How Growth Form Affects Invasive Plant Management

When developing an Invasive Species Management Plan (ISMP) for plants, it is critical to consider how the target plants grow. 'Plant Growth Form' considers the plant's life cycle (annual, biennial, perennial), and anatomical differences in perennials (herbaceous vs woody, "bunch-type" vs creeping, and tree/shrub vs vine). Plant Growth Form describes characteristics of target plants that aid in developing a management prescription. The plant growth forms described in this article are summer annual, winter annual, biennial, non-creeping herbaceous perennial, creeping herbaceous perennial, non-suckering woody, suckering woody, and woody vines. These are summarized in Table 1.

Plant Growth Forms

Annual 

A plant that germinates from seed, matures, flowers, sets seed, and dies within one growing season. Annual plants are further divided into two categories based on the time of year they germinate.

Summer annual - germinates in spring and completes its life cycle before winter. Examples: Japanese stiltgrass, mile-a-minute vine, Japanese hops.

Winter annual - germinates in late summer/fall, remains green during winter, and flowers/sets seed the following spring.

Examples: hairy bittercress, downy brome, yellow rocket, common chickweed.

Biennial

A plant that completes its life cycle over two growing seasons. It can germinate from seed in spring or early summer and overwinter as a rosette (low-growing cluster of leaves). In its second season, the plant 'bolts' (flowering stem elongates), flowers, sets seed, and dies.

Examples: garlic mustard, poison hemlock, common teasel, plumeless thistle, bull thistle.

All annual and biennial plants are herbaceous (non-woody) and reproduce and spread only by seed. Therefore, preventing seed production is a key to successfully managing these life cycles.

Perennial

A plant that lives for longer than two growing seasons. Perennials can be herbaceous or woody. Herbaceous plants have annual above-ground growth that dies to ground level each fall, while woody plants have persistent stems. Some reproduce/spread only by seed while many others can also spread through vegetative means. Management strategies will differ depending on which combination of these characteristics the plants have.

Herbaceous, non-creeping - a non-woody perennial that only spreads by seed. Individual plants typically increase stems arising from the root crown each year, but it is a single, discrete plant. Above-ground growth dies at the end of each growing season and regrows each spring from persistent roots.

Examples: purple loosestrife, Chinese silvergrass, dandelion, broad-leaved dock.

Herbaceous, creeping - a non-woody perennial that spreads by seed and vegetative means, such as spreading roots, rhizomes (spreading underground stems), stolons (spreading above-ground stems), and tubers (fleshy stem tissue).

Examples: Canada thistle, Japanese knotweed, orange daylily, lesser celandine, common reed.

Woody, non-suckering - a tree or shrub that does not spread by producing new shoots from spreading roots. Mainly spreads by seed, though some non-suckering shrubs, such as Japanese barberry or multiflora rose, can spread through ‘layering’ by rooting at points where drooping stems contact the ground.

Examples: callery pear, shrub honeysuckle, autumn olive, European alder, Norway maple, Japanese barberry, multiflora rose, empress tree.

Woody, suckering - a tree or shrub that spreads through seed and vegetative means, usually through spreading roots that send up new shoots. These plants tend to form colonies or "clones".

Examples: tree-of-heaven, Japanese angelica tree, privet.

Woody vines - most woody vines spread through seed and vegetative means. Their distinctive growth form gets its own category for management strategies.

Examples: Asiatic bittersweet, porcelainberry, kudzu, Chinese/Japanese wisteria, chocolate vine.

Management Strategies

Management practices to suppress undesirable plants are broadly categorized as cultural, mechanical, biological, and chemical (herbicides).

Cultural techniques include preventing the introduction of weed propagules through soil, mulch, and aggregate; and on mowing, harvesting, and excavation equipment. Practices that enhance the growth of desired species, such as proper seeding and planting techniques, and prescribed fire increase the competition against invasive species.

Mechanical techniques include cutting and uprooting plants (through pulling or digging). Annual plants can often be effectively controlled with mechanical methods. Perennial plants are typically not controlled by cutting, but it can be used to weaken or shorten plants prior to follow-up herbicide application.

Classical Biological Control is the introduction of diseases or insects from the target weed’s native range that will impact only the target plant. These introduced organisms are evaluated for years in their native range and under quarantine in their introduced range to be sure they do not impact native species. Another variant of biological control is the use of browsing livestock, such as goats, to reduce woody plants.

Herbicide methods include preemergence applications to prevent seedling establishment, foliar applications to emerged plants, stump treatment of cut woody plants to prevent resprouting, and woody stem treatments such as hack-and-squirt and basal bark. Hack-and-squirt is the application of small volumes of concentrated herbicide to spaced cuts exposing the vascular tissue along the circumference of woody stems, and basal bark is spraying an oil-based mixture of concentrated herbicide to the circumference of the lower 12- to 18-inches of woody stems.

Generalized methods and timing will be summarized here for the different life cycle and growth form categories (Table 2), and visually represented in the timing calendar in Table 3.

Annuals – must be removed or treated with herbicide before flowering/seed set. Postemergence controls before flowering can be repeated annually to gradually reduce the soil seed bank over time, but can be labor-intensive and requires persistence. Soil-active herbicides can be applied once or twice per year (depending on herbicide and product rate) to provide residual control of the seed bank. If plants are in flower, but not yet setting seed, at time of removal, bag the flowers and burn or dispose of in the trash.

Summer annuals – preemergence herbicides should be applied in late-winter or very early spring, before germination begins in target plants. Postemergence herbicides or mechanical controls should be applied to actively-growing plants before flowering (May – August, depending on plant species).

Winter annuals – preemergence herbicides should be applied mid- to late-summer, before germination of target plants. Postemergence herbicides can be applied to green plants during fall and winter whenever air and soil surface temperature is above freezing. Hand-pulling, digging, or cutting can take place in fall, winter, or early spring before flowering.

Biennials – must be removed or treated with herbicide before flowering/seed set. Biennials are easiest to control in the rosette stage. Due to a long germination window and remaining green through winter, rosettes can be found anytime of the year. They can be treated with herbicide anytime the air and soil surface temperature is above freezing. Garlic mustard can be easily hand-pulled while most other biennials have strong taproots and would require a shovel for mechanical removal.

Biennials that are bolting can be pulled, cut, or treated with herbicide before flowering or just as they begin to bud. Biennials that have flowers can be pulled, bagged, and burned or disposed of in the trash. Soil-active herbicides can help reduce the seed bank.

Perennials – primarily require control of reproductive structures. Systemic herbicides (herbicides that move within the plant) are required for most effective control. Repeated cutting/mowing is only effective for some perennials and must be done very frequently and persistently. Similarly, hand removal or digging is only possible for certain perennials and is only practical for limited infestations.

Herbaceous, non-creeping – seed production must be prevented and roots must be killed. Apply systemic herbicide or dig from the ground before flowers develop. If flowers are present, remove and bag them, and burn or dispose of in trash.

Herbaceous, creeping – emphasis should be placed on controlling the reproductive structures that facilitate spreading. For most species, this is best accomplished by applying systemic herbicides in the mid- to late-growing season (July – September) when herbicide will most readily move down to the roots/rhizomes/stolons/tubers. One exception is lesser celandine, which is a spring ephemeral and can only be treated during the few months (March-May) in which above-ground growth is present.

Woody, non-suckering – control requires destruction of the regenerating tissue in the root crown. Systemic herbicide can be applied to the foliage from June until onset of fall color, or applied to the woody stems any time of year (though applications in November-December may have reduced effectiveness, and applications to certain species during spring sap flow (April) will not be effective).

Efficiency of herbicide application may be increased, especially in large infestations, by mowing or mastication followed by foliar herbicide application to regrowth late in the growing season.

Woody, suckering – complete control of the root system is critical. Mechanical control will not be effective. Systemic herbicide must be applied to foliage or to stems only during the mid- to late-growing season (July-September) when it will most readily move to the roots.

Woody Vines – can be cut any time of year to kill top portions and force ground-level resprouting. Foliar or stem treatments with systemic herbicide should be applied mid- to late-growing season to kill the root system. Do not attempt to pull vines down from trees or structures.

Remember that this information is generalized and specific control methods and timings can vary for individual plant species. For more detailed identification and control information on specific invasive plants, see the articles and videos on the Penn State Extension Invasive and Competing Plants section.