Thursday, December 30, 2010

Protecting Milkweed in a Hard Freeze

It's chilly out! After above normal temperatures in November and December, the Phoenix area is bracing for a possible hard freeze tonight. (Hard to believe when we reached 76 degrees on Christmas Day!) But it's not too late to protect the monarch butterfly's favorite host plant, milkweed, from freeze damage with a few simple precautions. In November it dipped to 28 degrees one night, but monarchs and milkweed can withstand short intervals of freezing weather as you can see by this male monarch that visited a surviving lantana flower in my yard a few days later.

Different milkweeds have different degrees of freeze sensitivity. Below are listed some of the more common plants and their critical temperatures. Information is available at Monarch Watch's Bring Back the Monarchs milkweed profiles

Desert or Rush Milkweed
Asclepias subulata
10 to 20 degrees

Note: Some areas of the Valley may dip into the teens the next few nights - cover with frost cloth or cloth sheets or bring potted plants under a patio near your house.



Tropical or Bloodflower Milkweed
Asclepias curassavica
Frost tender at 32 degrees

Note: This non-native but monarch favorite is extremely frost sensitive and will freeze at expected low temperatures tonight across the Valley. Depending on where you live it will likely experience extensive frost damage or will not survive a hard freeze.

(See special directions to save A. curassavica cuttings below.)

Pine-needle Milkweed
Asclepias linaria
Freeze damage information not available, but a distribution map indicates this milkweed should be safe from the expected low temperatures.
USDA Pine-needle milkweed distribution in AZ




Narrowleaf or Arizona Milkweed
Asclepias angustifolia
10 to 20 degrees
Note: Some areas of the Valley may dip into the teens the next few nights - cover with frost cloth or cloth sheets or bring potted plants under a patio near your house.


Important! Check your milkweed plants for caterpillars that may not survive the freeze! Look especially for tiny larvae on fresh new growth of Desert or Rush Milkweed and Tropical or Bloodflower Milkweed. I started this pot of A. subulata in late summer but when I looked closely...




...I found three very small queen caterpillars on the new growth. Laura also found a queen caterpillar and Tatsuyo found several monarch larvae. Our warm weather prompted unusual egg-laying by breeding queen and monarch butterflies in the area. Your choices? Leave them and see what happens, or you can move them to a potted plant near your house or indoors.

 If you decide to bring your potted plants with larvae indoors, you can remove any aphids with disposable garden gloves - but watch for hidden small caterpillars. I found another caterpillar that I missed earlier. Placing your plant in a mesh laundry basket available at most grocery stores (I found mine at Fry's Marketplace) helps to limit your caterpillars from wandering away and also keeps any other insects on board contained.

Another option, besides covering frost sensitive plants or providing an alternate heating source such as stringing the old-fashioned, warm Christmas lights around your plants, it to make cuttings. Tropical or Bloodflower Milkweed, Asclepias curassavica, easily regrows from cuttings. Cut a 5 to 6 inch branch of A. curassavica at one of the "bumps" on the long stem and remove any leaves on the lower part of a 2 to 3 inch section. Place this portion in potted soil and water deeply. Some growers recommend keeping a plastic bag loosely over the cuttings for 2 to 3 weeks until they send out new roots. I always use a lot of cuttings since not all will survive. But some is better than none this time of year. Keeping them indoors during this temperature critical time will help with this process.

Will the monarch butterflies in the Phoenix area survive where they are overwintering at Rio Salado, Desert Botanical Garden or near Tempe Marketplace? Only time will tell. Monarchs can survive some freezing temperatures, but no one is sure how deep of a freeze we will have. In most of the small overwintering areas small streams or creeks are helping to moderate the temperatures and every degree of warmth can help their survival. Lets hope the monarchs among us dry off from yesterday's rain (we received 0.55" at my house) before the temperature plunge tonight.

Icicle in our backyard fountain
January 1st UPDATE: We dodge a bullet with the low temperatures the last two nights. Thursday night a cloud cover crawled in around 4:30 a.m. protecting us from temperatures below 27 degrees at my house in Chandler that was matched again last night. But the good news is that both Rio Salado and the Desert Botanical Garden were near 30 degrees, so the monarch butterflies there have a favorable window to make it through this cold spell. The low temperatures hovered for an extended period of time - as we can see by all the freeze burn all around us and today's high will once again remain in the upper 40's. One more night of freezing temperatures, then we'll be warming to daytime highs in the middle 60's and lows in the upper 30's in the next few days.

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