How to Grow Delphiniums From Seed

Delphiniums are some of my favorite flowers. Some varieties can grow to over 4 feet tall, with tall spires that fill with violet, pink, white, and purple flowers. They’re incredibly cold hardy and survive winters down to zone 3. 

Besides their tall and showy nature, they make excellent flowers for attracting pollinators. The only downside to them is their toxicity, so plant them far away from children or pets. 

They’re easy to grow from seed, by either starting them indoors or sowing them directly in your garden. Let’s dive into how to grow these wonderful flowers from seed and when to get started.

Pacific Giants Blend Delphinium Seeds

Pacific Giants Blend is an award-winning heirloom strain with tall, elegant spires of closely packed, large, well-formed blossoms in pink, violet, and many shades of blue flowers.

Buy at Epic Gardening

Step 1: Buy Your Seeds

The first step to grow delphiniums from seed is to purchase high-quality seeds from a reputable reseller. You can buy them online or at your local nursery. Find seeds that were packed less than a year before the day you’re planting them. 

Make sure the seed packets are in good condition. Check for signs they’ve gotten wet or left out in the sun for too long. Your seed packets should look untouched, without signs of wear and tear. 

Check the seed types and varieties you want to grow. There are many varieties to choose from with various sizes and characteristics. The most popular are Delphinium grandiflora and Delphinium elatum. 

Step 2: Find Your Planting Location

Whether grown from seed or transplant, delphiniums need full sun with at least six to eight hours of direct sunlight. Find a location without shade from trees or nearby buildings. 

Because these plants are toxic, I like to grow them in the background of my garden. Find a space that’s away from trails or places where pets or children play. 

Keep them away from other sun-loving plants so they don’t block the sun. They grow quickly and can reach heights of over 4 feet. They’ll tower over low-lying plants, so find a space near shade-loving annuals or ground cover. 

Step 3: Prepare Your Bed

Delphiniums love well-drained, slightly acidic soil with a pH of around 6.5 to 7. Amend the soil with plenty of organic material and compost shortly before or after your last frost date. 

Try using a light loamy soil mixture with compost. I use a simple organic garden soil amended with organic compost and worm castings. 

Make sure your bed has plenty of space to grow groups of flowers together. The more varieties and colors you can grow, the better. 

Step 4: Cold Stratify Your Seeds

Delphinium seeds will germinate healthier and quicker after a period of cold stratification. This involves placing the seeds in a moist paper towel inside a plastic bag and refrigerating them. You can do this for about two weeks before planting them. 

This process mimics the natural process of loosening the hard seed covering in nature. Cold temperatures signal to the plant that the dormant phase will end. When you remove them from the cold and plant them in warm soil, they’ll naturally enter their active germination phase. 

Step 5: Start Seeds Indoors

If you’re not directly sowing, start your delphinium seeds indoors. You can begin from 6 to 8 weeks before your last frost date. If you live in a warmer climate, you can get them started indoors in midsummer. These late-season starters will be ready for transplanting in the fall and will bloom in the spring after going dormant in the winter. 

Plant your seeds in starter containers with well-drained soil rich in organic compost. Plant them about a quarter inch deep and space them in groups of 3 to 6 seeds per cell. Cover them lightly with soil and water them. 

Step 6: Monitor Your Seedlings

As your seedlings grow, check on them from time to time to make sure they have everything they need. Keep them near a window with full sun or indoor grow lights. Keep them away from cold temperatures, especially if they’re near windows on a freezing night. 

Continue watering them but try not to let the soil become soggy. Keep up with their watering and keep the soil moist. 

After around 15 to 20 days, you should see the first seedlings start to emerge. Keep watering them and giving them enough light. 

Step 7: Thin Your Seedlings

When your seedlings are about 3 inches tall, thin out the weakest plants. Compost them or grow them separately in other containers. 

Leave the strongest plants in their containers before planting. You can also do this when you transplant your seedlings outside. 

If all the seedlings look strong and healthy, separate them into new containers or plant them all in your garden. Extra delphinium seedlings would make fantastic gifts for friends, neighbors, and family members with green thumbs. However, remember to warn them about the plant’s toxicity. 

Step 8: Transplant Your Seedlings Outdoors

Wait until at least two weeks after your last frost date to transplant your seedlings. Check the Old Farmer’s Almanac website to see your average last frost date. Enter your zip code, and you’ll get the dates your region’s last frost should occur. 

In your garden beds, space your seedlings apart by around 12 inches or more. This will give them enough aeration to help prevent powdery mildew and other diseases from forming. 

Dig holes around twice the diameter of your seedlings’ root balls. Gently place them in the hole and bury them up to their crowns in soil. Give the soil enough water to saturate it evenly. Be careful not to overwater your seedlings. 

Step 9: Fertilize After Planting

After planting your adorable delphinium seeds, give your seedlings a little fertilizer boost. Choose a balanced organic fertilizer to give them enough nutrients to put on healthy new growth. 

Make sure the fertilizer doesn’t touch the young plants or it may damage their plant tissue. Water immediately after fertilizing. Make sure there’s no fertilizer buildup in the topsoil. 

Step 10: Skip the Indoor Planting and Sow Directly

Delphinium seeds will grow just fine when directly sown into the ground. Like your indoor starters, cold stratify your seeds before planting. Alternatively, you can sow them in the fall to give them their natural cold stratifying times. 

Wait until 1 or 2 weeks after your last frost date, when the average outdoor low temperatures are above 50°F (10°C). The soil temperature should be above 60°F (16°C). 

Plant your seeds at a depth of about a quarter inch. Space them in groups of 6 seeds every 2 feet. Give them plenty of water after planting, but be careful not to overwater them. 

Step 11: Add Compost Around Your Young Plants

As mentioned before, delphiniums are heavy feeders. They respond well to nutrient-rich soils. 

Evenly amend your soil with organic compost. Spread the compost between the young plants and evenly water them. 

If you notice stunted growth or yellowing leaves, it may indicate your flowers are not getting enough nutrients. If you see these signs, give them an extra amount of plant food or compost. 

Step 12: Stake Your Plants

As your delphiniums grow, they may need some support to not fall over. These plants grow remarkably fast and can surpass 4 feet. Some varieties, such as ‘Pacific Giants’, can reach heights of around 6 feet! 

Place wooden or metal stakes in the garden bed beside your plants. Avoid staking the root ball which could damage the plants. As your plants grow, the stakes will help keep them upright. 

During severe windstorms, these tall flowers are some of the first to fall over. Extra supports will give them added protection from heavy winds. 

Step 13: Watch for Signs of Pests and Diseases

Delphiniums may be cold-hardy and easy to plant, but they can succumb to pests or diseases easily. Pests such as snails and slugs can feast on your summer plants. They thrive in warm, humid environments. Other common delphinium pests include cyclamen mites, which attack new growth. 

The most common diseases to watch out for include fungal and bacterial spots, powdery mildew, and root rot. On some plants, you may see southern blight, white rot, and mold. Severe infections can decimate your flowers. 

The best way to prevent these harmful pests and diseases is by keeping your plants well-aerated. Water them in the early morning or from below to keep their leaves dry. If you see infected plants or foliage, remove it immediately before it can affect healthy plants nearby.