Starting Seeds

Published On: March 15th, 2018


Growing plants from starting seeds will allow you to grow a wider range of varieties, and often at significant savings over purchasing more mature plants. It is also fun and satisfying! Here are our guidelines for starting seeds.

“Soil”

You will need a clean, well-drained medium for germinating your seeds. Seed starter blends are actually soil-less and are usually made up of perlite or vermiculite and peat moss.

Perhaps the most convenient way to start seeds is in a specially designed seeding tray, but you can use a wide range of containers as long as they have adequate drainage. If re-using a container it is important to thoroughly clean and sterilize it first.

The general rule of thumb is to plant seeds two to three times as deep as they are wide. When in doubt, err on the shallow side – planting your seeds too deep will inhibit germination. Some seeds need light to germinate so should not be covered by soil at all. Planting depth information is usually included on the seed packet.

Seeds need to be warm to germinate – most like temperatures between 65 &75 degrees Fahrenheit. If you do not have a warm spot in your house you can purchase warming germination mats to put under your seeding flats. Supplemental heat only needs to be provided until the seeds sprout. After that, the lights they are growing under should provide adequate heat.

Even a South facing window usually will not provide enough light to start seedlings. Full spectrum lights should be hung as close to the seedlings as possible, about 2”-3” above. Raise the lights as the seedlings grow to maintain this same distance. Once the seeds germinate the lights should be kept on 16 hours a day and off 8 hours a day. The easiest way to achieve this is to hook your lights up to a timer.

Keep moisture levels high for germination by covering your seed tray with a clear plastic top to lock in moisture. Once seeds germinate remove the plastic cover. Check your plants every day. Soil should be kept moist but never wet. Spraying with a mist bottle is a good way to water young seedlings.

Make sure to label your seeds! We usually simply mount the seed packet on a small stake at the top of each section of seeds.
starting seeds flower vegetable seed seedling container Heyden's Gardens cedarburg garden center
starting seeds flower vegetable seed terrarium hot house greenhouse Heyden's Gardens cedarburg garden center

Starting Seeds – Flowers

Plant When to sow seeds indoors Notes Best sown outdoors
Ageratum 6-8 weeks before last spring frost do not cover seed (need light to germinate)
Amaranth 6-8 weeks before last spring frost
Bachelor Button Direct Seed in May-can tolerate light frost
Calendula Direct Seed in May-can tolerate light frost
Castor Bean 6-8 weeks before last spring frost soak seed before sowing Seed outdoors after last spring frost
Celosia 4-6 weeks before last spring frost
Cleome 6-8 weeks before last spring frost Direct Seed in May-can tolerate light frost
Coleus 6-8 weeks before last spring frost
Cosmos 4-6 weeks before last spring frost Seed outdoors after last spring frost
Gomphrena 6-8 weeks before last spring frost
Hyacinth Bean Vine Seed outdoors after last spring frost
Impatiens 6-8 weeks before last spring frost do not cover seed (need light to germinate)
Marigold 4-6 weeks before last spring frost Direct Seed after last spring frost
Morning Glory soak seed before sowing Seed outdoors after last spring frost
Nasturtium Seed outdoors after last spring frost
Nicotiana 6-8 weeks before last spring frost
Nigella 6-8 weeks before last spring frost Direct Seed in May-cold tolerant
Petunia 8-10 weeks before last spring frost Tiny seed (mix w/ sand)/do not cover (need light)
Poppy (annual) Seed outdoors after last spring frost
Salvia 6-8 weeks before last spring frost
Snapdragon 6-8 weeks before last spring frost Tiny seed (mix w/ sand)/do not cover (need light) Direct Seed in May-cold tolerant
Sunflower 4-6 weeks before last spring frost Direct Seed after last spring frost
Sweet Alyssum Seed outdoors after last spring frost
Sweet Pea (annual) soak seed before sowing Seed outdoors after last spring frost
Tithonia 6-8 weeks before last spring frost
Zinnia Seed outdoors after last spring frost
starting seeds flower vegetable seed growing greenhouse Heyden's Gardens cedarburg garden center

Starting Seeds – Vegetables

Plant When to sow seeds indoors When to sow outdoors
Beans June
Beets June
Broccoli March/April May
Brussel Sprouts April May
Cabbage March/April May
Carrots May
Cauliflower March/April May
Corn June
Cucumber June
Eggplant March/April June
Garlic (bulbs) October-December
Kale May
Lettuce May-August
Melons May
Onions February-April May
Peas May-July
Pepper April May
Potatoes (tubers) April-May
Pumpkin April June
Radish May-August
Spinach April May-August
Squash (Summer) June
Swiss Chard April May-June
Tomato April N/A