Many other growth and habitat characteristics of horseweed and hairy fleabane are similar. However, field observations in the Central Valley show that hairy fleabane produces flowers and seeds at least 2 weeks earlier than horseweed.Horseweed and hairy fleabane seem to prefer lesser disturbed soil environments. In the U.S. Midwest and other parts of the world where growers have converted or are converting to reduced-tillage systems, the prevalence of horseweed and hairy fleabane is increasing . It was found that even minimum tillage of the soil in spring or fall effectively controlled horseweed . This explains why these weeds are observed more in undisturbed situations, such as orchard and vineyard berms, field margins, roadsides, and irrigation or ditch banks in California. In Australia, these species are considered the most difficult weeds to control in no-till systems . The longevity of horseweed seeds does not seem to be very long. Seeds stored under dry conditions or in the laboratory had longevity of only 2 to 3 years . Similarly, the seeds of hairy fleabane survived up to three years under field conditions . However, a study reported viable seeds of horseweed were found in the seedbank of a 20-year old pasture despite its absence in the vegetation . The seeds of these species do not seem to have a dormancy requirement to germinate . Once the seeds are mature, they germinate as soon as conditions are favorable. This probably explains why recently dispersed seeds of these species emerge where postharvest irrigation water is applied in certain orchards and vineyards.Very little data exists on crop yield and quality losses caused by horseweed and hairy fleabane. In Michigan,vertical farming supplies it was found that horseweed reduced soybean yields by up to 83 percent . However, horseweed and hairy fleabane can compete directly with young trees and vines for soil nutrients, water, and light.

They can reduce tree and vine vigor, especially in newly planted fields. In walnut orchards in California, for example, heavy competition from these weeds can reduce tree height the first year by a foot or more. In cases such as this, growers must either replant or live with a reduced canopy height, which can make it difficult for harvesting equipment to enter the field without injuring low-hanging branches. Horseweed is also known to be a host plant for the glassy-winged sharp shooter . These weeds can form dense stands that interfere with the distribution of water, particularly in low-volume sprinkler and drip irrigation systems. These weeds also seem to be an aesthetic nuisance both in agroecosystems and natural or man-made landscapes. Horseweed can be a major problem in vineyards. If not adequately controlled, horseweed will grow through the vine canopy, interfering with harvest and other field operations . Horseweed may be particularly troublesome for raisin growers who use a continuous tray harvest system. In this system, green grapes are harvested mechanically and are spread on long sheets of paper to dry. Invariably, some berries are damaged by the harvester and release juice that can coat the berries and make them sticky. At the same time, horseweed seeds shatter on contact with the harvester, and the dislodged seeds may adhere to the surface of the sticky berries. As the berries dry into raisins, the seeds may be enveloped by the folds of raisin skins.Since horseweed and hairy fleabane seeds do not survive for more than a few years under field conditions, controlling plants before they produce new seed is critical for long-term management. Therefore, control tactics should be employed when these weeds are young and more sensitive to chemical and non-chemical controls. As these weeds begin to grow upright, their woody stem makes control very difficult and expensive.

In home gardens and areas with sandier soils, these weeds can be easily uprooted by hand.Preemergent and postemergent herbicides can provide effective control, depending on crop or location used, method of application, rate, application timing, uniformity in soil and weed wetting, and other important factors. Several herbicides used in California are effective on horseweed and hairy fleabane . As with all pesticides, follow all label recommendations closely to achieve desired efficacy and crop safety. Since a large complex of weeds is often present at any given time, applying combinations or sequential treatments of different preemergent and post emergent products is usually needed for the best overall control. For preemergent herbicides to be effective, they need to be applied uniformly to the soil surface before these weeds emerge in late fall through early spring. Control with preemergent herbicides can be reduced if they are applied to soils covered with leaves, weeds, and other debris. Removing debrisfrom the soil surface in an orchard or vineyard before applying preemergent herbicides helps maximize control of horseweed, hairy fleabane, and other weeds. If small weeds are present at the time of application, adding an appropriate post emergent product to the tank can kill those weeds as well. Soil moisture is important for the activation of these herbicides ; timing the application of a preemergent herbicide as close as possible to rainfall or incorporating the herbicide by irrigation is necessary for improved efficacy. Horseweed and hairy fleabane can also be effectively controlled with post emergent herbicides. However, it is extremely important to treat these weeds when they are very young and to apply the appropriate herbicide label rate. Tank-mixing more than one post emergent product often provides the best overall control, especially if other weeds are present. The most consistent control will be achieved when horseweed and hairy fleabane plants are treated prior to the 14-leaf stage. Once they reach the rosette stage, regrowth often occurs following post emergent treatment. Furthermore, bolted plants have woody stems and form dense canopies, making wetting of the entire plant foliage difficult and reducing the effectiveness of control.

In general, applying post emergent herbicides in a spray volume of 30 to 50 gallons per acre provides adequate wetting of the weed foliage necessary for good control. This is especially important where contact-type herbicides are used. Older and larger horseweed and hairy fleabane plants are more tolerant to systemic herbicides , and thus, treating small plants significantly improves control. Repeat applications may be required due to the extended germination period of horseweed and hairy fleabane.In cases where soil tillage is allowed, control can be achieved without the use of herbicides. Various forms of cultivation are available for both annual and perennial cropping systems. Specialized in-row cultivation equipment such as the hoe plow, in-row roto-tiller, spring hoe weeders, and berm rakes are available for certain orchard and vineyard systems that can provide effective control of these and other seedling weeds. As with herbicides, it is important to use mechanical tools when these weeds are small and most easily controlled. Mowing is not generally a viable option for control of horseweed and hairy fleabane. Mowing tends to stimulate additional branching from the crown and only delays seed production. Mowing also hardens off these plants, vertical weed grow making control with post emergent herbicides nearly impossible. Some success has been achieved with repeated applications of propane flaming, but only with plants at the seedling stage. With current and potential air quality issues in the Central Valley, flaming may not be a practical option in the future.Very little information is available on biological control of these two weed species. Since both weeds grow on lesser-disturbed and managed natural ecosystems, their seeds can easily blow into agroecosystems. Therefore, it is important to find methods that prevent the continuous influx of new seeds from the natural ecosystems. The bacterium Pseudomonas syringae pv. tagetis has been reported to affect these weeds , but this has not yet been developed as a large scale bio-control agent. Similarly, certain stem borers and leaf-eating caterpillars have been observed to damage these plants, but very little information is available on the success of these insects in controlling these weeds.Glyphosate-resistant horseweed was first discovered in Delaware in 2000 . Since then it has spread to 16 states in the United States and has also been reported from Brazil, China, Spain, and the Czech Republic . Rapid seed dispersal, the expansive use of glyphosate, and the lack of tillage are major factors contributing to the quick invasion of glyphosate-resistant horseweed in the eastern United States . In other parts of the United States, glyphosate-resistant horseweed has been reported where glyphosateresistant crops have been used in conjunction with no-till production systems . In California, glyphosate-resistant horseweed has been reported in orchards, vineyards, roadsides, and canal banks . Worldwide, including several regions of the United States, horseweed has developed resistance to several different herbicides, such as glyphosate, paraquat, and atrazine . Similarly, glyphosate-resistant hairy fleabane has recently been reported from Spain and South Africa . Confirmed cases of glyphosate-resistant horseweed and hairy fleabane were reported in California in 2005 and 2007, respectively. In California, it was observed that the foliage of horseweed or hairy fleabane would initially show symptoms of injury to glyphosate, but the plants would recover in a few weeks and survive .

Even within the same population, portions of the population were controlled while others would survive the glyphosate application . Such observations are characteristic signs of herbicide resistance. Relying on a single herbicide or combination of herbicides year after year increases the likelihood of selection for resistant populations. Rotating other effective products or using tank mixes of products where possible helps prevent or delay herbicide resistance. For detailed information on herbicide resistance management, refer to the Prather et al. 2000.In order to use straw bales as a growing medium, they must first be prepared for plant use. This preconditioning process is a form of composting. During preconditioning the bales will become hot, and the pH will change as decomposition occurs. It is important to complete the initial compost process prior to planting into the bale or plants may be injured. Microorganisms will attempt to break down the straw bales as soon as the bales are moistened. During this process nutrients—primarily sources of nitrogen—will be tied up by the microbes as they decompose the straw. As a result, plants growing in root-zone environments high in carbon, such as straw bales, may not have essential nutrients available for growth, even if fertilizer is applied, until the straw has sufficiently decomposed.For optimal composting, a ratio of carbon to nitrogen of 30:1 should be present; however, on their own, straw bales have ratios ranging from 40:1 to 100:1. As a gardener adds fertilizer containing nitrogen, the ratio approaches the ideal. Active composting will then begin. This composting process generates carbon dioxide and a substantial amount of heat. A fresh, 3-string straw bale can exceed 140°F during conditioning. Older, seasoned bales may not exceed 100°F , but should still be preconditioned before use. Once the preconditioning is complete, nutrients will no longer be sequestered in large amounts and the bale will remain cool enough to plant.Select clean, weed-free straw bales to minimize future weeding. Straw bales are usually available at farm supply dealers or directly from local farms. 2-String or 3-string straw bales may be used. Place bales with the baling twine parallel to the ground. The twine will help to hold the bale together as the season progresses and the bale decomposes . The preconditioning process begins when the bale is moistened. High nitrogen levels are maintained initially by adding fertilizer. Midway through the conditioning process, nitrogen fertilizer inputs are tapered off as the bales continue to compost. Tables 1 and 2 provide two common preconditioning schedules. The process should take approximately 2 weeks to complete for either schedule. Throughout the conditioning process, bales should be kept wet. A finger inserted in the side of the bale should feel hot and damp. It is best to add water slowly to each bale to minimize runoff and to ensure that the fertilizer applied to the surface is moved into the bale. In Europe straw bales were originally preconditioned using solid, granular fertilizers; however, this led to salinity problems in some crops as roots came into direct contact with fertilizer granules. Wilson found that using water-soluble fertilizer sources eliminated this problem.