Invasive Weeds Waste No Time

Aggressively invasive exotic species become weeds.

With few exceptions, the most aggressively invasive weeds here are exotic. In other words, they are not native. They came from other regions where they were likely compliant participants of their respective ecosystems. At home, where they must compete with other members of their ecosystem, they may not be so aggressively invasive. Ecology is the opposite of a home field advantage.

Exotic species become invasive weeds in foreign ecosystems for a variety of reasons. For some, the climate is more favorable. Some grow and proliferate more freely without diseases, insects and animals that troubled them back home. There are also several that simply compete more aggressively for space and resources than native species are accustomed to. It is a jungle out there.

Most invasive exotic species are annuals. Many are biennials or perennials. Some are vines, shrubs or even trees. Most were imported intentionally, for a variety of reasons, and then naturalized. Forage and cover crops were some of the earliest of exotic species to become invasive. Other invasive species escaped from home gardens. Blue gum eucalyptus was imported for wood pulp.

Regardless of their origins or physiological forms, invasive species are weeds. They compete for the same resources that desirable plants use. They impede on the aesthetic appeal of gardens and landscapes. Some types of weeds become hazardously combustible. Even if not directly problematic, invasive weeds disperse seed that can be problematic nearby. Many disperse stolons.

Most weeds start early and grow fast to get ahead of their competition. They are more active at this time of year than at any other time. They are also vulnerable. While the soil remains damp from winter rain, they are relatively easy to pull intact. They have not yet dispersed seed for their subsequent generation. Later, they are likely to leave behind seed and bits of roots that can regenerate.

It is important to pull or grub out seedlings of unwanted shrubbery and trees, as annual weeds. They are likely to regenerate if merely cut.

Mock Strawberry

Potentially invasive mock strawberry fields forever.

The small yellow flowers are the giveaway. Real strawberries have white or maybe pink flowers. Otherwise, mock strawberry, Potentilla indica, looks very much like wild strawberry, with similar small red fruits and neatly serrate trifoliate leaves (that are palmately divided into three leaflets). The berries are edible, but do not taste like much.

Like real strawberries, mock strawberry spreads efficiently by stolons. If watered occasionally and sometimes fertilized, it is probably a better groundcover than the common ornamental varieties of wild strawberry. It is invasive in some areas.

Exotic Species Can Become Naturalized

Naturalized Himalayan blackberry is detestably aggressive.

Every palm tree in local landscapes is exotic. Simply put, all palms are originally from somewhere else. The desert fan palm, the only palm that is native to California, came from isolated colonies in desert regions many miles away. In fact, most plants in common landscapes are exotic. Landscapes composed of Californian plants likely include some plants from other regions of California.

With few exceptions, exotic plants are not a problem. However, some of those few exceptions have become very serious problems. Himalayan blackberry, blue gum, silver wattle, pampas grass, giant reed and broom are some of the more notorious examples. They naturalized to become prolific and aggressively invasive weeds. Some are more common than natives in many situations.

Naturalized exotic plants such as these are problems for local ecosystems, even if they do not affect refined landscapes. They compete with native plant species for limited resources, space and pollinators. A lack of pathogens from their homelands can be a distinct advantage. They alter the lifestyles of some of the native fauna. Some enhance the combustibility of the forests they inhabit.

The justifications for importing exotic species are as varied as the species themselves are. It might have been for lumber, forage, fruit, or vegetable production. Giant reed might have arrived here as packing material for cargo from southern Asia. Nonetheless, most naturalized exotic species, including the most aggressively invasive, came here simply for home gardening and landscaping.

Realistically, of all the countless exotic species that came here during the past few centuries, very few naturalized. Fewer are now aggressively invasive. Some with potential to naturalize may not have yet been able to escape the urban situations they inhabit. The problem now is that there are so many more exotic species readily available from all over the World than there has ever been!

Online marketing facilitates procurement of exotic and potentially invasive plant species from other regions, with minimal regard to regulation of such commodities.

Weeding Earlier Rather Than Later

Rain makes weeds grow like weeds.

Weeding is not much fun. Some of us might enjoy the relaxing monotony of productive weeding. Realistically though, most of us would prefer to do something else in the garden. There is certainly plenty of other chores that need to be done now, after earlier rain, and before the weather gets significantly warmer. However, such weather is why it is important to start weeding earlier than later.

By definition, weeds are weeds, because they are unwanted. They get to be unwanted by dominating space and exploiting resources more aggressively than wanted plants. Some innately grow faster and more aggressively than most other plants. Some are innately prolific with seed. Some employ multiple tactics to gain unfair advantages. Weeding is how we help our gardens compete.

Weeds grow throughout the year. Most slow down through the dry warmth of summer, and many die off then. However, there are always some weeds growing somewhere. When they die off, it is only after they have dispersed seed for their next generation. Some generate a few generation annually. Some are perennial weeds, or even shrubs, vines and trees, which survive for many years.

Weeding is more of a concern now because the majority of weeds grow so much more aggressively after the earliest rain of winter. Warming weather later in winter accelerates their proliferation. This is the time of year that weeds start to crowd desirable plants. If weeding is delayed for too long, weeds eventually bloom and toss seed. Some weeds extend stolons to disperse vegetatively.

The good news is that the same rain that promotes the proliferation of weeds also facilitates weeding. Weeds are easier to pull while the soil is still damp than they will be as the soil dries later in spring and summer. Also, while weeds are still fresh and turgid, they are less likely to leave roots or stolons behind in the soil. They are more difficult to pull intact as they begin to deteriorate later.

Furthermore, weeding should be done before weeds bloom and disperse seed for subsequent generations. Some are sneaky and quick.


Is this part of the secret to their success?

Montbretia showed up here several years ago. Of course, it did not take long for it to get very established. It is too shady for bloom, but not shady enough to inhibit vegetative proliferation. Those nasty stolons get everywhere, and grow into corms. They are so aggressive that they exclude English ivy! Seriously, they are the only species we know that can crowd out English ivy!

Some consider Montbretia to be the the genus name. Some consider it to be a common name for the genus of Crocosmia, or for a particular intergeneric hybrid. What is now so aggressively naturalized here might be Crocosmia paniculata. I really do not know. The few rare and sporadic blooms look like what I am familiar with in other landscapes, with branched inflorescences.

Now, I am aware of how aggressive their stolons are, and that their stolons swell into corms when they get to where they are going. I also know the physiology of simple corms, and that new replacement corms develop on top of old deteriorating corms. They might extend a few more stolons in the process, or put out a litter of cormels off to the side, but their technique is limited.

Well, it should be.

The technique demonstrated by this picture is weird. It seems to show a series of corms from the last twelve years. That makes sense if one corm replaces a previous corm annually. Longer accumulations can be found in older colonies. However, montbretia infested this landscape less than a decade ago, and took a few more years to disperse where these corms were unearthed.

Furthermore, after a decade, the oldest corms should be rotten and decomposed. Except for the stunted four year old corm, those that developed in the last six years seem to be suspiciously fresh.

Red Trumpet Vine

60420Red trumpet vine, Distictis buccinatoria, is more green than red. The tubular orangish red flowers with yellow throats are pretty while weather is warm, but not too abundant. Slightly distressed plants tend to bloom more abundantly. Each evergreen compound leaf is a pair of leaflets with a sneaky central tendril that will grab onto anything while holdfast discs get a more permanent grip.

Vines tolerate significant shade, but will find their way to sunnier situations where they grow more aggressively. They can easily reach the roof of a two story house, and grow out of reach in trees. Their holdfast discs will damage paint, and even shingles! Overgrown plants can be cut to the ground and allowed to regenerate.

Red trumpet vine wants to be watered somewhat regularly while young. Mature plants can disperse their roots well enough to find water if they do not get it directly.


60323Some of the biggest and nastiest weeds are thistles. The most common is annual sowthistle, which can get taller than four feet in just a few months. It is relatively easy to handle, since the bristly foliage and stems are somewhat soft, almost like coarse lettuce. Blessed milkthistle is much nastier, with sharp foliar spines that can penetrate boots! It can get more than five feet tall and broad!

Most thistles are biennials or perennials, with spiny lobed foliage. They produce low foliar rosettes during their first year, and then bolt and bloom on tall floral stalks during their second year. Biennials usually die after bloom, but sometimes regenerate from the roots later. Perennials are more likely to regenerate and bloom annually for several years. Some thistles get rather shrubby.

The roots of many types of thistle would not be too difficult to pull from well watered soil if only the spiny foliage were not so difficult to handle. Larger plants might be easier to pry out with a shovel. If foliage is merely cut off at the the surface of the soil, it will regenerate from the large tap roots left below. However, cutting down flower stalks before bloom interferes with seed dispersion.

Weeds Obviously Grow Like Weeds

90306thumbNot many of our favorite plants grow like weeds. We must help most of them along, and give them what they want. A few might naturalize and perform well on their own, but if they do too well and become aggressive or invasive, they too become known as weeds. Although we might prefer some of our favorites to be easier to grow, we are probably fortunate that more do not do too well.

Conversely, not many weeds are appealing plants when they invade our gardens. They might not be so disdainful if they provided fruit, vegetables or flowers, or were less aggressive with other plants. Instead, the conquer and occupy useful space, consume resources, and then toss their seed for the next invading generation. Their aggressive invasiveness is what makes them weeds.

There is no easy definition of ‘weed’. We know them only as unwanted plants, or plants where they are not wanted. Most are exotic (nonnative) plants that were once imported at a time when they were actually desirable. Some were vegetable or flowering plants grown in home gardens. Some were forage crops. Blue gum eucalyptus was imported for wood pulp. A few weeds are native.

Weeds become weeds because they have distinct advantages. Most get an early start at the end of winter, while other plants are still dormant. Then, many weeds bloom and toss seed for the next generation earlier than other plants. Many lack the pathogens of their homelands. Weeds generally survive on less resources, or complete their life cycles before resources are exhausted.

Most weeds are annuals. Many are perennials, Some are shrubby or vining. A few are trees. One commonality is that they should be pulled as soon as they are big enough to get a grip on. They are easier to pull while the soil is moist from winter rain, and before they have dispersed their roots much. Some of the short term annuals are pretty quick and sneaky about dispersing seed too!

Weeds that are woody shrubs, vines or trees need to get pulled like the rest. If merely cut to grade, they will likely regenerate from their stumps, and need to be dug later.