Gardening and Horticulture - too cold to plant?

the temp. here has been in the 60's and 50's during the day- but in the 40's and 30's at night. does the ground retain enough heat from the day that it is safe to plant in this weather? also, is there any connection between the temp's. of local rivers and lakes to ground temp's.? the ground , i know, has to be at least 50 degrees or so- does this mean i need to wait untill it is 50 degrees at night also? since it has been in the 60's and it is march, i have planted three rose bushes. how do i protect them? plastic bags? mulch? thank you!

zone7, I live in Z7 also and our average last frost date is March 15. If your roses are leafed out make sure you cover them with something if a frost is possible. I have always started a little early and have found that caladiums are all that don't like the less that 50 degree ground temp. I really don't think that planting early is any help having more later on because plants tend to wait until it is thier time...Mike