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Container gardening for pollinators

In the city, outdoor gardening space can be hard to find. Backyards are replaced by balconies, but this doesn’t mean you can’t still garden for pollinators. You just have to get creative about it. This National Pollinator Week, TPI has tips and tricks for how you can support urban pollinators with container gardening whatever your outdoor space looks like (even if it’s just a windowsill!).

1. Choose perennial native plants. Apart from a few species with deep taproots, many native plants will thrive in containers. Since they’re perennial, your plants will come back year after year, and support pollinators without any extra cost. Here are some good options that TPI has had success with:

  • swamp milkweed (Asclepias incarnata)
  • anise hyssop (Agastache foeniculum)
  • lance-leaf coreopsis (Coreopsis lanceolata)
  • smooth aster (Symphyotrichum laeve)
  • great blue lobelia (Lobelia siphilitica)
  • cardinal flower (Lobelia cardinalis)
  • blazing star (Liatris spicata)
  • virginia mountain mint (Pycnanthemum virginianum)
  • purple coneflower (Echinaecea purpurea)
Blazing star (Liatris spicata), aka gayfeather, is a favorite of pollinators and easily grown in a container! PC: Flickr, CC.

2. Combine plants with complementary bloom times to have the greatest impact. For your first container, try picking three plants: one that blooms in early summer, one in mid-summer, and one in fall.

3. Deep containers are your friend. Choose containers at least 16” deep to allow your native plants to build strong root systems and thrive for years to come.

4. If you only have space for shallow planters (<12” deep), annual or biennial native plants are a great option. These plants don’t invest in a deep root system and can survive in shallower soils. Black-eyed susan (Rudbeckia hirta), spotted horsemint (Monarda punctata), and partridge pea (Chamaecrista fasciculata) are three good choices.

5. Flowering herbs are also a great (and tasty) option for small spaces. TPI previously waxed the benefits of letting your herbs for pollinators, and the same applies here. Dill, lavender, thyme, mint, and cilantro are all popular herbs that do well in containers and favorites of pollinators.

Container herb gardens can be a win-win! Flowering mint feeds all sorts of pollinators, like this metallic green sweat bee (Agapostemon sp.), and you can enjoy mint leaves in sun tea and watermelon coolers! PC: Nick Dorian.

6. Water often. Soil in containers dries out faster than those growing below ground, and as your plants become root bound, they will need water more frequently. As always, water the soil rather than the leaves.

7. Don’t fertilize. Native plants are adapted to soils that are low in nutrients, and adding fertilizer will result in many big leaves and not many flowers. Amend store-bought potting mix with perlite and sand to create a well draining medium for your plants. Leave the manure and kelp fertilizer for your veggies!

8. Protect your container plants over the winter with 3-4” of leaf mulch, by moving your planter to a less exposed area, and potentially covering with a tarp. The key is to keep the soil warm enough so the roots don’t freeze through.

The Feathered Bees’ Spring Arrival

RTHU in flight. PC: Michael Janke.

The smell of spring permeates the air as the grass begins to poke through the sodden dirt and buds emerge from the tips of the trees.  Concurrent with the change of seasons is another great event – hummingbird spring migration!  While there are more than 330 different species of hummingbirds (Trochilidae), only around 12 to 15 species routinely nest in the U.S.  For us resident Bay Staters’ we can expect to see even less!  The Ruby-Throated Hummingbird (Archilochus colubris; RTHU) is the only breeding hummingbird in eastern North America.

Twice a year, these tiny powerhouses undergo an 805 km migratory route—some individuals coming from as far away as the Pacific slope of Costa Rica and flying up into Central Alberta, Canada.  Despite their small size (females averaging 3.5 grams and males averaging 3.0 grams), most fly nonstop across the Gulf of Mexico during spring migration.  In Massachusetts, you can expect to see these sparkling gems beginning in mid-April.  In fact, the first sighting was just reported on eBird—an online database of real-time bird observations—in East Falmouth, MA on April 27th.

Before embarking on their migratory routes, RTHU will gain about 25-40% of their body weight, fattening up on nectar and insect.  Once they have arrived in North America, RTHU get right to work replenishing their depleted reserves.  While hummingbirds are birds, they feed similarly to insects and have appropriately been dubbed “feathered bees” by the National Audubon Society.  Like insect pollinators, hummingbirds fly from flower to flower in search for nectar.  As a hummingbird dips its bill into flowers, pollen grains stick to their feathers and bill which get carried and distributed to the next flower.  It is estimated that nearly 8,000 plant species throughout the Americas rely on hummingbirds as their primary pollinator.

Female RTHU resting on flower. PC: Nick Dorian.

Hummingbirds and plants represent a classic example of a plant-pollinator relationship and demonstrate coevolution—the phenomena of intimately linked organisms influencing each other’s evolution.  Hummingbird-dependent plants have evolved several “pro-bird” traits including sucrose-rich nectar, brightly colored, unscented flowers (smell is important for insects to find flowers, while birds rely on vision), and tubular flowers that allow easy access for hummingbirds.  Like other pollinators, when native plants thrive, so do hummingbirds.  To sustain a heightened metabolism (their hearts beat as fast as 1,260 beats per minute after all), hummingbirds eat once every 10 – 15 minutes and visit between 1,000– 2,000 flowers per day!

Unfortunately, the preferred habitats of RTHU are changing along their migratory routes due to habitat conversion and climate change.  There has been an increase in the concern about the conservation of pollinators in urban environments.  A known way to mitigate the impact of urbanization is through native plants gardens which provide both habitat and resources for pollinators.  Such efforts can also encompass native plants with specialized hummingbird-pollinated flowers in urban landscaping.  In the northeast, planting red cardinal flower (Lobelia cardinalis) or trumpet honeysuckle (Lonicera sempervirens) in your garden may contribute to community restoration and ecosystem functioning by attracting RTHU—and who wouldn’t want these cuties flying around their yard?

Female RTHU drinking from L. cardinalis. PC: Max McCarthy.

Do you want to start your own pollinator garden this year, but don’t know what plants to get? Come to our Grab-n-Grow Native Plant sale next month to stock up on all thing native plants!  More information can be found at: https://sites.tufts.edu/pollinators/cultivate/native-plant-sale/.

The right way to leave stems for native bees

About 30% of New England’s native bees build nests above ground. Besides bee hotels (many of which have their own issues), a great way to support these above-ground nesting bees is to leave dead plant stems standing in gardens. Bees will lay and provision offspring in these hollow or pithy stems. TPI members are often asked by gardeners, “when is the best time to cut down stems?” The answer is at least two years (ideally never), which is longer than you might think. Let’s review bee and plant biology to understand why.

Year 1: Plant stems are growing. Native plants like joe-pye weed, elderberry, wild bergamot, mountain mint, and swamp milkweed produce hollow or pithy (e.g. soft, spongy tissue) suitable for nesting bees. Bees won’t nest in these actively growing stems. At the end of the growing season (December through March), cut the stems back to between 6-18” tall. Use sharp tools to ensure a clean cut. By cutting back the stems, you have created homes for next year’s bees.

Joe-pye weed (Eutrochium purpureum) stems were cut back in December 2020. The stems are hollow and will provide homes for twig-nesting bees during the 2021 growing season. TPI will leave them stems standing until the end of the 2022 growing season to ensure that all bee offspring have emerged.

Year 2: Bees active during this year will nest in the stems you left standing. They will lay eggs in the stem and provision each egg with a nutritious ball of pollen and nectar. Inside the stem, bees will develop from eggs into larvae and adults that hibernate through winter. Bees won’t emerge from stems until next growing season. Remember to cut back the new, green stems produced this year for next year’s bees.

Leafcutter bees (Megachile sp.) will live in your garden if you provide undisturbed stems for them to nest in.

Year 3: In spring of year 3, stems produced in year 1 still contain bees; stems produced in year 2 do not contain bees. Leave both generations of stems standing throughout the year. Spring-active bees will emerge from year 1 stems by June, whereas fall-active species might not emerge from year 1 stems until August or early September. During this time, new bees will nest in year 2 stems, so leave them standing!

While this may seem like an awfully long time to leave stubble in a garden, it is the only way to ensure that native bees find safe, undisturbed places to nest. Posting signage in your garden to inform visitors about how gardens can be managed to balance aesthetic and ecological goals can be helpful.