Promote native bee habitat by leaving some leaves and stems on the ground

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The onset of fall brings shorter days, cooler nights and a gradual decrease in gardening activity. The flowers are starting to fade, while the leaves are turning beautiful colors and starting to fall from the trees. For many gardeners, however, fall is not the time to kick back and put your feet up. That’s because common gardening advice tells us to rake up all those leaves, remove every dried stem, and maybe even start a burn pile for pruned branches and twigs at this time of year.

But a different, more liberal approach to fall gardening is on the rise: One can view these seasonal changes as opportunities to build healthier soils and create nesting sites for garden wildlife, especially native bees. In other words, there’s a great excuse to “leave the leaves” and let the dried stalks and stems and dead wood stay put. Of course, care must be taken to keep batteries and all dry combustible leaves and woody materials away from structures in order to be safe from fire!

“The Real Dirt” is a chronicle of various local Master Gardeners who are part of UC Butte County’s Master Gardeners.

Allowing leaves to decompose over the fall and winter months is a form of natural composting that adds nutrients, increases soil condition and nourishes organisms necessary for plant health. In the prolonged drought we are experiencing, a layer of leaves helps retain moisture. And the invertebrates that help control garden pests depend on leafy soil for shelter and survival through winter.

One way to gradually allow more leaves to do this useful work is to remove them from lawns or walkways and pile them under the canopy of ornamental trees, shrubs and perennials (but away from trunks and stems) or at the over vegetable beds, using them as you would any compost layer (larger leaves may need to be crushed or shredded to stay in place).

Besides letting the leaves compost naturally, there’s another reason to do less cleanup in our fall gardens. A very important group of invertebrates that benefit from and depend on these garden “leftovers” are our native bees. Most people immediately think of bumblebees and honey bees when the subject of bees comes up. But in fact, California is home to 1,600 native bee species of varying sizes, shapes, and colors, and all of them are important pollinators of flowering plants, shrubs, and trees. Some are even considered essential pollinators for orchards and commercially grown produce like tomatoes.

We can benefit in many ways from the immense diversity and beauty of flowering plants and the richness of vegetables through pollinators, especially bees. Bees and flowers evolved together, with flowers producing pollen and nectar needed for food at different stages in the life of all bees (they are vegetarians, while their cousins ​​the wasps are carnivorous). Bees help move pollen from one flowering plant to another, thus ensuring the successful reproduction of plants.

When we allow certain leaves to decay naturally in our fall gardens, we help provide nesting sites and shelter for native bees so they can survive the winter and emerge in the spring to start a new generation.

Almost all native bees are solitary; after mating, the female does the work of building and provisioning a nest, and then lays her eggs. There are no queens or worker bees and most do not care for the young, leaving them to complete their development into a new adult bee on their own.

The majority of native bees have a fairly short lifespan as active adult bees, often only living a few weeks or months.

Bumblebees, however, are social. They form nests with a queen and worker bees, but unlike honey bees (which are not native to North America and were introduced in the 1600s), the nest only survives a year with a newly formed queen. mated that flies off in the fall and finds somewhere, often in the ground, to spend the winter before emerging in the spring to start a new nest.

Different species of native bees emerge throughout spring, summer and fall, often appearing when the flowers they depend on are blooming. Our native manzanitas, California Lilac and Western Redbud, which bloom in early spring, are good examples of this seasonality.

Associated bee visitors emerge from their winter nests and appear in our gardens around the same time these natives bloom. Our home gardens can provide native bees with flowers to feed on and places to nest.

Our native bees have common names that may give a clue to the type of nest they build or a behavior they share. Carpenter bees chew wood or stems to form their nests.

Leafcutters cut circles or ovals out of soft leaves which they then use in a stem or tunnel for nest building.

Mason bees use mud to enclose their nests. Cellophane bees make a resinous gel to form their cells. Sweat bees like to drink sweat off exposed skin!

About 70% of our native bees nest on the ground. The remaining 30% nest in dried stems and wood, existing tunnels dug by beetles or annoying insects, rock cavities or (in the case of bumblebees) old rodent holes or even man-made objects like birdhouses. All their nests are in what can be called a linear space or a tunnel and do not resemble beehives. Some species of stem-nesting bees use hollow stems (like reeds); others prefer stems that have soft interiors they can chew on.

A good example of a stalk-nester that depends on stalks or dried stalks is Ceratina, the little carpenter bee. She uses these fluffy stems to build her nest, lay her eggs, and overwinter as an adult bee. Flowering perennials that have long, fairly strong stems with a softer or fluffier interior are Ceratina’s choice for nesting.

Leaving dried stems on the ground instead of removing them provides opportunities for these nests. In the spring, the female, after mating with a male bee, begins to chew through the top of the stem or sometimes from the side to form a tunnel. She gathers a ball of pollen and nectar and supplies this tunnel, laying her egg above the food source. She then seals this in a cell and builds another. Usually five to six cells are formed in each tunnel. Ceratina is a bit unusual because it does not close the top of the nest and protects it from predators. The next generation develops into adults and overwinters inside the nest tunnel, emerging in the spring to begin the cycle again.

Not cutting dried stems in the fall could mean saving a nest and allowing it to overwinter. Waiting until early spring to cut off the seed heads will open the stems and hopefully allow the newly emerging females to have a nesting place. You can watch these stems cut for any activity! Most of our native bees go about their business without caring about us. They are a fascinating and often beautiful group of insects, mostly reluctant or even unable to sting. They live their lives harmlessly and often unnoticed unless we take the time and effort to seek them out.

Last year’s thatch with old nests will quickly decay in the spring and new perennial growth will hide much of it. Additionally, leaving the dried seed heads to remain often provides benefits to birds and other wildlife that eat the seeds. Where possible, leaving tree stumps or brush piles in place can also provide nest sites for tunnel nesters and other hibernators.

Ground-nesting bees need areas of bare soil or soil that is lightly mulched or covered with a thin layer of leaves. They dig tunnels to build their nests, also with chambers and cells, supply them and lay their eggs. They too need these nesting sites to survive the winter. If possible, not tilling the soil and avoiding pesticides helps them nest successfully.

Pesticides are problematic because these bees use their mouths to dig into the ground, putting them in contact with all the pesticides in the dirt near their nests.

Ground-nesting bees can use just about any ground texture that will hold together. A garden that has stepping stones with dirt between them is an excellent potential nesting site for these bees, with the added benefit of providing protection to the nest. Scientists from the Xerces Society (a national non-profit organization that studies invertebrates and works for their conservation) have observed that ground-nesting bees often nest at the edge between lawn and garden. This may be because bees, especially the smaller species, often do not fly far from their nests to forage, so the proximity of their nest to flowers, their food source, is beneficial.

Making some or all of these small, but potentially important changes to our gardening habits (and aesthetics) each fall has the potential to encourage population growth of these important insects while improving soil health in our gardens. . Tell your friends and neighbors that you are “leaving leaves” and stems, for the benefit of healthier plants and for the sustenance of wildlife, including our all-important pollinators — native bees.

The UC Butte County Master Gardeners are part of the University of California Cooperative Extension System, serving our community in a variety of ways, including 4-H, agricultural counselors, and nutrition and health programs. ‘physical activity. To learn more about the UCCE Butte County Master Gardeners and for help with gardening in our area, visit https://ucanr.edu/sites/bcmg/. If you have a gardening question or problem, call the hotline at 538-7201 or email [email protected]

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