Answer: I suspect your Oriental Poppy (Papavar orientale) plants are going dormant for the summer, which is normal. Since they were planted only this year, they may be a little bit off on their timing, but next year should run closer to the expected schedule.
Oriental poppy plants normally go dormant for the summer, which means they turn brown, dry up and disappear. (Mark their locations so you don't dig them up by accident!) Hot, humid weather can accelerate the process. In the fall, they sprout new rosettes of leaves. Late the next spring, they burst forth into big bold plants with beautiful blooms. Then the cycle starts all over again. The plants generally last for many years.
These plants do best in a sunny spot with deep rich soil that is well-drained (not soggy) and do not like to be moved or transplanted. However, if you absolutely must do it, do it in late summer when they are dormant. The spacing really depends on the particular variety you have, as some are taller and wider than others, and how well they grow for you because many within the same variety may vary in height from say 2 to 4 feet. To some extent, the spacing is also a matter of preference -- if you like the full look or a sparser look -- but the smaller types could go in about 2 feet apart and some of the largest types as much as 3 or 4 feet apart.
Seed grown poppies grown as annuals, such as Iceland Poppy (Papavar nudicale), can be touchy, but once established in a garden will often self sow from year to year. Some gardeners surface-sow seed in the garden in late summer or early fall and hope for germination either in fall or very early spring. In my experience, this has been the most reliable method.
Enjoy your poppies!
Q&A Library Searching Tips