouble. Here are some factors that can cause low germination rates (or none at all), along with what to do about them.

Soil temperature

Seeds need an appropriate temperature to begin the process of germination. The stored energy in the seed, in the form of carbohydrates, needs to be broken down into glucose by enzymes. Once in glucose form, it can be used by the embryo in the seed to grow. If the temperature is too cold, the enzymes may not activate. Too hot and they can be damaged or destroyed.

Plants vary in their optimum germination temperature, but, in general, cold temperatures slow germination by slowing the chemical processes that break down stored carbohydrates. Even cool-weather crops will germinate more slowly in cold weather than in more moderate temperatures.

For seeds, the soil temperature is more important than the air temperature. Take a thermometer and measure the soil temperature. If it’s colder than 60℉, wait to plant warm-weather crops like cucumbers, squash, or melons.

Seed planting depth

This one is pretty forgiving, but still important. It’s easy to unintentionally plant seeds too deeply. Many small seeds are planted ¼” deep or less. Thinking they’re doing the seeds a favor, some gardeners plant these seeds half an inch deep or more, tucking them in safely. 

Remember, all of the energy needed to put out a root and to push up through the soil and unfurl a set of seed leaves must come from that stored in the seed. Tiny seeds simply don’t have as much stored energy. A snapdragon or carrot seed planted too deeply might just run out of steam before ever reaching the surface. In reality, the seed germinated, but you never saw it because the seed’s energy was exhausted before the sprout could reach the sunlight. 

Pay close attention to the seed depth recommendations on the packet, and follow the general rule: small seeds are planted shallowly, and large seeds are planted deeper.

Soil crusting

Soil crusting can be a problem outdoors, but usually only when planting in the ground, not in containers. Both raised beds and traditional garden beds can be susceptible. 

Soil exposed directly to the actions of sun and rain can often form a hard crust at the soil surface. You may have noticed this when weeding. The crust over the soil underneath can be too much resistance for a small seedling to break through. It’s a particular problem for delicate carrot seedlings. 

To avoid soil crusting issues, apply the thinnest possible layer of fine mulch. Don’t use bark or wood. Clean grass clippings, shredded leaves, or straw rubbed between your gloved hands (to break it up into smaller pieces) works well. Apply the mulch lightly enough that you can still make out the soil underneath, not so thick that it’s a blanket. All we need to do is lightly shade the soil for a few days, not make a new barrier for little seedlings to struggle through. After they’ve popped up, the mulch can be applied in a thicker layer to suppress weeds.

Moisture swings

Seeds need three things to germinate. Oxygen (normally not a problem for us unless the soil is water saturated), the right temperature, and the right soil moisture. We talked about the temperature above. Soil moisture is equally important.

Seeds have an outer coating that protects the inner parts. When water soaks through the seed coating, it softens the shell, making it easier for the new root and shoot to break free, like a chick breaking out of an egg. Moisture also initiates the biological processes that lead to germination. Without moisture, the seed just sits there, waiting, even with oxygen and the proper temperature.

What our seeds need is not too much moisture, and not too little. With overly dry conditions, the seeds won’t absorb any moisture, and the process won’t start. With too much, the seeds could rot before they finish germination. 

When directly sowing seeds outdoors, whether in containers, raised beds, or in the ground, feel the soil with your fingers. If the soil feels damp, that’s usually enough moisture. If it feels dry or you aren’t sure, water the seeds well after planting. It’s a trick many gardeners miss, and it will speed up your germination. 

Old seeds 

Seeds have a lifespan. As time passes, the germination rate, or the number of seeds that will still sprout when planted, drops. Onion and parsnip seeds, for example, experience a sharp drop off in germination rate after only a year or two in storage. Tomatoes, cucumbers, and beans commonly store well for three years or more.

Proper seed storage (cool, dark, and low humidity) goes a long way toward maintaining viability, but if your pack of sweet corn seeds is five years old and didn’t sprout well or at all, fresh seed could solve the problem.

Pathogens, pests, and seed thieves

Sometimes we do it all correctly but still have problems. In that case, troubleshooting can be a matter of observation and elimination.  If the soil moisture and temperature were in the proper range, the soil wasn’t crusted over, and the seeds weren’t old, it could be a pathogen or pest. 

The same pathogens that cause damping-off can also attack seeds, killing seedlings before they reach the surface. These fungi and fungi-like organisms are more common problems in overly damp soils in spring. If your seeds should have popped up but nothing is happening yet, gently dig around and look for a few seeds under the soil (you did mark your rows, right?). If you see the seeds sitting there undamaged, cover them back up and wait, and water if necessary. If they’re mushy, have a dead-looking sprout, or are blackened, it may have been a pathogen. Try replanting in a new bed or transplanting larger seedlings.

Pests can sometimes be a problem for seedlings, leading you to think you had germination issues. A couple of years ago, seemingly none of my zinnias came up. The environmental conditions had all been satisfactory, and I was puzzled. The seeds were fresh, but I planted a new round anyway. Still nothing. A third planting revealed the problem. Daily observation revealed that although the seeds were germinating quickly in the warm soil, slugs were eating the new sprouts as they popped up! A bit of slug bait, and my zinnias (the fourth sowing) were on their way.

Sometimes, you’re the victim of simple common thievery. All winter long, I’ve taught my backyard birds to eat sunflower seeds at the feeder. Guess what they do in spring when I plant a patch of sunflower seeds? I’ve seen them hop right down the row, snatching them out of the soil. Chipmunks, squirrels, mice, and birds are all seed thieves. While netting can help with the birds, I’ve found that a thin layer of grass, pine straw, or hand-rubbed straw (as above) will greatly reduce seed predation. The larger seeds that are commonly choice food for these critters don’t have any problem poking up through a half inch of mulch, and you’ll keep a few weeds down too. If you’ve had problems with animals eating your seeds, try a thin layer of mulch immediately after planting.