All seeds should be started between 3 and 10 weeks before your last frost date. You can get the last frost date in your area by looking at the USDA zone map or contacting your local County Extension Office.
For peppers, I like to start them about 8 to 10 weeks prior to planting in the garden. For tomatoes 6 weeks works best for me, but can be started earlier if you are willing to keep potting them into larger containers. For other vegetables, you can sow them directly in the ground after the last frost date or start them about 3 to 4 weeks before last frost date. All squash and cucumbers will do significantly better if you start them inside a few weeks before your last frost date. I prefer to start just about everything in containers inside and then plant out. I have observed that my garden performs better when I do this.
The best information I can give you is this: Do not plant your seeds in PEAT PELLETS or SOIL. Real soil will have too many nutrients for new seedlings and could possibly have pathogens that can infect the seedling or cause the seed to rot. Peat pellets have probably caused more problems than I can count. When I had a retail nursery I not only sold the things, but I tried many times to use them in the greenhouse for plant propagation. One year I had every single tomato lost due to damping off and experienced low germination rates as well. Upon further investigation, I found the seeds were rotting in the peat pellets before they could sprout. The biggest problems I have encountered with peat pellets have been with people actually planting the seeds too deep. For some reason most people poke the seeds way too deep into those soggy biscuits for them to ever emerge. So please avoid using peat pellets or peat blocks.
The following is based on tomato plant seeding. Not all of the information will apply to larger seeds like squash or cucumbers.
So, how do you go about planting your seeds? First always use clean containers. If you are reusing something from previous years, then you should soak them in a mixture of bleach and water for at least 10 minutes. This should kill any plant pathogens that have lingered on the containers. Your containers must have drainage holes in the bottom or your seeds could rot.
Once you have clean containers fill them with a quality soilless seed starting mix. Or if you have a soilless potting mix you can sift it through a screen until you have a very fine fluffy medium to use. I use either Promix or Sunshine potting mix and sift it. I have also used Jiffy Seed Starting Mix and had good results. Just don’t use dirt, soil, compost, or peat pellets.
Once your trays are full, make a label for each variety you are going to plant. It helps to mark them with the date too. Once your label is made, place it in one of the containers, or cells and open the appropriate package of seeds. Gently tap the surface of the soilless mix to give the seeds a flat bed to rest on. Do not pack the soilless mix. Place the seeds on the surface of the soilless mix in the container or cell where they are not touching each other. You can use a toothpick or pencil to arrange them if necessary. Plant more seeds than you will need. Planting one or two seeds is a rookie mistake that you should avoid and will never give you positive results. I recommend you plant them all. You can always find someone that will take a free plant and you can save fresh seed from this crop for next year. Of course, if you are planting an F1 hybrid you cannot save seeds and have them come true the next year.
After the seeds are in place, sprinkle them with just enough of the soilless mix to cover them. Most seeds should not have more than twice the thickness of the seed of mix on top of them. For tomatoes, eggplant and peppers, they should not have more than about the thickness of a Nickel coin of soilless mix on top of them. Once the seeds are covered, place your next label in the container or cell and open the next seed package. Never work with more than one package open at any given time. This prevents seed mix-ups and confusion.
Once you have all of your seeds placed and covered, you will need to add moisture. I like gently spraying water on the surface until they are saturated. The mix is saturated when water runs out of the bottom. Be careful not to splash the surface and make the seeds jump out. Misting is preferred, but you can let water soak up from the bottom by placing in a tray of water if you prefer. This takes longer for the medium to become wet and can over soak as well, but can be done if you pay attention to what you are doing.
I usually use small cell containers that can be purchased at discount stores instead of the large commercial trays. This allows me to do small batches a little at a time and not be overwhelmed by the volume of seeds I have to plant each year. Also, these small trays can easily fit inside a gallon size plastic zip bag. This is like a tiny greenhouse. I highly recommend you try to find some small trays and use the plastic bag greenhouse method. If you can’t find the small trays, then cover whatever you are using to start your seeds in with some sort of plastic or wax paper cover. This holds in moisture and keeps the seeds from drying out. If seeds dry out after they have been wet they will not germinate. There are also trays with lids available at many home improvement and discount stores. These work great as long as you use the ones that allow you to fill with your own soilless mix.
Next, place your seed containers in a warm location. I use a heat mat, but you have to be careful with heat mats or you will bake your seeds. Only use heat mats with a thermostat. If you have the kind that just raises the temperature a few degrees above room temperature you will most likely get the seeds too hot. This will be ok for peppers, but for most other things and especially tomatoes it could be disastrous.
Most seeds do just fine with room temps that are above 70 degrees. If you have a spot in your house that is extra warm you can place them there. My friend Larry put his tomato seeds on top of a satellite receiver and they popped up in just a few days from the bottom warmth. If you come up with a creative way to warm your seeds until they germinate, I would love to hear about it.
Most seeds will sprout within 10 days with some a bit sooner and some a bit later depending on variety or other factors. You should have most of your seeds germinate within 3 weeks but I have had peppers take a month. If you don't see germination within the 3 week window, chances are you have done something wrong.
Once your seeds sprout and emerge from the soilless mix, you should remove the cover. Your seeds will then need a great amount of light. A sunny window usually is just NOT enough, but if that is all you have make sure they get as much light as possible. We suggest you place your seedling container under a florescent light fixture and keep the light on for at least 12 hours per day. If your seedling don’t get enough light they will get long and spindly. If your seedlings get leggy from the lack of light they usually will be ok, just behind on when they produce.
As your seedlings grow, keep them moist but not soaking wet. I only water every few days in the early stages. Knowing when to water takes some experience. You don’t want them to ever dry out but you don’t want them too wet either. You will see the surface of the soilless mix become lighter in color as it dries. Usually the entire surface of the soilless mix will become somewhat uniform in the lighter color when it is time to water. Just don’t let them wilt. As I said, this takes some practice.
When a seedling emerges, it has two cotyledons, or seed leaves. These are not true leaves. A plant will need some time in the warm light to make real leaves. When the first set of true leaves appears, then you can transplant your seedlings. If you followed the instructions so far, you should have multiple plants growing in a single cell or container. To separate the individual plants, just pop them out of the cell and by gently holding them by the stems. Dip them repeatedly in a cup of water until the roots are dangling and all the soilless mix is gone. You should then be able to easily pot the individual tomatoes. For the first pot up you should stick to using a soilless mix to keep from dealing with damping off or other diseases. After I transplant the seedlings I usually water them in to remove the air pockets in the potting mix. It is ok to use a 50% diluted water soluble fertilizer at this point. If you use an organic fertilizer like fish emulsion, use at about 75% strength.
After all danger of frost has passed, then you can plant your tomatoes in the garden or container.