Native vs Non Native gardening

Butterfly & Zinnia cm
Giant Swallowtail Butterfly on a Zinnia – the flower is definitely not native to Southern Ontario

I recently started following the Royal Horticultural Society on Twitter (@The_RHS); I’m not sure how this feed came to my attention, likely it was Twitter itself, that clever creature, that suggested it.  It was a good suggestion.  Even though it’s a British organization, and the information they share is abut British gardening and British plants and British garden shows, the RHS does, not infrequently, post tidbits that are relevant over here.  Also, their picture are very pretty.

This morning the Society tweeted that the second paper from their Plants for Bugs program has at long last been published.  (The first paper was published in 2015.)

Plants for Bugs is a “a four-year study into wildlife gardening, which was undertaken at RHS Garden Wisley in Surrey…”  as in England, just southwest of London.  The scientists set out to determine what sort of plant typically found in an English garden does best at attracting insects: native or non-native.

Garden commentators in North America have, for many years now, been battling it out (sometimes quite heatedly) over this question.  There are extreme gardeners who ONLY plant natives and those who couldn’t give a whit if a plant is native or not, as long as it looks pretty.  Most commentators fall in the middle – wanting to use natives when possible but not closing the door to aliens (aka non-native plants).

The arguments for using natives run along the lines of they are more drought tolerant (true, but only if it’s a drought tolerant plant to begin with: you wouldn’t stick a native Marsh Marigold – Caltha palustris – in gravelly soil on a south slope); they’re better suited to our climate (until you factor in climate change and how Prince Edward County’s Plant Hardiness Zone has gone from zone 5b to zone 6a in the past 20 years); and they provide better food – nectar, pollen and roughage – than non-natives.

It’s this last assumption that the folks in England wanted to measure.  In 2009, they created 36 planting beds, each nine square metres, and planted some with a mix of plants native only to the UK; other plots had plants that only grow elsewhere in the northern hemisphere while the rest had plants that grow in the southern hemisphere.  Then they waited, and counted the invertebrate life that showed up.

“By the end of December 2013 (four full years of recording) approximately 80,000 invertebrates had been counted and more than 300 species identified.”

The peer reviewed papers that have so far resulted from the study have led the RHS to come up with key messages for the home gardener.  To quote from the RHS site:

From the first paper, that studied pollinating insects:

  • The best strategy for gardeners wanting to support pollinating insects in gardens is to plant a mix of flowering plants from different countries and regions.
  • Emphasis should be given to plants native to the UK and the northern hemisphere, though exotic plants from the southern hemisphere can be used to extend the season
  • Regardless of plant origin (native or non-native), the more flowers a garden can offer throughout the year, the greater the number of bees, hoverflies and other pollinating insects it will attract and support.

From the second paper, that studied insects that chomp on plants, suck the juices from plants or eat other bugs that suck or chew:

  • The best strategy for gardeners wanting to support plant-associated invertebrates is to plant a predominance of plants native to the UK.
  • Plants native to the Northern hemisphere are likely to support only marginally fewer (less than 10%) invertebrates in some functional groups (including herbivores and some predators) than UK native plant schemes. And exotic plant schemes based on Southern hemisphere plants will still support a good number of invertebrates, albeit around 20% fewer than plants from the UK.
  • Regardless of plant origin, the more densely your garden is planted or allowed to grow, the greater the abundance of invertebrates of all kinds (herbivores, predators, detritivores and omnivores) it will support.

 

I’m guessing these messages will appeal to many North American gardeners as well.

For me, the takeaway is to plant densely using a diverse mixture of annuals, perennials, shrubs and bulbs to create as long a flowering season as possible.

Sunflower August 2017

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