Sometimes seeds need a little help germinating. Seeds with a tough outer shell will benefit from scarifying the out most part of the seed. Other seed to be cold treated in a method call vernalization to trick them into believing a cold winter had happened. And during dry summer months it can be beneficial to pre-soak your seeds.
Some seeds that have a tough outer shell that can benefit from deeply scratching, or scarifying, the outside of the seed. Here we opened up a sweet pea seed to expose the inner seed and show an outer shell that is similar to a coconut shell. Deeply scratching the surface helps water enter the seed and increases your germination rate.
One way is to take a metal fingernail file and carefully create a nick in the seed that you can feel with your fingernail. You don't want to go down too far into the seed.
The method we like to use is a fingernail clipper to do the same and cut through the outer shell. We find this is quicker and once you get the hang of it you can get more consistent results. Be sure to only nick the surface of the seed until you can feel it with your fingernail and try not to go down to the lighter seed area.
Some seeds need to be cold treated prior to sowing to break their dormancy cycle. This does not apply to all flower seeds, and cold treating some seeds such as sunflowers, zinnia, and nasturtium will kill the seed.
To cold treat seed we put some potting soil, sand, or screened peat into a zip lock baggie or plastic container and then mix in your seeds. Add about a tablespoon of water, or a very moist paper towel inside a baggie and gently mix again. You want your soil slightly moist but not visibly wet. Place the baggie or container in your refrigerator, not your freezer, for between 30 to 60 days (depending on the seed type). We use sand for seeds that need light to germinate and peat for seeds that are going to be 1/4" or more under the soil. After the cold treatment remove from the baggie and sow.
For fall, winter, or spring sowing you don't need to pre-soak, but sometimes if sowing in dry spells or the hot summer months soaking will help seeds germinate. We always soak for about 8 hours and in a shallow dish with the water just covering 3/4 over the seeds, but not submerging the seeds. Some seeds are finicky about being submerged for long periods of time and they will actually germinate less if left underwater for too long of a period. We then pat dry the seeds with a towel, or paper towel, and sow in the garden normally.