HORTICULTURE TIPS by Josh Steffen: Dormant Oils

Forty degrees Fahrenheit seems an important yard and garden threshold, and with air temperatures set to trend upwards again this week, there a few things stirring out there. Forty degrees is an important milestone for soil organisms, for one. When soil temperatures stretch eight degrees north of freezing, soil organisms-friend and foe alike-begin to awake from their long winter’s nap. Decomposition in the compost pile and the flower bed pick up where they left off last autumn. This becomes a good time to move that pile around, harvest the best, finished compost (found at the bottom of the stack) and spread it an inch or more thick. Give those hungry microbes something to cheer about.

Beyond those organisms one cannot see, there are those who start to emerge and move around with warmer temperatures that are visible. One such is a sap sucking insect called oyster shell scale. This particular insect occupies several Wellfield Botanic Gardens woody species including Cornus sericea (red twig dogwood), Aesculus (buckeye) and Cercis (redbud).
One part of a multiple step IPM strategy being employed at Wellfield is spraying a dormant oil. Dormant oil can be effective in coating scale eggs and nymphs (i.e. crawlers) and suffocating them. The oil may not get them all if the infestation is particularly heavy and scale insects are piled on top of each other, preventing the oils from reaching the intended targets. The majority of dormant horticultural oils on the market are derived from petroleum distillates, which are not permitted near drinking water sources. The only oil option available to a Wellfield gardener is one derived from vegetative matter. Until more recently, no such dormant oil existed. I am testing a new product to me, a canola oil based spray and shall see its efficacy. The key is spray timing. Spray dormant oils when day temperatures reach forty degrees with no rain for twenty-four hours. Be sure to follow label instructions since there are some species that do not take kindly to dormant oils.

Incidentally, now is a great time to apply oil sprays to dormant fruit trees such as apples and pears. A good organic spring tree spray program may include a round of dormant oils, followed by copper (not a labeled product permitted at Wellfield) and wettable sulfur. It is important to time sprays to not overlap, causing toxic results in your garden coming from mixing chemicals. Follow label instructions. Of course, the best ways to minimize disease pressure in small scale fruit production is planting disease resistant varieties and good fall debris clean up.

Hopefully, all this talk is warming you up and encouraging you to get outside and see what is happening in your yard.