Now, here's an interesting little chart. I found it to be quite the eye opener. This information can be put to good use: What can the gardener do to ensure good germination when direct sowing? There are several methods that can be employed.
1. Pre soak or even germinate your seeds, and sow them on a warm day. If you use any of these methods, you need to be careful that you don't soak them too long. That will encourage rot. And, if you pre-sprout your seeds, they must get planted immediately at the first sign of germination. Pre-sprouted seedlings are very delicate, and it's easy to break the root or cotyledons off... which will kill the sprout.
2. Pre-heat your soil. I employ this practice when ever possible. I hang on to large sheets of plastic, and use them multiple times. Covering the soil with plastic before planting works best if the plastic is in direct contact with the soil. When it's time to plant, it's best to remove the plastic so you don't scorch the seeds when they sprout. Even a day or two of extra solarization will do wonders to warm up a bed before you plant it. Then, you can move that sheet of plastic on to the next bed.
Depending on the temperature, and the crop, I may trench the rows under the plastic. This works especially well with corn. Dig a 4" furrow, and plant the corn at the bottom of the furrow. You can then leave the plastic in place. The furrows result in little inverted grow tunnels. Of course, you must monitor soil temps and remove the plastic if things heat up too much. Then, as the corn grows, it's a simple matter to use a hoe to pull the soil up around the plants. Thus, hilling your corn, and killing any weeds that sprout in those little trenches.
3. Sow in a low tunnel, or erect a low tunnel over your row/bed after you've sowed your seed. You can also use milk jugs, or wrap a tomato cage in plastic. I even lay tomato cages on their side down the length of a bed, and lay the plastic over the cages. This makes an instant low tunnel if you stake down/ weigh down/ or bury the sides of the plastic with soil. Beware: tunnels and plastic containers quickly overheat. You must monitor the climate inside!