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Invasive plants: yes, they matter

  • Marie Narlock
  • Imagine an out-of-town friend has arrived for a visit. The first few days are fine, but slowly things change. Turns out your friend's a slob. She's loud. She's rude to your neighbors. Worst of all? She won't leave.

    For thousands of years, California's native plants have co-evolved alongside our local butterflies, small mammals, birds, bees, and other pollinators – creating one of the most heavenly diverse ecosystems on the planet. Our sublime conditions also allow us to grow numerous plants from other regions of the world. Most are not a problem. But what happens when, like your house guest, one of these out-of-towners misbehaves and overstays its welcome?

    Weedy, invasive plants cost California $82 million annually and cause a death spiral in biodiversity because they out-compete native plants. They clog streams and cause flooding, disrupt wildlife, raise havoc on farms and ranches, and create voluminous quantities of fuel for fire. Today, California's invasive plant problem is considered severe.

    Some of these plants arrived with European settlers, others smuggled in as seeds on ships, packing crates, or on soles of shoes. Others have proliferated thanks to climate change. All invasives have three things in common: they're not native, they reproduce and spread quickly, and they cause harm to the environment, economy or human health.

    Many are familiar – broom, pampas grass, giant reed grass, yellow star thistle, and periwinkle, to name a few. Unfortunately, some are still sold in nurseries, which puts the onus on shoppers to avoid purchasing problem plants. In the right conditions, even the simplest, most innocent act can snowball into environmental chaos.

    Here's how that unfolds.

    Let's say you plant ivy. A few months later it's spreading into your herb garden and marching toward your roses. Next, it's heading for your neighbor's yard.

    Soon it creeps into nearby wildlands, where it snuffs out an assortment of native plants. It forms a thick, ground-hugging monoculture – an area dominated by one species – making it almost impossible for trees to establish. The reduced shade increases stream temperatures, which affects fish and other wildlife.

    Insect populations decline because the plants they need decline. Ground-feeding birds have a tougher time finding food through the thicket and ground-dwelling bees have a tougher time nesting underground. Animals relocate because their food and cover has gone missing.

    The ivy blooms. Birds gobble up the seeds and spread them afar, excreting a dose of fertilizer to help each seed germinate. New patches emerge on farms and parks and ranches, where it's toxic to cattle. As ivy crowds out the farmer's crops and rangeland forage, land value declines and management expenses skyrocket.

    New ivy sprouts like Whack-a-mole – only there's no one there to do the whacking. It overruns a stand of milkweed and lupine, leaving fewer monarchs and mission blue butterflies. It steamrolls native honeysuckle and trillium and redwood sorrel deep in the forest. The altered area becomes a weedy tangle. This, combined with ivy's penchant to clamber up trees, creates massive fuel for wildfire. Residents hold their breath, hope for the best.

    All because of one little ivy plant gone rogue.

    Yes, it can happen.

    Should you feel guilty if you have ivy? Absolutely not. Innocent planting mistakes don't always go this bad. We've all planted the wrong thing – and many of us inherited gardens with ivy and other issues. Just do what you can. Snip off ivy flowers to limit spread, don't let it girdle trees, and dig out a little at a time. Fortunately, many plant invasions can be interrupted or stopped -- even horribly infested areas.

    The next time you have spots to fill in your garden, choose plants that benefit the environment and don't spread. The easiest way to eliminate invasive plants is to prevent them in the first place.

    Visit the UC Marin Master Gardener "Choose plants" webpage for helpful plant lists. Visit Cal-ipc.org or Marinrcd.org to learn more about invasive plants.