I think the reason your plants stop growing has more to do with short winter daylight than with temperature. All the cool weather crops I know are delighted to grow, without protection, in the 30-70 F range. Night time lows in the 30's shouldn't faze them unless freezing temps are accompanied by dessicating winds. If you're getting freezing winds, then I would consider using a row cover. The heavy row covers, with their added light barrier would seem counter-productive and unnecessary if your temps stay at or above freezing.

Even in sunny California, the hours of daylight are greatly reduced between Holloween and Groundhog Day (the mid-points between Equinoxes and Winter Solstice). In your climate, cold hardy plants will still grow during that time but much slower. Once you notice this, you'll realize that Groundhog Day (Candlemas) is actually really significant for gardeners because that's the point when the rate at which the days get longer, accelerates. Plants respond accordingly with faster, noticable growth.

faithling, sorry but I disagree. I am in a microclimate that gets more frost then others nearby. Most zone 9 or 10 gardens have no problem growing all winter. Gardens a few miles from here (zone 9) are just enough warmer to grow in winter. Gardens a few miles the other direction (zone 10) are frostless, and definetly grow all winter. Even tender plants.
Here I have frost most mornings, and stay at freezing for 12 hours sometime. It just doesnt go much below 30 often.

I should know better than to offer advice for California! But I can't resist offering more.

I would try a full covering of the regular row cover first before opting for the heavy duty type. The latter seems like overkill, given your warm daytime temps and losing 50% of the light seems like too much. I use regular row covers on hoops during frosty weather in the fall and spring and it works very well down to the teens.

I think the heavy duty covers are most useful for overwintering hardy plants in cold climates where you don't expect to have growth in winter. A double layer of the regular weight works pretty well for that purpose too.