Are you having trouble with difficult seeds? Try these tricks to improve germination.
Some seeds take a long time to absorb the water they need to germinate. This
process is speeded up by soaking them in water overnight. Don't soak seeds for more than 24 hours, unless specifically recommended on the packet.
Nick, chip, or scarify:
Seeds like sweet peas and morning glory have hard seed coats that block the uptake of water into the embryo. To speed germination, you need make a tiny hole in the seed coat. You can either nick the surface of the seed with fingernail clippers (much safer than a knife), or rub the seed against a piece of sandpaper. Be careful not to damage the embryo by removing too much of the seed coat. It's best to nick or scarify the seed on a side away from the growing point. For example, the growing point on a morning glory seed is the pointed tip, so scarify it on the rounded side.
A percentage of the seeds will sprout without nicking. If you don't want to nick all of them, just soak them overnight, plant the ones that swell up, then nick the remaining seeds and soak them again.
Refrigerate, pre-chill, or stratify:
Some seeds like columbine and penstemon germinate best after a period of cold and wet that simulates the winter season. This is called cold-moist stratification. The easiest way to stratify seeds is to sow them into their pots, water them, cover the pots with plastic, and place them in the fridge for the recommended amount of time.
If fridge space is limited, try this method: Place the seeds in a paper napkin, then fold it and moisten with tap water. It shouldn't be sopping wet, just damp. Place the napkin in a ziploc bag, and put it in the fridge. After the recommended amount of time, remove the seeds from the fridge and pot them up. This method won't work for very tiny seeds, which are difficult to handle individually. You can try stratifying these seeds in a bag with moist peat moss, which can then be scattered over the surface of the soil. The only drawback to this method is that you can't tell how thickly you're sowing the seeds.
After stratification, place the seed pots in a sunny window or under lights for germination.
Some seeds like Eryngium giganteum need alternating warm and cold temperatures. It's best to sow these seeds outdoors in the fall and let nature work on them over the winter. They can be direct sown in the garden, or they can be sown in pots and kept in a cold frame or a protected location like the north side of the house. Check the pots frequently, and add enough water to keep them moist but not soggy.
When in doubt, experiment with different germination methods. Then you'll know what works the next time around.