Holiday Lights for Frost Protection: Which to Choose?

 
"For frost protection of citrus, use the large, C9 Christmas/holiday lights..."

Now, there's a phrase we've been tossing around a lot lately.





To generate as much heat as possible into a citrus tree's canopy on a night when the temperature dips below freezing, stringing those C9 lights around the tree's interior is a good idea. Christmas lights work better than a single, large light bulb in the center of the tree canopy. It eliminates cold corners and edges. Those bulbs can add four degrees of protection. The combination of bulbs with a frost cloth will give you seven or eight degrees of extra warmth (be sure not to let the bulbs rest against the fabric or the leaves).
   
But after doing a little research, does it come as a surprise that not all C9 holiday lights are created equal?

The original C9 incandescent bulbs generated about 10 watts per bulb. Now, the selection of C9's (and the slightly smaller C7's) vary greatly: from 10 watts to less than 1 watt per bulb.

C7 (5 watts)

C9 (9 watts)

LED C7 (less than one watt)

If you're buying C9 lights for frost protection, be sure to read the label. You want as much heat as possible.


The output among various style C9 and C7 holiday lights from one online retailer:  
.6 Watts - LED C7    
1 Watt - LED C9   
2.5 Watts - Low Energy C7  
3.7 Watts - Low Energy C9
5 Watts - Traditional C7 
7 Watts - Traditional C9 
10 Watts - Traditional C9


And the information on the package can be tricky. One box of Christmas lights I found in the garage was labeled "C9 Style". Turns out, only the covering of the bulb was C9 in style; the output of the bulb itself was .75 of a watt.


Besides being festive this time of year with your citrus, you can extend the colorful light show to your cold frames.
  
In trials conducted by Colorado State University, one 25 light string of C-7 (mid-size) Christmas lights per frame unit (four feet wide by five feet long) gave 6 degrees F to over
18 degrees F frost protection.  Lights were hung on the frame under the plastic and turned on at dusk and off at dawn.