When should we cover the hostas and perennial flowers such as asiatic...
Hostas are very winter hardy. If you have just planted them this fall, you should water them well - until the ground freezes, or until you decide to disconnect your hose - and then after a good hard freeze, cover them with about four inches of leaf mulch. In the ensuing years, just make sure that they are well watered going into the winter. Hostas rarely need mulching after they have become established.
Asiatic lilies may need a bit more care. Like hostas, an established plant may not need any extra mulch for the winter, but because I'm not sure what kind of lily you have I'm including a link that may be helpful for you:
I'm not sure what kinds of Poppies you are referring to, but I assume you mean Oriental poppies. Oriental Poppies are very cold hardy and survive routinely even into zone 3, but care must be given to plant them in well drained soil to ensure survival. Excessive moisture, especially during summer months, can lead to their demise. In addition to having well drained soil, full sun, to half a day of sun is also important for success. In the right location they are very low maintenance. Some sources recommend covering them with 3-4 inches of mulch After the ground has frozen - making sure to remove that mulch as the weather warms up.
Other kinds of poppies - such as the "California" ones are annuals, and do a good job of self-seeding. The trick is to remember that these plants have created seedlings, and to recognize the bluish-green sprouts in the spring-time - and don't weed them out!.
I hope this information is helpful. Please contact AaE again if you have further questions.