Six general methods exist for controlling weeds. Although all have at least limited application in Christmas tree plantation management, some have more utility than others for most commercial growers.
Fire - Of the six methods, use of fire to control weeds in established Christmas tree plantations is of least value. However, it can be useful to eliminate dense weed growth and low brush cover prior to planting. Use of fire depends on the type of cover and characteristics of the planting site, especially the soil. Although fire can be a useful tool, it can also be extremely dangerous. It requires special techniques to achieve the desired results, and in most instances it should be done only with consultation of a professional forester.
Biological Control - Use of livestock is the only type of biological weed control currently of potential use in Christmas tree plantations. The decision to use livestock depends on a number of factors, including species of trees being raised, type of livestock available and skills of the grower.
Mulch - Mulches can effectively control weeds around the bases of trees and reduce evaporation of moisture from the soil. However, obtaining and spreading mulches on a large scale can be difficult and expensive. They are probably best used as an alternative method of weed control for small growers, hobby growers and growers who do not want to use herbicides. Many materials can be used for mulch, but coarse textured materials such as straw should not be used because they may attract mice or other pests. Sawdust is one of the most commonly used mulches. It should be applied only to the soil surface. If mixed into the soil, it can cause nitrogen deficiencies unless about two percent (by weight) nitrogen fertilizer is added to the sawdust.
Cultivation - Although it is not extensively used, hand or mechanical cultivation (rototilling, disking, harrowing) can effectively control weeds around the bases of Christmas trees (Fig. 34). Several important factors should be considered when considering cultivation as a weed control technique. First, it takes two to four or more cultivations per year to effectively control weeds. The exact number depends on weather, soil fertility and type of weeds. On sites with severe weed problems cost of effective weed control by cultivation may be quite high. Second, cultivation should be relatively shallow to prevent root damage (probably not more than two or three inches). Third, cultivation should not be done on steep topography where it might substantially increase erosion.
Fig. 34: Rototilling to control weeds in Christmas tree plantings. Tillage should be shallow to prevent damage to roots of trees.
Mowing - Mowing is the most commonly used method for controlling weeds in Christmas tree plantations, except around the bases of young trees where almost complete control is generally desired for at least the first few years after planting. Mowing does not eliminate weeds, but it does reduce their size, and it may also change species composition.
Although it is desirable to keep weeds closely mowed to minimize their impact on tree growth, mowing too frequently is unduly expensive. The number of mowings needed in a plantation in any year depends on weather, soil fertility and the type of weeds. Three are usually sufficient-one in mid to late May, about a month later and about the third week in August.
The kind of mowing equipment used depends on the type and size of the Christmas tree operation and the capital resources of the grower. While walk-behind mowers may be perfect for some growers (Fig. 22), others may need a small riding tractor with an attached mower or a large orchard or agricultural tractor with an attached mower (Fig. 23). Relative times required for mowing using three types of tractors are listed in Table 6. In choosing equipment, factors other than initial equipment cost must be taken into consideration, including cost of labor to do the mowing, expected short-term and long-term future changes in labor costs and expected maintenance costs. Considerations should also include reliability of equipment and the availability of service.
Chemical Weed Control - In most Christmas tree plantings, use of chemical herbicides provides the most effective and economical way of reducing or eliminating weeds around the base of Christmas trees during the critical early years of establishment and growth (Figs. 29, 33).
|Table 6: Examples of Labor and Equipment Requirements for Various Cultural Practices1|
|Practice||Time Requirements, Hours per 1,000 Trees|
|Mowing:||30 HP Tractor||1.25||1.25|
|12 HP Tractor||1.84||1.84|
|6 HP Tractor||2.33||2.33|
|12 HP Tractor and 50 Gal. Sprayer||0.81||0.81|
|30 HP Tractor and 100 Gal. Sprayer||0.74||0.74|
|Insect Control:||Hand Sprayer||0||3.11|
|12 HP Tractor and 50 Gal Sprayer||2.02||2.02|
|12 HP Tractor and Spreader||0||0.58|
|1. Information taken from Leuschner.W. A. and W. A. Sellers. 1975. The economics of producing and marketing Christmas trees. Virginia Polytechnic Institute and State University. Division of Forestry, FWS-1-75. 47 pp.|
|Rates listed are for comparison only and can be expected to vary considerably from one Christmas tree operation to another.|