Pollinator Garden Design
Pollinator Garden Design
for WSARE: Sustainable Agriculture & Education
By Gary Nabhan & Caleb Weaver
When you think about planting hedgerows or gardens to attract pollinators, think of coherency, color, contrast and continuity between seasons. Think of the specific “life history” needs of particular pollinators as reconciliation ecologists do (Rosenzweig 2003)— from larval host plants and nectar sources of a pipevine swallowtail (species within the genus Aristolochia or pipevines) to the nesting substrates and foraging distances of cactus bees. It is their needs (not ours) which need to be met through the structure and seasonality of the groups of plants that you integrate into a comprehensive design. Such habitats should not be mere amalgams of randomly-selected plants, placed next to one another for no “reason” apparent to either humans or to pollinators themselves! Instead, this is an opportunity to develop a cohesive habitat structure attractive not only to humans, but to bees, bats, butterflies and hummingbirds–the charismatic microfauna so abundant in healthy habitats of Western North America.
A good design should meet those needs both spatially and temporally (or seasonally). Let the earliest tide of plants in flower plants gradually ebb and flow into the mid-season tide, the late season tide. It may take some time and study to adequately select plants for a pollinator garden or hedgerow design that creates the greatest phenological or seasonal continuity. Nevertheless, it will be well worth the effort if the knowledge you have acquired can help to better support a safe haven for foraging, nesting or roosting pollinators. At the very least, a pollinator garden or hedgerow should offer insects and birds enough nectar and pollen to survive. At best, these plantings can enhance the health of the entire biotic community they reside within – including their human neighbors.
Design with Pollinators’ Perceptions and Needs in Mind: Sustenance, Shelter, and Safety
Ideally, we would like to design pollinator habitats by being able to see, smell and feel their microclimates as hummingbirds, bats, bees and butterflies or night moths do. If we were successful, we’d be able to create environments that pollinators could easily find nectar and pollen foraging resources, comfortable roosts and potential nesting or mating sanctuaries with ease. Because their eyes and other senses are different than ours, their umwelt, or perception of space as they move to find resources critical to their survival is very distinctive from our own.
Pollinator Syndrome Generalizations
(There are many exceptions!)
|Flower Color||Dull white, green or purple||Bright white, yellow, blue or UV||Dull white or green||Scarlet, orange, red or white||Bright, including red & purple||Pale and dull to dark brown or purple; flecked with translucent patches||Pale and dull red, purple pink or white|
|Odor||Strong musty; emitted at night||Fresh, mild, pleasant||None to strongly fruity or fetid||None||Faint but fresh||Putrid||Strong sweet|
|Nectar Content||Fructose & Glucose dominated||Sucrose dominated||Sucrose dominated||Sucrose dominated||Fructose & Sucrose dominated|
For starters, consider the fact that each group of pollinators has sensory mechanisms different than our own, so different that they simply do not see a planting the way we do. Bees, for instance, have compound eyes. Each of these eyes is comprised of thousands of light sensitive cells which help the bee navigate between different colors, light intensities and directional cues gathered from the sun’s ultra violet rays. Nocturnal Lepidoptera, such as white-lined sphinx moths and hawkmoths (whose caterpillars can be found on Lycopersicon or tomatoes, Mirabilis or four o’clock, Malus or apples, Oenothera or primrose, and Vitis or grape) have an extraordinary capacity to follow certain fragrances to their associated flowers well before they see the blooms themselves. Butterflies may be attracted to sharp contrasts between yellow, red or orange patches in the foreground, and larger masses of greener, darker foliage in the background. A single orange blossom next to an equally small patch of green foliage may not lure them in visually, nor may it emit and hold a fleeting fragrance nearly as long as a cluster of orange flowers protected from the wind by an extensive windbreak of dense foliage. See the Xerces Society’s (1998) book, Butterfly Gardening, for more clues into the umwelt of particular pollinators.
Because you may lack the capability to fully sense the world as they do, at least remember that pollinators must meet three essential needs to survive and reproduce: sustenance, shelter and safety.
Sustenance: Most pollinators are rewarded for their fertilization services to the flower with a kind of nectar suited to their nutritional needs. Flower nectars vary in their ratios of different kinds of high-energy carbohydrates—fructose, sucrose, glucose, etc. — and these roughly tend to match the needs of their most allegiant pollinators. But nectar alone does not provide enough liquid for pollinators – they must also drink free-standing water, either from surface water or a damp, seep-like patch of soil. Meanwhile, pollen, which is gathered by many bees, supplies the essential amino acids that are used to maintain body tissue and assist in raising young.
Pollinators prefer many varied food sources. It is best to have three or more different plant species flowering at a given time. It’s especially important to maintain a healthy pollinator population in the face of climate change by choosing plants that flower early in spring and late in fall. Pollinators are adapted to feeding from flowers and are thusly healthiest when native flowers dominate their diet. However, artificial hummingbird feeders may have their place in some contexts. They can bring life to areas that are not conducive to planting large gardens, assuming they’re cleaned and refilled on a weekly basis.
Shelter: Some plantings intended to aid pollinators tend to temporarily concentrate a few kinds of common bees or hummingbirds in one location, but without actually increasing the size or diversity of their resident or migrating populations. To actually expand and diversify these populations requires achieving a critical mass of foraging, nesting and roosting habitat. Creating shelter to buffer from extreme cold or heat spells and catastrophic events may also help pollinator populations build through time rather than decline. Dead branches and hollows on trees and shrubs can provide roosting and nesting habitat, so they should not necessarily be pruned away. In short, woody plants with different, complex morphologies (and their dead standing crops) can provide habitats for a variety of pollinators which then service different flowering plants growing nearby.
Similarly, bunch grasses are useful for bumblebees, which nest within or beneath the clumps of dead stalks. Native solitary bees may lay eggs in trimmed hollow-stemmed grasses. In the desert and spilling uphill into the sky islands, yucca, sotol and agave stalks often house carpenter bees. A pile of dry, loose sand in an area that receives warm sun in the morning invites ground-dwelling bees. Artificial dwellings, such as bee blocks and bee boxes can bring some creativity into a pollinator garden yet need to be cleaned every year to minimize disease transmission.
Safety: Gardens are best when they’re a place where both humans and pollinators can be safe from threats. Pesticides cause the pose a dangerous threat to pollinators, especially to insect pollinators like bees, butterflies and moths. The USDA cautions that inappropriate use of pesticides may be one of several contributing sources of Colony Collapse Disorder, which has in recent years ravaged honeybee populations across the globe. If you wish to maintain populations of native bees as well as honeybees, you (and your neighbors) should avoid or at least minimize the use of pesticides and herbicides to maintain healthy pollinator populations. You may wish to explore non-confrontational means to educate neighbors about the potential harm done by pesticides, so that your community can work together to restore ecosystem health.
Basic Design Considerations for Pollinator Plantings in the Arid West
With regard to the basic design principles for habitats that attract pollinators and other beneficial wildlife, we’ve taken much of our inspiration from Bill Mollison’s Introduction to Permacultureand Janine Benyus’s Biomimcry. Mollison reminds us that in designing for any habitat, we should:
- Try to mirror nature – Practice thoughtful observation of the intricacies of the natural systems and avoid thoughtless action. (More on this below, from Benyus (2003).
- Focus on getting most of the work done through use of local, renewable resources – Follow the flow of rainwater to irrigate plants instead of pumping or hauling water onsite with fossil fuels. Compost and build soil from nutrients that are available locally instead of mining and importing petro-chemical fertilizers.
- Maximize the value of edge effects – Extend the length of the edges of the garden with curves to increase habitat perimeter length, biodiversity and plant growth.
- Accelerate natural ecosystem processes – Create depressions in the ground where water naturally pools to harvest rainwater. Fill these depressions with plant mulch and bring the soil to life while increasing its water storage capacity. Plant pollinator-attracting plants, to establish the base of the ecosystem’s food chain.
- Stack functions – Stratify biodiversity into different vertical layers by selecting plants that form many functions: nurse plant canopies that provide roost for pest-regulating birds and for shade and frost protection (Celtis or hackberry); shrubs and trees which pump nitrogen (Prosopis or mesquite); ground-hugging vines which shade the ground and reduce desiccation; and nutrient pumps (Achillea or yarrow), etc.
In Biomimicry Janine Benyus (2003) has laid out the principles marked below in italics, which one of us (Nabhan) has then elaborated in Growing Food in a Hot, Dry Land as key principles for adapting to climate change. Here are a few that are pertinent to pollinator plantings:
- Nature fits form to function – Native plants are already adapted to the environment, exotic plants often require extra work including supplemental water and petrochemical fertilizers to artificially enhance the environment to fit the crop’s needs.
- Nature rewards cooperation – You and your pollinators never really farm or forage alone; optimize the collaboration between diverse pollinators, soil microbes, beneficial insects and other neighbors that can be there as allies farming with you!
- Nature banks on diversity – If the pollinator habitat within your foodscape is designed to be rich in growth forms, species, varieties, microbes, and strata, it will offer much of the resilience you need in the face of shifting seasons and climatic disruptions.
- Nature demands local expertise – While seeking inspiration for redesigning your pollinator habitat and food system, draw upon the particular examples of local place-based species, habitats, and plant associations or guilds. For instance, near the U.S/Mexico border, many Mexican farmers still plant and maintain living fencerows, hedgerows and windbreaks in roughly the same habitats that their American counterparts farm in without the integration of hedgerows into their farmscapes. Investigate what these neighbors are doing, rather than reinventing the wheel. Let your collective homeland be your mentor, your genius loci.
Assessing Your Site
Pollinator gardens and hedgerows need to build on existing conditions at the site, but create micro-climates and protective niches that better attract and keep pollinators over the seasons. Ample access to sunlight is essential for a thriving pollinator garden. Sites receiving plenty of sun will warm up ectothermic flying insects, increasing their activity and pollination potential. Make sure to include a few rocks and sticks with full solar exposure for basking butterflies. Most pollinating insects need unobstructed views of the sky to navigate so keep shady trees on the north, east and west perimeter of the garden. While heavily shaded landscapes, like dense forests, are not conducive to maintaining diverse pollinators, including some trees in hedgerows or gardens is important for allowing hummingbirds to view and defend their territories.
Fierce winds make foraging difficult for pollinators. If hedgerows are to be designed as both functional windbreaks and as pollinator forage habitat, additional design considerations must come into play. The width and height of the hedge must be designed in relation to the prevailing direction and intensity of spring and summer winds relative to the more sensitive plants, such as crops. Use existing buildings as windbreaks if you do not have enough space to let the plantings perform this function.
Enhancing access to water is probably the most important resource for successfully sustaining populations of pollinators in a hedgerow or garden, especially in dry climates. Keep a slow drip or trickle flowing into troughs or seeps below rainwater-harvesting cisterns or faucets. You can direct runoff from rainstorms into basins, rather than allowing this critically-important resource to flow away as an erosive force. Making water more accessible to flowering plants will increase nectar production and will eventually develop into budding pollinator populations.
Getting the Right Plants for the Job
There’s no wrong way to select plants for a pollinator garden. However, putting in a bit of thought will drastically increase the desirability to flyers-by. Remember the following principles when selecting plants:
- Locally-procured ecotypes of natives – Research shows that bees are four times more attracted to native plants while native plants increase butterfly and moth diversity three-fold in comparison to exotics. Native plants require less maintenance, nutrients and water while providing the best food source for native wildlife. Choosing and collecting cuttings and seeds for nursery stock from your watershed— ones that grow well and flower at your elevation and in your soil conditions— will maximize plant survival. For instance, many local populations of purple tansy, scorpionweed, or lacy-leaved fiddleneck (Phacelia tanacetifolia) perform well in native wildflower mixes within the landscapes where they originated, but some commercial wildflower catalog listings offer a single selection that is allelopathic; it suppresses the germination and growth of other species with which it is planted. Rely as much as possible on your local seed sources.
- The more the merrier – Researchers have found that pollinator diversity and total abundance is significantly higher with a minimum of 8 different flowering plants. Pollinator diversity continues to rise with an increase in plant diversity.
- Every shape & size – Alter shapes and colors of flowers to attract diverse pollinator populations. Hummingbirds frequent red tubular flowers while fragrant white flowers entice bees.
- Clusters of flowers – Research suggests that pollinators are more attracted to clumps smaller plants to a width of 3-4 feet rather than solitary individuals. Clumping saves pollinators energy by reducing travel distance.
- Contrast colors – Use dense mats of dark green foliage to contrast clusters of bright flowers. Keep in mind the vantage point where most pollinators will approach flowers.
- Out with the old – Prune to promote flowers, or reproductive growth, rather than more woody stems and leaves, vegetative growth. (However, keep some dead branches and standing dead trees as roosts.) Yard clippings make great mulch.
- Baby food – Chose plants for the entire life cycle. Flowers are of course necessary for adult pollinators, but don’t forget caterpillars and eggs. As a general rule of thumb, caterpillars prefer eating milkweed and vines. Bees lay eggs in grass, sotol and bear grass in my neck of the desert.
- Plant guilds – Choose plants that hang out together in nature. Hiking around in nature is the best way to find these guilds.
- Redundancy – Replicate plant clusters and guilds throughout the garden to sustain more hummingbird territories and pollinator populations.
Finally, Focus on Specialty Plants for Specialized Pollinators
Despite the many concerted efforts by landscape architects, farmers and ecological restorationists to plant hedgerows which attract and sustain pollinator diversity to food-producing landscapes, few of the efforts successfully support the rarest of pollinators or diversify the pollinator guild beyond what is in existing “background” habitats. Instead, to date, they have only increased the abundance of already common pollinators, and not the most threatened (Winfree et al 2011). One way to increase success is to focus on the peculiar life history needs of the rare species (Rosenzweig 2003). In particular, focus on the specialized larval host plants of pollinators such as pipevine swallowtails by planting native pipevines (Aristolochia watsonii) or erect fences of sotol and agave stalks that can be readily colonized by carpenter bees. Plant sacred Datura (Datura wrightii) to attract hawkmoth (Manduca sexta) larvae which will eventually pollinate both Datura and Agave (Agave palmeri). The Soapweed Yucca (Yucca glauca) creeps south from the prairies of interior Canada through Texan flats and houses a moth no other plant can support (Tageticulla yuccasella). Including any of these plants will introduce habitat for these specialized pollinators.
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