Hmmmm......the average minimum winter temperatures - what they base USDA hardiness zones on - for zones 5 and 6 range between -20F and -10F. Are you saying you do not drop below 20F? If so, sounds like you are in a very specific microclimate. And that is a good thing if growing pampas grass, Cortaderia selloana, as it is not considered reliably winter hardy below zone 7. And it is an evergreen grass - it should retain a very similar appearance year round.

What is your normal annual rainfall? Pampas grass is an especially drought tolerant species and once established, generally requires no supplemental irrigation. It is naturalized throughout many parts of southern California, which is experiencing an extended drought, and survives splendidly (too much so, as it is listed invasive in that state) with virtually no water.

Because of cooler temperatures in winter, there is typically far less moisture evaporation from the soil and regular watering is not the requirement it may be in summer. If you remain very dry throught the winter months, a deep watering monthly may be all that is necessary. But that also depends on how well established your grasses are. Newly planted (this season) plants may require more frequent irrigation.

I took the USDA zone from a poor map.

There are no official numbers here. A retired weather man who worked for the gold mines called the weather a "shadow of the mountain" phenomenon.

Thanks. I'm guessing that your guess of monthly is close enough.

Well, whatever zone I may be in it got down to 6°F last December. The average low for December and January was about 33°F and it snowed and snowed and snowed. No one at the Senior Center remembers anything like it.

Anyway, my pampas grass which had a clump 2-3 feet in diameter and got above the six foot fence last year now has one little green shoot. The rest of the plant seems rotten.