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How to Grow Wildflowers
In California and areas with mild winters, plant annual seeds in fall, winter, or early spring to benefit from seasonal rains. Rain will encourage seed germination, but most cool season growth is directed downward, to the roots. In spring when the weather mellows the above ground growth begins. Late spring and summer planting of annuals is also possible, but only in areas which receive irrigation.
Perennial wildflower seeds may be sown almost year-round in mild winter climates. Again, planning your project to coincide with seasonal rains eases your need for supplemental watering. These seeds may be sown directly on the ground or started in flats for transplanting.
In climates with heavy frost, or for late season bloom, seeds of annuals should be planted in spring or early summer. They may need irrigation during the growing season to compensate for a shallow root system.
Cold actually helps some perennial wildflower seeds germinate, and it is possible to do a fall sowing of these seeds in autumn, even in areas with heavy frost. After tilling the soil broadcast your seeds evenly over the surface and then cover them with up to 1/4" of soil and mulch. The mulch protects the seeds from washing away. Sow your seeds in late fall, when the weather is cold. Slightly frosting weather is okay. The idea is to put the seeds out when it is too cold for them to germinate (below 45° F). They overwinter on the ground, and then open in early spring. If unsure of this method it is better to wait for a spring sowing.
When broadcasting wildflower seeds in the landscape sow them thinly. You can mix seeds with sand or vermiculite for more even distribution, and to make them more easily cover a large area. Protect the seeds from birds by covering with an 1/8" layer of soil, peat moss, straw, or by using some other protective material. Covering them too deeply will interfere with germination and may cause project failure. If a lawn roller is available, passing over the prepared area after sowing will greatly enhance germination. If you cannot get a roller, pressing a board or walking over the prepared surface will do.
Remember to keep your seeds moist after sowing. The most common cause of seed failure is desiccation (drying out). If you have an irrigation system, a five-minute interval at the beginning and end of each day will help. If you are depending on seasonal rains, supplemental water during dry spells the first six weeks after sowing will greatly enhance your results.