The inner life span of plants | Garden Notes

Douglas Meadowfoam (Limnanthes douglasii) is a long-blooming annual wildflower with special value to native bees. It reseeds to begin a new annual cycle each year.
Douglas Meadowfoam (Limnanthes douglasii) is a long-blooming annual wildflower with special value to native bees. It reseeds to begin a new annual cycle each year.
Photo courtesy of R.W. Smith, Lady Bird Johnson Wildflower Center
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“If a tree dies, plant another in its place.” – Carl Linnaeus

Plants have a predictable life span. Virtually all of a plant’s cells, tissues, and organs age and eventually die. This complex process integrates genetic factors and environmental signals to determine a plant’s longevity. 

Unlike animals, many plants have “indeterminate growth” — that is, they continue to grow throughout their lifetimes. They have meristems, or bundles of undifferentiated tissue like stem cells, that enable them to get taller, form buds, and spread. Of course, that doesn’t mean they’ll live forever even under perfect conditions. 

Each plant’s inherent characteristics, including its growth habits and ability to withstand stress, help determine its expected total life span. The plant’s life cycle refers to the number of growing seasons needed to complete its journey from seed germination to maturity and eventual death. 

Understanding life span and life cycle has a number of benefits. It can help determine seasonal and year-to-year progression in the garden. It can help gardeners decide when to replace an aging plant. It’s also useful in determining strategies to deter specific types of weeds. 

Annuals are the shortest-lived plants, completing the life cycle within a year, from seed germination through flowering, seed production, and death. Many are self-seeding, like California poppy (Eschscholzia californica) and Meadowfoam (Limnanthes douglasii). If you want to control more aggressive annual spreaders such as hairy bittercress (Cardamine hirsuta), purslane (Portulaca oleracea), or chickweed (Stellaria media), remove them at the seedling stage, before they self-seed.

Some plants like tender geraniums (Pelargonium spp.) act as perennials in warmer zones, but are considered annuals if they are killed by winter frost. Gardeners can bring them indoors over the winter to extend their life span beyond one season. 

Biennials are usually small the first year as they develop leaves and store food. After a cold dormant period, they bloom and set seed to complete their life cycle the second year. The first year’s small plants or rosettes can be easy to overlook, so it’s helpful to make sure they have enough space to accommodate the second year’s growth and flowers. 

Foxglove (Digitalis) is one genus where desirable varieties like rusty foxglove (Digitalis ferruginea) produce tall spikes of small, rust-colored flowers in the second year, then gently reseed to begin a new biennial cycle. 

More prolific reseeders like common foxglove (Digitalis purpurea) produce a flurry of new plants from seed. To deter fast-spreading biennials, including weeds like bull thistle (Cirsium vulgare) or poison hemlock (Conium maculatum), remove blooms before they mature. 

Occasionally, biennials go from seed germination to seed production in one growing season due to drought or temperature extremes. This is known as bolting, and can occur when biennial plant starts are exposed to cold before being planted in the ground. 

Perennials generally go dormant in cold weather and return with seasonal warming, year after year. Most take a few years to establish. Some, like lavender (Lavandula), rock rose (Cistus), or daphne (Daphne odora) have relatively short life spans. They can become leggy or woody after several years, showing less vigor and fewer flowers. Aging plants can become more susceptible to environmental stress and pest damage. At this stage, replacement may be a better option than continuing to invest in remedial actions. 

Perennial weeds like Canada thistle (Cirsium arvense) or field bindweed (Convolvulus arvensis) are more difficult to control than annuals or biennials because they can reproduce both vegetatively and by seed dispersal. A combination of techniques will be most effective, including careful hand removal and use of inorganic and coarse organic mulches.

Trees and tree-like shrubs follow similar patterns, with predictable growth cycles and life spans. Typically, fast-growing species develop weaker wood, leading to shorter life expectancy. Native bitter cherry (Prunus emarginata) and non-native black poplar (Populus nigra) are fast-growers with average expected life spans of 30 to 50 years, depending on conditions. As they age, they may become less resilient. 

In contrast, the Great Basin bristlecone pine, found in the mountains of California, Nevada, and Utah, can live for thousands of years. Its slow growth produces dense wood, helping it adapt to harsh conditions and resist threats. 

Of course, ideal conditions can help extend a plant’s life. Environmental factors can play a major role. Biotic influences include fungal or bacterial pathogens, viruses, and insect damage. Abiotic factors include drought, fire, flooding, mechanical damage, or herbicide drift. Observing these factors can help diagnose plant problems and determine appropriate responses. 

Finally, when a plant dies, there are some silver linings: the opportunity to continue the cycle by creating wood chips, compost, or a wildlife snag — or by planting something new. 

If you have questions about plant life spans or life cycles, the Master Gardener Plant Clinic offers live Zoom sessions on Thursdays from noon to
2 p.m. through September. Sign up or submit a question at extension.wsu.edu/jefferson/gardening-2/plant-clinic. 

(Barbara Faurot is a Jefferson County Master Gardener and Master Pruner, working with other volunteers who serve as community educators in gardening and environmental stewardship.)