Sowing a row of sunflowers by the driveway is simple. But some flowers are a little harder to germinate. Many perennial and some annual flowers benefit from being started indoors and then transplanted as seedlings. 

Why start flower seeds indoors?

Weather, variety, precision control, and little feathered seed thieves are all good reasons to start flower seeds indoors. Starting flower seeds indoors not only provides controlled conditions for better success with difficult seeds, but also gives you access to a much larger selection than the few varieties for sale at the garden center.

Get a jump on the season

We often think about starting seeds indoors to get a jump on the spring growing season. Slow to germinate or slow to grow flowers can be started indoors, shaving weeks or even a couple of months off the time to bloom once they’re transplanted outside. 

It’s easy, but you will need to plan ahead. In spring, many flower seeds can be started 6-8 weeks before you’ll be ready to transplant them, ensuring nicely sized young plants to use when filling your hanging baskets, borders, or containers. However, don’t neglect the need for space. Two dozen geraniums or marigolds will take up a surprising amount of room as they grow, so plan ahead on where you’ll keep them until planting day. 

Cool-weather flowers for fall can be started indoors, too. The heat of summer may be too much for flowers like sweet peas and poppies. Starting them inside, away from the heat and intensity of summer, can be a perfect way to have them ready for autumn blooms.

Make your own starter plants for baskets, planters, and beds

Buying flats of petunias, geraniums, and marigolds at the garden center is my idea of shopping, but it can get expensive. Since I can never seem to resist adding a few more pots around the house, I start many of the flowers from seed indoors in late winter. 

By spring, they’re ready to be transplanted into pots, just as if I’d purchased them. It leaves my gardening budget more flexible to buy something that catches my eye, since the bulk of the flowers I need for my containers are already growing at home.

Exercise exact control

Some seeds are just plain difficult, no two ways about it. Whether they need exact warmth, moisture, or lighting, the conditions are easier to manipulate inside, where you have total control. 

When starting seeds indoors, you can easily control moisture. Most flower seeds germinate well at room temperatures, but it’s simple to provide a heat mat if needed. Supplemental lighting on a timer provides the right amount of energy to young seedlings.

Avoid problems with squirrels and birds

Sunflowers germinate readily, grow quickly, and are usually planted directly outdoors where we want them to grow. However, the chipmunks, nuthatches, blue jays, and squirrels like to visit my garden in spring and pluck the seeds right out of the soil. I don’t know how they find them, but it happens so often that I frequently have trouble. To outfox them, I succession-sow sunflowers directly in the garden and also start a few dozen indoors, where, hopefully, there are no squirrels.

What you’ll need to start flower seeds indoors

Starting flower seeds indoors is easy and doesn’t require many supplies. You’ll need seed starting mix, containers, and some light.

Seed starting mix

While a seed doesn’t need soil to germinate, they do need a medium to grow in, and that’s where the seed starting mix comes in. While a true seed starting mix is slightly different from potting mix (it won’t have fertilizer and is finer in texture), it’s typically sold in small bags and can get expensive. I start hundreds of flower seeds indoors every year in a regular potting mix, and it works fine as long as I use clean containers and start with a fresh bag.

Avoid any soil from your garden. Garden soil, when subjected to frequent watering in a container, can become very compact. It can also harbor soil-borne fungal diseases that will attack new seedlings. 

Clean Containers

Containers can be standard flats of cells, often sold as 72- or 50-cell trays, or just about anything else. Seed starting kits will include a container, a tray underneath for watering, and often even a clear plastic lid to serve as a humidity dome when the seeds are planted.

Peat pellets are easy to use and work well in trays. They swell up with water and cradle the seed, and can be planted directly into the soil when the seedling is ready.

Even an old plastic salad container can work for starting flower seeds. The key is that you want something with drainage that is large enough to hold your young plants. I try to avoid potting them up to a larger container more than once to reduce stress on them and me.

Proper Lighting

This is the part that trips people up. Our eyes are a poor measuring tool of what light is enough for plants. A sunny window, while a good spot to read, rarely makes good quality seed starts. Seedlings grown in a windowsill often look pale and leggy, reaching for the light and developing spindly stems.

To provide enough light, you’ll want to place your seedlings under either a fluorescent or, better yet, an LED light. Full spectrum lights, often described as “daylight,” are inexpensive, easy to find, and work well. The light fixture should be only 4-6 inches above the plants, so you’ll need to rig a way to raise it as the plants grow.

Remember to harden off seedlings

Once your flower seedlings are growing and have a few true leaves, they’ll be ready to go outside. That is, if the weather is right. For most flowers we start indoors, the safe time to plant them outside is a week or so after the last frost in spring. The soil is warming, and the weather is more supportive of plant growth. But we can’t just take them from their nice, cozy, warm shelf under the light and plop them outside. Well, we could, but we shouldn’t. They need to be hardened off.

Hardening off is a simple process to help young plants adjust to the more extreme weather conditions outside. When planted outdoors, they’ll be exposed to sunshine much brighter than your grow lights were, the pelting impact of rain, temperature swings, and wind. It can be a shock to little plants grown indoors.

To harden off seedlings, we set them outdoors, in a somewhat protected location, for successively longer periods each day. Ideally, start with an hour or two on day one, and by about day ten, they are set outside in the morning and brought inside in the evening. Skip stormy weather or cold days. 

At the end of a program of about ten days, your plants will be much more adapted to outdoor conditions and ready to be planted in the ground or in their containers.

Good flowers to start indoors

While nearly any flower can be started indoors, here are some common choices to get started. Grab a few packets and get planting!

Annual flowers to start indoors

Many of these annuals grow rather quickly (especially geraniums), so you’ll likely need to pot them up at least once into a larger container or cell as they grow. Snapdragons can be planted 8-10 weeks before the last frost, as they are cold-tolerant and can be transplanted outside a few weeks before your last spring frost. 

• Snapdragons

• Geraniums

• Impatiens

• Marigolds and calendula

• Petunias

• Cosmos

• Nasturtiums

• Zinnias

• Pansies 

Every year, I start about 100 snapdragons (sometimes more), as well as some African marigolds, geraniums, and cosmos, indoors for an early start to the season. Feel free to experiment, as a packet of seeds is relatively inexpensive. This year, I planted coleus from seed, and they germinated and grew wonderfully. Now, I have 50 coleus plants to find homes for.

Perennial flowers to start indoors

Start these perennials early, as they typically grow slowly at first. Beginning 8-10 weeks before your last spring frost is fine. Many of our favorite herbs we use both as decoration and in the kitchen can also be started from seed, including lavender, thyme, chives, and sage.

• Coneflowers

• Black-eyed Susans

• Columbine

• Sweet William

• Anise hyssop

• Lupines

• Delphiniums

Some perennials need to be stratified, or exposed to damp, cold temperatures, to break their dormancy. Many species need 30-60 days of cold and moist conditions, so be sure to check the package well in advance for any specific requirements. 

Don’t forget about ornamental grasses and plants. Native grasses like big bluestem, switchgrass, side oats grama, and ornamentals like decorative kale and basil, can be easily started indoors from seed. Their bright colors and attractive forms make great accents in containers.