Growing Sweet Peas

Getting Started
When to Plant
Site Selection
Germination
Soaking/Chipping

Germination Rates
Growing On
Cultivation
Problems
Containers

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When to Plant

Before you start, consider the proper planting time in your area of the United States or Canada.  Unlike England, where the climate is more or less uniform throughout the country, we are blessed with climates ranging from sub-tropical to Arctic.  Indeed, within a given planting zone there may also be many microclimates.
The best thing to do in your area, and I say this in all seriousness, is to find a gardening friend who has good luck with Sweet Peas and ask them when they start their seeds.  Lacking this resource, inquire at a reputable garden center in your neighborhood about the best time to get started.
If all else fails, consider the following general guidelines for starting your seed:
Sweet peas are cool weather annuals. Sweet peas germinate when the ground temperature is a uniform 55-65 degrees.  Once germinated, the plants thrive at temperatures from 70 to 80 degrees. Above 85 degrees, they really struggle. They hate hot, dry winds. If you live in an area where the transition from cold winter temps to hot summer temps happens quickly, you should consider starting your seeds indoors and transplanting. Start your plants six to eight weeks before you plan to transplant them. 
In an average weather-year the general rules for the USA for starting sweet peas are:
California- August to September for spring bloom- Generally the winter flowering varieties should be used for this planting time. If you have a cool, wet autumn this will work. If your fall is hot and dry, this will be challenging.  (Make sure you keep your soil moist at all times.  Don't even think of letting the soil dry out, much less letting it commence!!!!!!!!!!!!!!)  If it is hot and dry, consider waiting until the fall rains begin in November, December or January. 
November to January for summer bloom- Plant summer blooming varieties. Of these, Cuthbertson's blooms first, Old Fashioned are most heat resistant, and Spencers, which have the most spectacular blooms, bloom three weeks later.
Southwest. South Texas and South Florida- All of the literature I have read suggests September and October if climate is hot- typically in the lowlands.  Early spring in mountainous areas of the Southwest.  Remember to keep them moist.  I need feedback from people in these climates.  Please read about California (above)
Lower Southern States- November to January- This large geographic area is difficult to predict accurately, but remember that the ground temp must be consistently 55-65  degrees for the seeds to germinate. Consider transplanting.  I need feedback from people in these climates.
All other areas- January to April - Because most climates in the US have hot summers, I advocate starting plants early indoors and transplanting.  I need feedback about your successes and failures so I can help others.

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Site

Choose a flat site that gets plenty of sun.  Rows should run north to south.  Soil should be well drained.  Plants like the soil moist but not waterlogged. Protect from wind.  Avoid planting near trees or hedges, where there will be competition for the roots.  If you cannot meet all of these requirements, choose a site that resembles it most closely.
It is a good idea to prepare the sweet pea bed in the fall.  When spring comes you will be all ready to go.
Soil preparation- Soil should be deeply dug- no less than 12" deep. I prefer 18". Add well- rotted compost or other organic matter to the soil. Soil should be well drained. When your plants are in the ground the soil must be kept moist, but not waterlogged. If given the opportunity, sweet peas will develop a large root system. This makes sense if one considers the amount of growth above the soil; healthy plants will grow 8-10 feet.

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Germination

Method 1.  Paper towel method (no picture).  Many people like to germinate seeds between paper towels.  This is an effective and easy procedure.  It is neat and can be done indoors using minimum space and no special equipment.  (It's also a great kid project). Using a cookie sheet (with 1/2 inch sides) place 3 or 4 paper towels flat on the cookie sheet.  Next, spread your seeds evenly on top of the paper towels.  (I usually wet the towels down at this point to keep the seeds from rolling around).  Cover the seeds with 4-5 more paper towels and wet the whole thing down.  Put them in a nice, cozy spot.  Check the seeds daily.  Make sure you keep them moist.  They should germinate within one week.  Once they have germinated you may transplant them either into small pots or into the ground.  Which of these you do depends on your weather.  Be careful not to damage the tender new roots or cotyledon.   Once germinated they need plenty of light.  (I used this method exclusively in 2001- because we were moving and I had no alternative.  The germination part went great, but I had trouble with transplanting.  I knew I would not be around to take care of my newly transplanted sprouts.  I was at the mercy of the weather.  Of course the weather did not cooperate.  The new sprouts required constant moisture and, unfortunately,  we did not have enough rain in April.  Many of my transplants dried up and disappeared.  -See above-  Quite a few survived- even though they were neglected.)

Method 2. Here is how I started mine indoors six to eight weeks prior to transplanting. 
  (If this flat of seedlings looks especially full, it is because I put four times as many plants as I should have.  Fifty plants should be plenty.)  Using an Anderson propagation flat (this is about 17 inches square and has small holes on the bottom so that the potting soil will stay in the flat), I filled the flat full of potting soil. I soaked down the potting soil, which made the media settle down so the flat was ¾ full. I then placed my seed on top of the potting soil.  I neither soaked nor chipped the seed.  A flat this size will grow about 30 plants quite nicely.  Next I covered the seed with potting soil so the flat was again filled to the top. Next I watered the flat thoroughly and let the flat drain of excess water. When the flat stopped dripping I brought it inside and put it on top of my file cabinet. I put down a plastic garbage bag to protect the cabinet. I put newspaper on top of the flat to keep it from drying out. The seeds do not need light to germinate. When the flat seemed to be getting a little dry, I took it outside and soaked it again. When it finished dripping, I returned it to its cozy spot on top of the filing cabinet.  I checked daily to see if the seeds had germinated. The seedlings began emerging within a week. As soon as they started coming up I removed the newspaper and put them where they could get as much light as possible.  The plants need light to grow well. Without it they will become long, pale and leggy.  Once all of the seeds germinated, I moved the flat to my unheated greenhouse, where there is adequate light. There they stayed until it was time to plant. They went through, and survived nicely, several frosts.  For the individual, however, I suggest bringing the flat indoors when a frost is in the forecast. If, however, you forget to bring the plants in before a freeze and your plants appear to be frozen, the chances for survival are best if you let them thaw out on their own outside.

Method 3.  Use individual pots instead of a single flat.  I use this method all of the time for plants that I am growing to sell.  I use 2 1/4 inch pots and find them to be quite adequate.