This section is from the book "Manual Of Gardening", by L. H. Bailey. Also available from Amazon: Manual of Gardening.
A hardy, tendril-climbing annual, universally prized as an outdoor garden plant; also forced to some extent by florists. On any occasion the sweet pea is in place. A bouquet of shaded colors, with a few sprays of galium or the perennial gypsophila, makes one of the choicest of table decorations.
Deep, mellow soil, early planting, and heavy mulching suit them admirably. It is easy to make soils too rich in nitrogen for sweet peas; in such case, they will run to vine at the expense of flowers.
Sow the seeds as soon as the ground is fit to work in the spring, making a drill 5 inches deep. Sow thickly and cover with 2 inches of earth. When the plants have made 2 or 3 inches' growth above the earth, fill the drill nearly full, leaving a slight depression in which water may be caught. After the soil is thoroughly soaked with water, a good mulch will hold the moisture. To have the ground ready in early spring, it is a good plan to trench the ground in the fall. The top of the soil then dries out very quickly in spring and is left in good physical condition.
In the middle and southern states the seed may be planted in fall, particularly in lighter soils.
Frequent syringing with clear water will keep off the red spider that often destroys the foliage, and attention to picking the seed pods will lengthen the season of bloom. If the finest flowers are wanted, do not let the plants stand less than 8 to 12 inches apart.
A succession of sowings may be made at intervals through May and June, and a fair fall crop secured if care is taken to water and mulch; but the best results will be secured with the very early planting. When the plants are watered, apply enough to soak the soil, and do not water frequently.
 
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