Broccoli seeds are best started indoors 7 to 9 weeks before the last frost, at a temperature of 70 to 75 degrees F. They can also be sown outdoors 2 weeks before the last frost. For a winter crop in zones 8 and warmer, sow in late summer. Expect germination in 10 to 14 days. Warm soil will speed up broccoli germination, while cold soil will slow it down. If you want your seeds to germinate faster, put your seedling trays on a seed starting heat mat to keep the soil nice and warm. … Broccoli seedling lighting ? Broccoli seedlings need a lot of light or they will stretch and grow leggy.