Good question. If you are keeping mason bees in cold storage—whether in a garage, shed, or refrigerator—it is time to get them outside. Although conditions differ with latitude, nature tells us when the time is right.
As a rule of thumb, when things begin to bloom, the mason bees should be free to emerge. The blooming plants you see should be within a couple hundred feet of the mason bee housing. Unlike honey bees, mason bees will not fly long distances to find food, so it has to be close.
Early blooming plants here include crocus, scilla, vinca, skunk cabbage, and snow drops followed quickly by forsythia, dandelion, and oemleria. Once you see a few things start to bloom, you can safely put your bees outside.
If you don’t trust yourself to remember by looking at plants, just think March 1. Spring mason bees (Osmia lignaria) are generally active March, April, and May, so March 1 is a reliable date to use.
Although some people will keep mason bees in cold storage through April, I think it is better for the bees to be active when their wild counterparts are active. This will give your bees the best opportunity to mate with local wild populations and maintain maximum genetic diversity.
You can minimize the danger of wind and cold with a few simple steps:
- Mount your mason bee house so it faces south or southeast. This will provide maximum sun exposure.
- Attach a sloping and overhanging roof to your bee house to shed excess rain, or mount the bee house under an eave.
- Plant early blooming flowers close to the mason bee housing. The males will emerge first and need nectar-producing flowers right away. The females will emerge later, and they will require both nectar- and pollen-producing blooms.
I saw my first mason bee last week on March 13. He was drinking nectar from a Vinca minor while all around him honey bee workers and queen bumbles were working the Scilla.