The brilliant colours and nodding heads of poppies dot roadsides, gardens and fields in U.S. Department of Agriculture plant hardiness zones 2 through 10 depending on number — and you’ll find a lot of varieties both Papaver spp. and Eschscholzia spp. Which are called poppies. From yearly Shirley and Flanders poppies (P. rhoeas) to the short-lived perennial California poppy (E. californica) and perennial Iceland (P. nudicaule) and oriental poppy (P. orientale), the flowers grow readily from seed and often self-sow when the lighting is right.
Outdoor Poppy Culture
Poppies of all kinds are cool-weather plants which bloom in late spring and early summer when temperatures are mild. Sow seeds or plant poppy seedlings in well-drained, evenly moist soil in full sunlight — at least six hours every day, though 12 hours is best. The same light conditions required for germination carry on for their active growth phase. Once germinated, the plants set down deep taproots which help sustain them once they go dormant in the heat of summer. Sow seeds or plant seedlings from the coolest microclimate in your lawn; some shade in midday keeps poppies blooming longer.
Light for Indoor Poppy Seedlings
Although most commonly sown in position outside, poppies can be started indoors to get a jump on the season. Once seeds germinate inside, the developing seedlings have to be placed in bright sunlight in a south-facing window — more effectively — beneath fluorescent light tubes. A bar-style store light which holds two extended tube bulbs is ideal — use one warm and one cool bulb. Lights should be no closer than 6 inches from the tops of the plants — moving further back as they develop to avoid leaf burn. Lights will need to stay on for 14 to 16 hours every day.
Transplanting
Since poppies develop taproots, they have a reputation for being hard to transplant and will need to move to their permanent place while still young — 2 to 4 inches tall. Get them used to the change in temperature and lighting over the span of a week or so they can die from the combined shock of transplanting and the drastic change in light. Once outdoors, move them gradually from a protected, shaded area to complete sun.
Factors
When poppies grown inside do not get enough light, the plants can elongate and develop spindly in a process called etoliation. Etoliated poppies develop a fragile taproot that when planted outdoors in adequate light is a poor foundation for growth and can readily snap. Poppies planted in light shade may blossom, but only lightly. California poppies can shut up on overcast days or in the shade.