Pollinator-friendly garden

What plants are sterile and not suitable for a pollinator-friendly garden?

When aiming to create a thriving pollinator garden, it’s crucial to understand that not all “flowering plants” are created equal in terms of their benefit to bees, butterflies, and other vital insects. Many cultivated varieties (cultivars) have been bred for human aesthetic preferences, often at the expense of their ecological value.

Here’s a breakdown of what makes a plant sterile or unsuitable for a pollinator garden, and examples of such plants:

What makes a plant “sterile” or unsuitable for pollinators?

• Double Flowers (Flore Pleno): This is the most common culprit. In double-flowered varieties, the stamens (male reproductive parts that produce pollen) and sometimes the nectaries (structures that produce nectar) have been bred to transform into extra petals. This results in a very full, lush bloom but:

  • Little to no pollen: Pollen is a vital protein source for many pollinators, especially bees.

  • Inaccessible nectar: Even if nectar is present, the dense layers of petals make it difficult or impossible for pollinators to reach.

  • Confusing structure: The altered flower shape can make it hard for pollinators to recognise or navigate the flower.

  • Extended bloom time (sometimes): While a long bloom time sounds good, if the flowers offer no reward, it’s just a prolonged deception for pollinators.

• Sterile Cultivars (Bred for Reduced Seed Set): Some plants are intentionally bred to be sterile or have very low seed set, often to prevent invasiveness or to encourage continuous flowering (as the plant isn’t putting energy into seed production). While this can be a good thing for controlling invasive species, it can also mean reduced pollen and nectar.

  It’s worth noting that some “sterile” cultivars might still produce nectar, and some pollinators might still visit them, but they won’t contribute to the full life cycle support (pollen for larvae, seeds for birds, etc.).

• Highly Modified Flower Forms: Beyond just “double” flowers, some cultivars have drastically altered flower shapes that make it difficult for pollinators to land or access the reproductive parts.

• Foliage Color Breeding: Plants bred for dark red or purple foliage often contain high levels of anthocyanins, which can be deterrents or even toxic to some herbivorous insects. While this doesn’t directly relate to flower sterility, it indicates a focus on aesthetics over ecological function.

• Plants with Inaccessible Nectaries: Even in single flowers, if the nectar is too deep within a long, narrow tube that doesn’t match the tongue length of common local pollinators, it won’t be beneficial.

Examples of plants/types of plants generally not suitable for a pollinator garden (or should be chosen carefully):

• Many Roses (especially hybrid teas and most “double” varieties): Most modern roses, particularly those with very full, multi-petaled blooms, offer little to no pollen or accessible nectar.

  • Good alternatives: Opt for “single” or “semi-double” varieties, old-fashioned roses, or species roses (e.g., Rosa rugosa, Rosa gallica) which have open flowers with visible stamens.

     

• Most Hydrangeas (especially Hydrangea macrophylla cultivars): Many common mophead and lacecap hydrangeas have a high proportion of large, showy, sterile florets. While some lacecaps have fertile florets in the center, many cultivars are bred to be almost entirely sterile.

  • Good alternatives: Some native hydrangeas (e.g., Hydrangea arborescens) have a higher proportion of fertile flowers.

• Some Peonies (especially double-flowered varieties): Similar to roses, very full double peonies often have transformed stamens into petals.

  • Good alternatives: Single or semi-double peonies.

• Some Camellias (double forms): Many ornamental camellias have dense double flowers.

  • Good alternatives: Single-flowered camellia species.

• Many Marigolds (Tagetes spp. – particularly dense double cultivars): While some marigolds can attract pollinators, many of the very dense, pom-pom-like varieties are less useful.

• Certain Chrysanthemums (dense double varieties): Similar to marigolds, the very full, double-flowered chrysanthemums offer little to pollinators.

• Some Cultivars of Coneflower (Echinacea) and Dahlia (double forms): While the straight species of Echinacea are excellent, many modern Echinacea and Dahlia cultivars have been bred for complex double blooms that make it hard for pollinators to access.

• Plants with “fl. pl.” in their botanical name: This stands for flore pleno and directly indicates a double-flowered cultivar.

Key takeaway for a pollinator-friendly garden:

When selecting plants, always look for:

• Single flowers: Where the reproductive parts (stamens with pollen, pistil) are clearly visible and accessible.

• Simple flower structures: Open bowls, flat clusters, or tubes that match the tongue length of your target pollinators.

• Native species or close cultivars: These are often better adapted to local pollinators.

• Diverse shapes and colours: To cater to a wider range of pollinator species.

• Visible pollen: If you can see the yellow pollen on the anthers, it’s a good sign!

By being mindful of these characteristics, you can ensure your garden provides a true feast, not just a pretty display, for our essential pollinators.