Ground Cover Management

Weed control is very important to producing a high quality Christmas tree. Competition from grasses and other vegetation can kill young trees, or limit their growth by blocking sunlight and competing for soil moisture and nutrients. Tree quality can be reduced by vegetation that restricts lower branch growth or from vines growing into trees and bending tops. Some weeds, such as briars, thistles, or poison ivy, can also restrict worker's ability to care for trees.
    However, totally eliminating all weeds and leaving the soil bare is just as bad. Bare soil is prone to erosion and deterioration of soil structure and organic matter. The warmer soil exceeds the temperatures at which Fraser fir roots can function, causing the tree to draw nutrients and water away from the soil surface where fertilizers are applied. Bare ground also increases weeds like pokeweed, ragweed, field bindweed and others that require bare soil to germinate. Properly managed ground covers can also promote water penetration rather than runoff, and provide a firm surface for walking or driving.

Properly managed ground covers

    Eliminating all weeds may also eliminate native predators. Predators feed on insect and mite pests, but most of them can also feed on, or may even require, pollen and nectar to complete their life cycle. By having some flowering weeds and grasses in the ground cover mix or field borders, predators will be more likely to stay in the trees to feed on pests. The best pollen nectar sources are large open flowers like wild carrot, yarrow, and buckwheat. Green lacewings require grass pollen to reproduce.
    The goal of ground cover management is to promote the growth of native plants as a ground cover to protect the soil, promote good tree root growth, and provide a habitat for natural insect and mite predators without hurting tree growth and quality. This is done by mechanical mowing, or using herbicides to either kill undesirable weeds, or stunt weeds that are tolerable so they don't out-compete the tree.
    How competitive a weed is with a Christmas tree is often determined more by its rooting than its aboveground growth habits. In general, plants with thick, fibrous root systems, like cool-season grasses, will exclude and outgrow Fraser fir roots from growing and are, therefore, highly competitive. A weed like wild carrot that only has a deep taproot is much less of a problem. Other plants, such as wild violet, produce prolific rhizomes that can once again exclude fir roots.
    Understanding weed groups. Weeds that infest Christmas tree plantations have one of four life cycles. Become familiar with the life cycle to know at what time of year the most susceptible growth stage is occurring.
•  Summer annuals emerge in the spring, generally flower in the summer, and set seed before the first frost in the fall.
•  Winter annuals germinate at the end of summer and overwinter as small, dormant seedlings.
•  Biennials are similar to winter annuals, but they genninate earlier in the summer. As days lengthen and temperatures rise in the spring, both winter annuals and biennials are stimulated to flower, set seed, and die before the end of the summer.
• Perennials, as the name suggests, survive more than two growing seasons, and generally reproduce by seed or vegetative means.

    Weeds also are commonly classified into broad groups based on their growth types: grasses, sedges, and broadleaves. This is useful in determining herbicide selectivity. Broadleaves can be further classified into different groups including composites (stickweeds), mints, briars, vines, hardwoods, legumes, and others. Sometimes, identifying a weed to one of these broad groups is sufficient. Other times, it is important to positively identify the plant to the actual species.
    Weeds are classified based on many characteristics of the mature plant including leaf shape and size, height, and especially flower type and structure. However, a scout often needs to be able to identify small, immature weeds. There are several good weed identification books that help with the identification of both immature and mature weeds.
    Goals of weed scouting. Scouting is crucial to ground cover management. Only through scouting will a grower know what weeds are in a field, which decides the herbicide selection. In addition, soil moisture and weed height affects how well herbicides work. Through the spring and summer, the scout can help choose the proper choice and timing of herbicide applications.
    The first step to scouting is to understand which weeds are desirable and should be protected from being inadvertently killed by a herbicide, which ones are tolerable but need to be suppressed, and which weeds are undesirable and should be eliminated.
    Desirable weeds. Good ground covers seldom grow tall enough to interfere with tree growth. Some of these weeds are tolerant of low rates of post-emergent herbicides, however, certain herbicides, particularly Goal, may need to be avoided to protect these weeds.
    Manageable weeds. Most weeds in this group grow too tall, have too thick a root system, or are too woody or too dense to be allowed to grow unchecked in a Fraser fir Christmas tree field. However, these weeds can be managed through chemical mowing, to provide a good ground cover.

Horseweed, a manageable weed

    Ground cover suppression or "chemical mowing" is a term used to describe the practice of applying postemergent herbicides at low rates to stunt or suppress weeds and grasses. Some post-emergent herbicide mixes can be applied over new growth of Fraser fir without damage. This more affordable mowing practice was developed to provide growers with a low-cost, soil-conserving alternative to broadcast application of pre-emergence herbicides or mechanical mowing. Chemical mowing can be used as a broadcast application or as a between-row treatment where trees were previously banded in the row with pre-emergence herbicides. A single treatment usually gives 6 to 8 weeks of control.
    Cool season, perennial grasses are the most competitive native ground covers. Perennial grasses should be eliminated from the tree row since they can grow up through the bottoms of young trees, and limit the space where Fraser fir roots will grow. Suppress tall growing perennial grasses found between the rows to minimize competition. However, some of the native perennial grasses, such as timothy and nimblewill, are lower growing and can be desirable between the rows. These grow in the summer and fall.
    Annual grasses include foxtails, panicum, and bromes. These are usually taller, with wider leaves than perennial grasses, and are usually easily suppressed with either Round-up or Vantage.
    Composites, which include ragweed, asters, thistles, horseweed, and sow thistles, are less competitive than perennial grasses, but they can grow tall enough to shade trees and woody enough to interfere with shearing. These weeds, which have flower heads made up of many tiny flowers, are often called stickweeds. Many members of this group produce a low-growing cluster (rosette) of leaves in the spring, but grow rapidly in warmer weather. As a rule, this group of manageable weeds should be eliminated in the tree rows if trees are less than 4 feet tall, and suppressed between the rows.
    Undesirable weeds. Highly competitive weeds, which reduce Fraser fir growth or make it difficult to shear or harvest trees, include pokeweed, briars, tree saplings, and vines. These weeds will need to be eliminated if possible and can be most easily controlled in the fall.
    Briars are thorny shrubs or cane-producing plants. The most common briar is blackberry. It does not compete with the tree, but it will interfere with workers. Briars should be eliminated. Tree saplings are typically a problem in fields recently cleared of timber or brush.
    Vines are difficult to control because most are woody perennials with deep root systems. It is critical that vines be identified before planting trees or when they are still small enough to control. Timing is critical in controlling vines; consult your county extension agent for timing recommendations. Once the vines are growing in the trees, the options for control become limited. While vines are not very competitive for water and nutrients, they will grow up into trees, distorting growth, and making shearing difficult.

Vines, a undesirable weed

Table 1. List of common weeds in Fraser fir plantations in western North Carolina.

Desirable Weeds - allow and encourage, may need some control at times.	Manageable Weeds - can be tolerated but control in-row and suppress between rows.	Undesirable Weeds - may be difficult to control. eradicate if possible.
Chickweed, cinquefoil, dandelion, creeping buttercup, clover, groundsels, wild mints, nimblewill, strawberry, red sorrel, plantains, violet, and other low stature, low competitive species.	Asters, ragweed, horseweed, yarrow,mustards, pigweed, lambsquarters, wild carrot, horsenettle, cool seasongrasses (fescue, bluegrass,orchardgrass, etc.), summer annual grasses.	Briars and brambles, bindweeds, burdock, bedstraw, poison ivy, trumpet creeper, evening primrose, wild buckwheat, Virginia creeper, dock, pokeweed, dayflower, goldenrod, tree saplings, thistles, ironweed,, nutsedge, clemantis.

 

    Pre-plant weed scouting and control. Many problem weeds are easier to control before trees are set. Assessing problem weeds the year before planting a new field allows the most opportunity for controlling weeds during the proper time of year.
    Taking a weed inventory. A weed inventory is a list of the predominant or problem weeds in a field. The best time to take an inventory is late summer or early fall when most of the weeds are flowering and easiest to identify. However, a scout experienced with identifying immature weeds can make a weed inventory any time throughout the growing season.
    Taking the inventory is simple. Walk each block in a wide zigzag pattern or a random walk pattern. Note the general weed population and record relative densities. Take particular note of heavy infestations of a single species, perennial weeds, species you do not know (could this be a serious weed in the future?), and weeds that may be new to the area. Note which weeds appear to escape the treatment strategy that you have been using and may be becoming more of a problem. Some weeds may only be common in one portion of the field. This should also be noted. It is also a good idea to note whether a particular weed is more common in the row or between rows.
    Whenever trees are scouted for other pests, the weed inventory can be updated. Building a complete inventory is usually a season-long process as different weeds become apparent at different times of the year.
    Planning pre-emergent herbicide applications. From the weed inventory, next year's pre-emergent herbicide applications can be planned. Banded pre-emergent herbicide applications are recommended in tree rows for the first 3 to 4 years trees are in the field. The bands should not be over 2 1/2 feet wide. Pre-emergent herbicides are usually applied in February through April, depending on the choice of herbicide. Fall applications of some pre-emergent herbicides are showing promise. Contact your local county extension agent for current recommendations.
    Scouting and determination of chemical mowing timing. Suppressing, or chemical mowing, the ground cover left in a block after post-emergent herbicide treatments may be necessary from April through August depending on the make-up of the ground cover and the weather. Scouting the ground cover to determine average weed height will be important throughout this time to determine the best timing of applications. This ground cover scouting can be done while scouting for insect and mite pests.
    Perennial grasses and some perennial overwintering broadleaves can begin growing and reach the chemical mowing threshold of 6 to 18 inches in height any time in April with normal weather. If this happens in a field, there will be time to make a first chemical mowing before Fraser fir breaks bud. This gives more herbicide choices since there will not be as many concerns about burning tree foliage with the herbicide.
    However, if the weather has been cool, the ground cover may not reach the chemical mowing threshold until mid-May or later. Delaying the first chemical mowing application until after Fraser fir bud break can be beneficial. A large group of summer annuals usually germinate and emerge in late May. These include ragweed, lambsquarters, smartweed, asters, and several other stickweeds. At approximately the same time a group of late emerging perennials come up, including pokeweed, bindweed, dock and others. If the first chemical mowing treatment can be delayed until June, either because of cool weather or because these spring growing perennials are not an important part of the weed mix, then a single treatment can control all weeds.
    If the first chemical mowing treatment is made before bud break, it is important to schedule an early June scouting trip to monitor summer annuals and perennials. Reviewing the weed inventory can help anticipate problems with these weeds. Subsequent scouting trips to look for insect or mite pests can help the scout time further chemical mowing applications, which should be made whenever weed growth reaches the threshold (6 to 18 inches height) and should continue so that control is effective through August.
    Note: Chemical mowing should always be made as whole field applications. Attempts at "spot" applications have often proved unsuccessful. To avoid damaging desirable weeds in the ground cover, choose the proper herbicide.
    Fall herbicide window. Continue scouting in late August to early September, paying attention to the types of undesirable weeds present. Between September 1 and October 15, there is a window where Fraser fir tolerates certain post-emergent herbicides, such as Round-up and Garlon, and most undesirable weed species can be controlled.
    This fall herbicide application should be a spot treatment to avoid damaging desirable or manageable weeds, and the herbicide should be used at killing rates. Make this application only where undesirable weeds occur, or in a band over tree rows to assist in upcoming banded pre-emergent treatments next year. Be careful not to kill all the ground cover in the fall or else you might expose the soil surface to erosion through the late winter and early spring.