Exotic Weeds Are The Worst

English daisy gets prolific in lawns.

Exotic species are not native. It is that simple. There is nothing fancy about it. They came from elsewhere to live here. They are not necessarily rare, unusual or innately desirable. Most plant species within most refined landscapes and home gardens are exotic. Native species that inhabit unrefined areas, although trendy, remain unpopular for landscaping.

Not only are exotic plant species not necessary rare or desirable, many are too common or undesirable. With few exceptions, the most aggressively invasive of weeds are exotic. For a variety of reasons, they proliferate faster than native species. Some are aggressive because they are endemic to competitive ecosystems. Most lack natural pathogens here. 

A weed is merely any undesirable plant. Some might be desirable in some situations but not others. English ivy, for example, which is a practical ground cover within landscapes, is also an aggressively invasive weed within coastal forests. Most weeds are annuals or biennials, but others are perennials, shrubbery, vines, aquatic plants or substantial trees. 

An unfortunate reality about exotic weeds is that they are not here by mistake. Generally, their importation was justifiable at the time. Most were ornamental plants for gardens and landscapes. Some came as fruits, vegetables, herbs, cover crops or forage for livestock. Some came as timber. Species that escaped cultivation and naturalized became weeds.

Such weeds compete for the same resources that desirable plants utilize, and are mostly visually unappealing. Some enhance the combustibility of landscapes and forests. A few weeds produce seed structures that are hazardous to pets and wildlife. Even if problems are not directly obvious, weeds disperse seed to share their innate problems elsewhere.

This is typically the best time of year to pull or grub out weeds, although more weeds will grow later. Annual weeds are mature enough to get a good grip on. Soil should be damp enough from winter rain for roots to pull out relatively easily. Weeding will likely require a bit more effort this year because of the extended dry and warm weather since December. 

Not all weeds are unwelcome.

Unplanned acacias certainly put on a show when they bloom. It is not easy to dislike something so impressively colorful.

Even the best tended gardens get weeds. Most weeds, particularly annual and perennial weeds, meet a quick demise by getting pulled out or sprayed with herbicide. A few shrub and tree weeds though, are sometimes allowed to mature into functional members of the landscape.

Many sneak into the garden by growing within overgrown or otherwise concealing shrubbery, where they can hide long enough to get established. Others are left to grow because they are recognized as desirable plants. The main problems is that many end up in situations where they eventually become problematic.

Mexican fan palms are distinctive trees. Unfortunately, many grow below utility cables, because that is where birds drop the seed as they eat the fruit. Unlike other trees, palms can not be pruned around utility cables, so must be removed when they get too tall.

Silk tree, black locust, tree of Heaven and various oaks, pines, acacias and eucalypti are some of the more common trees that can sneak into gardens. Sometimes, they happen to land in good situations. More often though, they get too close to foundations, eaves, pavement or other features that they damage as they grow. Like Mexican fan palms, they are easier to remove while young, before they become problematic.

Pittosporums, cotoneasters and privets are commonly seeded shrubbery with less potential for problems. The main problem with glossy privet is that it can be too prolific and aggressive, so that it can crowd out more desirable plants. Most pittosporums and cotoneasters that get seeded are from plants that are ‘straight species’ (not cloned cultivars or varieties). Those that happen to be from cultivar or variety plants will not be ‘true to type’, which means that they will be more like the straight species than like their parents.

The few fruit trees that can sometimes grow from seed have the same problem, since only some of the more genetically basic types may resemble their parents. Fancier types and (non-sterile) hybrids probably will not. Fruit trees that grow from root suckers of grafted trees instead of from seed will be nothing like the parents, and may produce useless fruit.

Pampas grass is a prolific and sometimes welcome perennial weed near untended parcels or forested areas where pampas grass has naturalized. Broom is a shrubby weed that is even more prolific, but never welcome.

Algerian and English ivy rarely grow from seed, but can be really nasty weeds if they get where they are not wanted. Bear’s breech (or breaches), Jupiter’s beard, calla, mint and various yuccas that were planted may be very difficult to eradicate if they are no longer desirable, or if they migrate into areas where they become problematic.

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

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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

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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

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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.

Corm-ucopia

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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.

Thistle

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.