SWEET PEAS do well in mild cool climate, so it is no wonder the gardener here never thinks them possible.
But I have seen excellent sweet peas grown in Mississippi, which has a climate much like Washington (at least north of Vicksburg it does) only more of it.
The real trouble with sweet peas is they hate hot weather, so you want them to bloom in the spring before summer heats arrive. That means, of course, the sweet pea plants should be as large as possible in April so they can spend May in a great lather of bloom.
If our winters were only a little milder, the sensible thing would be to plant sweet pea seeds in September. They would make small plants before winter, and be all ready to "take off" when spring arrived. And in some years that will work, especially if the plants are mulched well in November.
But in other years, the winters will prove too much for sweet peas and they will be just mush by February.
Early spring planting (is February spring?) is all right, so is March. Plant the seeds indoors then, and insofar as possible -- though I know how impossible this is for many gardeners -- give them plenty of light and a very cool temperature. An unheated sun room does well, if you happen to have one.
Seeds planted in individual pots usually do better than a multitude grown in a flat or large pot, since the young plants rather resent disturbance. Sweet peas make longer roots than seems reasonable, so deep pots are a fine idea. Often I have thought of starting them in those cardboard tubes inside toilet paper (using up the paper first, of course, and filling the tubes with potting soil). It is one of those things I shall never get round to doing, naturally.
Ideally, since the young plants cannot be relied on to come through winters here, the seed could be sown individually in pots which were then carried over the winter in a cold frame. Whenever you fool with a cold frame, or any other form of glass protection, you let yourself in for the chore of watching ventilation. A cold frame can get very hot on those winter days in which the sun is intense, even though the outside air is cold.
Some gardeners can remember, almost automatically, to prop up the sashes of the frame, lowering them about 3:30 in the afternoon, so all will be snug by night. Others forget to open the sashes, or forget to close them, or both.
And needless to say, the cold frame treatment does not work if you have no cold frame, or if you are one of those gardeners that travels about in January.
Let us suppose, then, what is true of most gardeners here, that you are neither going to plant the seed outdoors in the fall, nor winter plants over in a frame.
There is still a chance to have some nice sweet peas. I suggest planting them indoors, in individual pots, and setting them out the end of March with some protection, but that presupposes you have grown them "hard," which is a bit optimistic of me. For I have done this sort of thing often enough myself to have an excellent idea what most gardeners do, and that is grow indoor seedlings of a pale weak sort, barely holding on, and usually rather spindly to about 10 inches. Such plants, set out in March, are almost certain to be a disappointment, since one good sleet storm and they collapse. Still, if you can grow the seedlings hard, firm, close-jointed, tough, they may indeed be set out in March and you will be embowered by late May.
Failing all this (and most of all this will fail, I have noticed over the years) you can plant the seeds outdoors, where they are to grow, in mid-March.
If you do this, it truly is important for the soil to be good. It should be dug this very instant, and given a nice layer of manure, if possible, then broken up in early March and allowed to settle before the seeds are planted.
There is not much point hacking a hole in cold wet clay on March 1 and expecting sweet peas or anything else to grow in it. (Though I have planted gladiolus corms this way, which is shameful, but which worked all right since the corms, after all, have a great supply of food already built into them).
Assuming, then, that you can get friable earth in your place (and full sun for the sweet peas, please) you might plant them outdoors late in February, and again a couple of weeks later.
There should be pea sticks (branched breastwood from tree prunings) for them to grow on, or else those efficient plastic nets. Or a frame with ordinary strings. The plants need something to climb on, since they will reach five feet.
I should say I have seen sweet peas blooming in half-shade from seed planted in April. In other words, very slothful approaches sometimes work. And the summers up here are so deficient in heat, sometimes, that sweet peas may bloom right on through them.
Still, even if one does not intend to do it really right (fall planting in pots and cold frame treatment for winter) we might at least do our best to make sure our ground is in good shape by early March, and to plant the seeds then, not waiting until the weather is comfortable. Comfortable for us, but not for the sweet peas, who like it cold. Perverse little creatures, needless to say.