Answer: Sow seeds indoors, in a seed-starting soil mix, about three weeks before the last hard frost for your area. This is probably mid- to late-March in your area. Once they're up and there's no danger of a hard frost, transplant seedlings into the garden. Sweet peas will tolerate cool soil and a light frost.
Whether you sow them indoors or directly in the garden, soak the seeds in rainwater or springwater overnight before planting. This softens the seed coat and speeds germination.
If your water has a high alkali content, that usually means it is high in sodium ions and these can inhibit plant growth. The soil amendment called gypsum can help remove accumulated sodium ions from soils. But especially while plants are small, water with rainwater.
Your sandy soil will dry out quickly meaning you'll need to water plants frequently. Once plants are 3 to 6 inches tall, spread an organic mulch around them to help retain moisture.
Sweet peas prefer a pH of around 6.0-7.0, so if your pH is also high, the plants won't thrive after germination. Consider having your soil tested. The testing labratory will follow-up with ways to improve your soil.
Hopefully the mountain soil you imported will improve the soil, but amending with compost--home made or not--usually helps, especially in sandy soils.
Sweet peas are heavy feeders, and thrive rich, humusy soils. Alternately, you can grow your peas in large planters so you have better control over the soil environment.
For best performance, sweet peas need even moisture and cool soil. Once the peas are up and growing, climbing their trellis, mulch the soil to keep it cool and to conserve moisture.
Finally, you can coat the seeds with nitrogen-fixing bacteria, called Rhizobium. These organisms are found naturally in soils and they work in a mutually beneficial relationship with legumes like peas. The bacteria helps the plant by providing nitrogen, and the peas "pay back" the bacterium with carbohydrates. You can purchase seed innoculants from Peaceful Valley Farm Supply in Grass Valley, California: http://www.groworganic.com
I hope this helps!
Q&A Library Searching Tips