How To Grow Delphinium From Seeds

To get Delphinium flowers from seed in one season, sow the seeds indoors in early spring.

TIP: Before planting refrigerate or pre-chill the seeds in a container until ready to plant. 

Delphinium seed germinates best even at low temperatures as 60-75F (15-20C), but keeping the soil moist throughout the germination period is important. The germination time is on average 14 to 21 days, but can range from 5 days for D. grandiflorum and very fresh seed of other delphiniums to 5 weeks or more. 

Delphinium can be transplanted outside after frost, or sown directly in the garden in summer, or grown from potted plants.

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Sowing Delphinium Seed Indoors:

• Sow seeds indoors 8-10 weeks before last spring frost.
• Sow the seeds on the surface of the pot or tray in a soil-less sterile seed compost which is thoroughly moistened before use.
• Then, cover with a thin (3mm) layer of fine compost or vermiculite 
• Keep the soil moist at 60-75 F (15-23C). Do not allow temperatures to exceed 26°C (80°F).
• Keep the pot covered with transparent kitchen foil until germination starts, so that the compost stays moist. Drying of the soil surface at this stage usually stops further germination and is probably a common cause of failure.
• Seedlings emerge in 14-28 days.
• As soon as the seedlings emerge, provide plenty of light on a sunny windowsill or grow seedlings 3-4 inches beneath fluorescent plant lights turned on 16 hours per day, off for 8 hours at night. Raise the lights as the plants grow taller. Incandescent bulbs will not work for this process because they will get too hot. Most plants require a dark period to grow, do not leave lights on for 24 hours.
• Delphinium seedlings do not need much fertilizer, feed when they are 3-4 weeks old using a starter solution (half strength of a complete indoor houseplant food) according to manufacturer’s directions.
• If you are growing in small cells, you may need to transplant the seedlings to 3 or 4 inch pots when seedlings have at least 2 pairs of true leaves before transplanting to the garden so they have enough room to develop strong roots.
• Before planting in the garden, seedling plants need to be “hardened off”.
• Leave them out for a few hours each day, gradually increasing the length of time over the course of two weeks. 
• Then accustom young plants to outdoor conditions by moving them to a sheltered place outside for a week. Be sure to protect them from wind and hot sun at first. If frost threatens at night bring containers indoors, then take them out again in the morning. This hardening off process toughens the plant’s cell structure and reduces transplant shock and scalding.
• Then, transplant them 12 inches apart in a sunny bed with rich, fast-draining soil once the soil warms up.
•  The plants grow quickly and are ready for planting out by the end of May, or earlier in a warm spring.

• Sow in the open ground in May or June but choose a shady spot or use shade netting to keep the soil moist and the temperature low.
• In milder areas, sow directly in the garden in mid to late summer.
• Remove weeds and work organic matter into the top 6-8 inches of soil; then level and smooth.
• Sow seeds evenly and cover with 1/8 inch of fine soil.
• Firm the soil lightly and keep evenly moist.
• Seedlings will emerge in 21-28 days.
• Thin to at least 18 inches apart when seedlings have three sets of leaves.

Planting Potted Delphinium Plants:

• Choose a location, light shade with a cool, moist organic soil.
• Prepare the bed by turning the soil under to a depth of 6-12 inches removing any debris, and lightly raking as level as possible.
• The addition of organic matter (leaf mold, compost, well-rotted manure) will be an additional benefit.
• Plant on a cloudy day or in late afternoon to reduce transplant shock.
• Dig a hole for each plant large enough to amply accommodate the root ball.
• Unpot the plant and gently loosen the root ball with your hands to encourage good root growth.
• Place the top of the root ball even with the level of the surrounding soil. Fill with soil to the top of the root ball. Press soil down firmly with your hand.
• Use the plant tag as a location marker.
• Thoroughly water and apply a light mulch layer on top of the soil (1-2 inches) to conserve water and reduce weeds.