If you are sowing for decoration you can sow successive batches from the time that the soil has warmed up to 7°C (45°F) in the spring, up to midsummer - giving yourself a summer long display. Papaver somniferum can be sown in the early autumn, in regions of USDA zone 7 or warmer the small plants will overwinter and flower in late spring and early summer the next year.

Dig over the area you want to grow poppies on, rake and firm. Sprinkle the seeds thinly and evenly on the surface and lightly rake them in. Water with a sprinkler, a gentle spray or a watering can with a fine rose.

Sorry I don't have any idea about the alkaloid content of our different poppies, I did a quick internet search and came up with the following experiment which indicates that nurture is more important, for alkaloid content, than nature - this has also been found to be the case with other medicinal plants.

Here is an abstract from scientific studies of the morphine content of 2 varieties of Papaver somniferum - it was concluded that growing conditions and the delay before harvesting were more important than the variety grown.
Seeds of Papaver somniferum tend to follow the shade of the flower - white seeds come from pale coloured flowers, dark seeds from dark coloured flowers. This could suggest that larger podded varieties are preferable. Hen and Chicks do have more surface area to their pods, and are likely to be left standing longer as they are decorative.

(Or it could be deliberately misleading disinformation - I have seen extraction from "Poppy Straw" mentioned, so maybe there are high quantities of alkaloids in the dried seed-pods. Mrs Grieve describes various cultivation and harvesting techniques. And says that the whitest seeded varieties were preferred when Papaver somniferum was cultivated medicinally for the capsules in the UK.
In hotter climates it is likely that greater alkaloid content is contained in the latex, European production was not considered cost effective and the British Pharmacopoeia directs that medicinal opium must come from Asia Minor)

Experiment to determine the better plant spacing and harvest time after flowering
Two varieties of opium poppy were grown under irrigation for one season and their capsules analysed for morphine content by gas chromatography. Harvest of the capsules at 29-33 days after flowering (mature seed) gave 6.70 kg/ha morphine as compared to 5.36 kg/ha morphine in capsules harvested at 4-6 days after flowering. With constant row spacing (30 cm between double rows on ridges 102 cm apart), a plant spacing of 6 cm gave 20 per cent more morphine than either 12- or 18-cm spacing. There was no statistical difference in morphine content between a blue- and a white-seeded variety.