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How Uncommon Weather Patterns Affect Invasive Plant Growth

California has officially ended its historic drought with back-to-back years of heavy rainfall, positively impacting the landscape of the Irvine Ranch Natural Landmarks but posing challenges for invasive species control. 🌧️ #InvasiveSpecies #UncommonWeather

As any Southern California resident can tell you, the state has experienced another rainy season, already following an abnormally rainy winter last year. According to the National Weather Service, Southern California has witnessed the “second-wettest back-to-back years since records began in 1877.” As a result, California has officially ended its several-year-long historic drought, with 0% of the state’s area facing drought conditions. While it’s great that the state is no longer in drought, these uncommon weather patterns can have a significant impact on the landscape of the Irvine Ranch Natural Landmarks.

Irvine Ranch Conservancy (IRC) staff and volunteers are continuously monitoring the Landmarks to keep track of the changes in the natural landscape. The goal is to preserve the natural environment while combating invasive species. Since it has been difficult to predict each year’s conditions, IRC staff remain vigilant, constantly adjusting plans based on the best response to weather patterns. This close monitoring is essential due to the challenges posed by unpredictable weather.

The recent Hurricane Hilary that hit Southern California has heightened the need for IRC staff to be alert at all times, compared to specific times of the year. The abnormal summer rain brought about various issues, including a massive germination of invasive castor bean (Ricinus communis) and early germination of invasive North African knapweed (Centaurea diluta). Because of this, IRC staff had to alter their plans and begin weeding much earlier than usual, starting in the fall, to address the impact of the excessive summer rain.

Furthermore, due to the past two years of heavy rainfall, both native and invasive plants have experienced faster growth and increased quantities. However, the plants matured faster last year with a shorter life cycle due to the heavy rain followed by dry, sunny conditions. This year, with rain occurring in increments and cloudy weather in between, the plants are maturing at a slower pace. It is important to note the pace of the plant’s life cycle because this predicts the timing that the seed drops from the plant. For invasive species, IRC staff want to weed out and remove the plant before it drops seeds.

The slower lifecycle allows IRC staff to weed earlier in the process, but scheduling has become a challenge. The increase in rain makes accessing the sites difficult as certain roads may be closed due to muddy trail conditions. Additionally, the rain can damage the trails, requiring IRC staff to dedicate time to repair and reopen them to the public, reducing the time available for weeding. Finding a dry time frame for weeding between sporadic rain has also proven challenging. It has taken staff a lot longer to accomplish their tasks this year due to the uncommon weather patterns we are experiencing. 

It could be several years until we see the impact of the heavy rains but in the meantime, IRC staff and volunteers are still surveying the Landmarks every year to monitor and understand the balance between native and invasive plant cover conditions and to identify new invasive problems. This valuable data helps IRC make decisions regarding the priority for the removal of invasive species. If you are interested in contributing to the invasive control work, please visit for upcoming activities.