Willamette Valley bumblebees and their native plant associations
Want more bumblebees? Check out this plant list.
As I pull back the blackberry and ivy invasive plants throughout my property, I’m finding I have space to add more habitat plantings. I wanted to know more about how to support our native bees, so I did a little deep dive on bumble bees and thought I would share my results with you.
For a primer on why bumble bees are particularly important, see this page from the Xerces society.
My first reference book was Bumble Bees of North America which was recommended as an identification guide by OSU in this online video. My second reference was the Bumbe Bee At last in Oregon website.
Both of these resources are broad and covered bees that are in our area, but also climates quite different than the Willamette Valley ecoregion. So my first task was set to figure out which bumble bees are in our region, and then what their most common plant associations are.
As I read I found that many species of bumble bees are noted as stable in their population. I was a little surprised by this, as I thought I had read multiple times that *all* bumble bees were declining due to such factors habitat loss and climate change.
As I looked closer, there are approximately 5 bumble bees that are in decline or possibly extinct in our region, with about 15 species that occupy our region in total (my sources varied just a little, so can’t be exact).
In this article I will profile those species that are in decline, and then list out the most commonly associated plants in the Willamette Valley habitat.
Bombus Occidentalis used to be the most common bumble bee in our area, but has not been seen in the Willamette Valley for many years. I noted that this particular bumble bee seems to be easier to identify than most others in our area, and researchers would like to know if you’ve seen one. Here is a guide on identifying it.
Unlike other bees that are in decline, there is one solid factor that has impacted the Occidentalis’ demise. Decades ago they were selected to be bred for use in greenhouses. When they came back from overseas after breeding, they spread a pathogen that our native bees had no resistance to.
The Bumble Bee Atlas notes that it’s most commonly used native plants are: lupines, spiraeas, and goldenrod. Bumble Bees of North America notes that its most common native plant associations are: ceanothus, thistles, geranium, grindellia, lupines, rubus species, goldenrod, and trifoliums.
Bombus Fervidus (AKA Californicus) is declining in our area when compared to historical records. The Bumble Bee Atlas notes its favorite host plants as: thistles, larkspurs, clovers and penstemons. Bumble Bees of North America notes favorite plants as: astragulus, thistles, lonicera, lythrum, penstemon, trifoliums, and vicia.
Bombus Calignosus is noted as a declining species, but that could in part be because it is easy to confuse this species with Vosnensenskii. One would need a microscope to tell them apart. The most noted native food plants by the atlas are: raspberry, blackberry, Prunella and trifoliums. Bumble Bees of North America notes favorite native plants as: ceanothus, thistles, clarkia, keckiella, lotus, lupines, rhododendron, rubus, trifolium, and vaccinium.
Bombus Pensylvanicus is a species that is being considered for the Endangered Species Act. It is not common here, and the atlas notes that the reports in Oregon of its sighting are unconfirmed. Bumble Bees of North America lists: astragalus, thistles, trifolium and vicia as favorite food plants.
Bombus Flavidus is found less often in the Willamette Valley than the historical records, but is noted as stable in other regions. The atlas notes that favorite native food plants are goldenrod, fleabane, thistles. Bumble Bees of North America notes that its favorite native food plants are thistles and senecio.
Notable plants
As I went through both of these books, I made a list of the overall food plants most appealing to all bumble bees, and then a list of those plants that are most favorable to the species in decline. I found some interesting insights to share with you.
* The asterisk denotes the plants that are *also* hosts to butterfly caterpillars in our area. Butterfly host plants are plants that caterpillars use exclusively as food. See my full list of associated plants here.
The plants are listed in descending order, starting with the plants most commonly used.
Native thistles*
By far the most common mentioned plant throughout Bumble Bees of North America are cirsiums, or thistles. It is used by both bumbles in decline, and those that are stable. I noted that the book listed this as an important plant to nearly all species in our region, while the atlas shows it very important to at least two species in decline in our region.
Sadly, most thistles we encounter are horribly invasive devils, and most folks react strongly to their presence. But there are *native* thistles to be aware of.
Clackamas Soil and Water Conservation District has a very detailed outline of our native thistles and their importance in our habitat, along with identification guide. Check it out here.
I’ll be trying to protect my little patch and collect seed so that I can expand its presence. The only native thistle I’ve seen available commercially is Cirsium Occidentale, which only does well in gravelly soil. I tried to plant it and give it gravelly soil, but it croaked pretty quickly.
Fireweed
Another plant that is exceptionally important to both declining bumble bees and those that are stable is fireweed. Fortunately, it is a plant that is easy to grow. It blooms all summer, extending into late summer. I have a bit of this on my property, but I’m going to expand it to a few big patches, like those I’ve seen on the roadways in Alaska. Stunning.
Fireweed is used heavily by at least 11 species in our region, 3 of which are in decline.
Lupines*
Are also in the category of highly favorable to most bumble bee species. Most commonly grown in our area is Lupinus Polyphyllus, or Big leaf Lupine. I have grown this species in the past, but sadly, it is a short-lived plant. It is supposed to self-seed, but it did not do so. I have also tried to set out seed, only to have them devoured by slugs. This might be a plant that needs to be started indoors and set out after they’ve grown past seedling stage which is when they are most vulnerable to slugs. Or just buy 4” starts or gallons, as they are commonly available in nurseries.
Another common lupine is Lupinus Rivularis, which is a species commonly found among sandy beaches on our rivers. Unfortunately my soil is clay, and I don’t think it will do well here.
The other common lupine in our area is Lupinus Bicolor, which is an annual native to gravelly soils.
Lupines are used by at least 10 bumble bee species in our region, 2 of which are in decline.
Penstemon*
Another group of very important plants for Bumble Bees is penstemons, with at least 10 bumble bees that regularly use it, including 1 that is in decline in our area.
It is a group of plants I’m not terribly familiar with, except for one species that I know fairly well, which is Penstemon Cardwellii. It is a plant I picked up over 10 years ago, that was falling out of the cliff faces of the Clackamas River. I plopped some rocky soil on the ground, stuck the penstemon in it, and it has gotten big enough for me to divide. I find it roots right along the ground.
There are multitude of other types of penstemons, both native and non. They don’t seem particularly easy to grow, unless you’ve got exceptionally sharp draining soil. One that looks particularly interesting to try would be the native Penstemon Ovatus, which is profiled well on Xera’s website.
Now that I know how important penstemons are, I’ll be protecting my patch a little more intensively. Right now the trailing blackberry is trying to choke it out, and God knows there is no shortage of trailing blackberry.
Rubus*
Rubus refers to a whole family of plants. Most common in our area: blackcap raspberry, dewberry (AKA trailing blackberry), thimbleberry and salmonberry.
Note that salmonberry in particular is a very early bloomer, and thimbleberry is just a little later than that. I wonder if these plants in particular are important early forage.
At least 8 species regularly use rubus species, with 2 of them being species in decline.
I note every time my red raspberries are in bloom, the bumble bees just love the flowers.
Trifolium
Trifolium is the Latin name for clovers. There are a whole bunch of non-native clovers. There are also several native ones. Last year I attempted to add a native one to my property, but they were quickly devoured by slugs. Speaking to other habitat gardeners, they’ve told me that they had the same problem.
At least four species of bumbles regularly use clovers, two of which are in decline in our area. And I mostly leave any non-native volunteer clover alone on my property, as at least they provide some benefit, unlike other problematic plants I need to get rid of on my property.
Delphiniums
Oregon has a multitude of Delphinium species, some of which are themselves endangered (especially white flowering varieties).
Most commonly encountered in native garden centers is Delphinium Trolliifolium. It is a gorgeous plant. I tried adding it last year. It bloomed for me quickly, but then succumbed to slugs.
Delphiniums are exceedingly difficult on properties like mine (shady wetland). If I win the lottery I might seek out a large, multi-gallon pot of this species and try again. I believe I have seen some like that in West Linn, at Bosky Dell Nursery.
Delphiniums are used by at least 5 bumbles, two of which are declining in our region.
Snowberries*
There are 2 common forms of snowberries that grow in our region. There is a low-growing, spreading ground cover type, and a taller shrubby type. I’ve got quite a bit of the taller shrub type, which I’ve divided multiple times. I’ve got roughly 20 of these shrubs now, and I note that when they are in bloom, the plants are absolutely abuzz with bees.
My snowberries get terribly infected with powdery mildew in late summer. But it doesn’t seem to impact the plant in the long run.
At least five species of bumbles use snowberry, with one of them being in decline.
Snowberries sure look cool late fall/winter when they’re decorated with pure white berries.
Goldenrod
Goldenrods are food for at least 5 bumble bee species, 2 of which are in decline. Goldenrods are important in that they bloom late in the season, which is when most of the above plants have already finished. The most common form of Goldenrod grows into a fairly tall, floppy plant that is difficult in small spaces. One cool goldenrod native to the valley that I planted last year is Solidago Elongata, which only grows a foot tall.
Spiraeas*
The most common Spiraea in our area is the big pink flowering plant you see next to wet ditches on the sides of the roads called Douglas Spiraea. There are a couple of other spiraeas that are native to our area available in less wet soils and better for garden situations, such as Spiraea Betulifolia and Spirea Splendens.
At least 4 species of bumbles use this plant, 2 of which are in decline.
Phacelias
The phacelia I have in my garden, and which is most commonly available, is Phacelia Hastata or possibly Heterophylla. I’m not sure how to tell the difference. Either way, it blooms for an incredible length of time, and is absolutely covered in bees of all kinds. It takes sun or shade and likes to seed around a bit, not in an obnoxious way.
I also had planted Phacelia Californica into my garden. I gave it a prime spot with excellent drainage. Sadly it died after just a couple of years, and did not reseed. Though this plant is a touch more colorful, I recommend Heterophylla for longevity.
Ceanothus*
Surprisingly, ceanothus wasn’t mentioned much in the bumblebee atlas, but was mentioned extensively with 7 species of bumblebees that use ceanothus.
Some ceanothus species are used as ground covers (such as Point Reyes) but more often they are larger shrubs. For an interesting selection of available plants, check out Xera’s collection. I planted “Umpqua Sky” last year. It grew about a foot with no irrigation. Hope it grows like gangbusters this year!
Aster
Asters are used by at least four bumble species, two of which are in decline. They bloom relatively late in the season and keep going until fall. Be forewarned however, that our native asters are big floppy plants that like a lot of room to spread.
Lavender
Lavender is the one plant I’ll talk about here that is not native to our area. Though it isn’t native in this habitat, it is strongly favored by our bumble bees, with five species using it regularly, two of which are in decline. Given that lavender has had absolutely no propensity to spread into a weedy nightmare, I think this is one non-native we can feel really good about keeping in our landscape. Plus the aroma is like food for the soul. Yum.
Other plants mentioned that are used by at least two species, but less commonly than the favorites above: agastache, dipsacus, vicia, aconitum, thermopsis, astragalus, hypericum, senecio, prunella, pearly everlasting, rhododendron, manzanita, eriogonum, Ribes, Huckleberries, stachys.
Early/late blooming flowers.
The above list is simply those species that are most commonly used. It doesn’t mean other plants are not important. I’m a little surprised to not see Oregon Grape on this list, as it is one of the plants that bloom most early in the season, useful for when queens are first waking up and getting their nest started. I did see that Ribes (currant, such as red flowering currant) listed as being used somewhat in the Bumble Bees of North America book.
My understanding is that bumble bees have long activity periods, and they really need flowers throughout the season. Most of the above flowers are mid-summer flowering species and special emphasis should be on making sure that late and early species are available.
Important early species are: salmonberry, thimbleberry, manzanita, Ribes. Some ceanothus species also bloom particularly early.
Important late species: goldenrods and asters. Grindelia and fireweed are often still blooming into September as well.
Happy habitat planting!
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