Answer: Perennials drop seeds at different times throughout the growing season, depending upon the species. The fate of those seeds usually follow two different directions. Some seeds remain in the soil throughout the winter and germinate in the spring, while others perennial seeds will germinate shortly after dropping on the ground and grow quickly so as to have some food storage before dying back for winter, then flower the following year. The ones that drop their seeds and don't germinate until spring have tough outer husks that can withstand winter, then germinate in the spring and flower.
Many cultivated perennials are easily and optimally started from seeds in July and August. This allows the plant a chance to grow and get their root system established before dying back in winter. Of course, only the top part of the plant above ground is affected while the root system is safely tucked away underground and quietly waits for the warm days of spring to start growing it's top again. Seeds that you save from your own plants often will not produce the exact same plant as what you got the seeds from. So expect instead to save money compared to purchasing plants from divisions, and enjoy witnessing plant genetics in progress!
Generally, late-summer or fall flowering perennials are planted in the spring, while spring-flowering perennials are planted in late summer or early fall. Phlox flowers in the summer, so you can sow your seeds outdoors in the spring, or scatter them in the garden in the fall where they will go through a natural cold period (stratification) prior to germinating. If you want to start the sowing process now, you can artificially treat the seeds and then sow them in containers. (Artificial stratification - this process helps recreate the natural seasons so that the seed knows it's time to germinate. For cold stratification, place the seeds in moist peat moss or vermiculite in the refrigerator. For warm stratification, place the container in a warm spot. After the first month or so, examine the seeds regularly for signs of germination. As soon as the small white primary root appears, plant the seed in soil.) Regardless of the time of planting, perennials should be allowed sufficient time to establish themselves before blooming or the onset of cold weather.
How to start seeds: Seeds can be sown directly into a prepared bed. Or, sow them directly into prepared containers. Sow the seeds generously to compensate for unpredictable germination percentage and thin-out afterwards. Maintain frequent irrigation until seeds have germinated and seedlings become established. Protect the seedlings with a light mulch to enhance winter survival. In most cases, you can expect flowering the following season.
Don't let the soil dry out completely or your seeds will dry. Keep seedlings covered with shade cloth or remay while they are young and tender to prevent sun burn or breakage from wind, animals, etc. As with any new plant introduced to the garden, watering is very important so they can establish a good root system. Be very vigilant about weeds so they don't take over or compete with your new seedlings.
Perennials will come back year after year, but often it is a good idea to replace a few of them each season as they usually will not look good and do well after three years in one spot.
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