For best weed control effects by ground cover plants, it is advantageous for them to rapidly establish a dense foliage cover over the desired area. In most amenity horticulture scenarios, a most critical step for the establishment of ground cover plants is to completely clear the planting area of all weeds and vegetation, including roots in the soil (MacKenzie 1997). This can be facilitated by the use of a non-selective translocated herbicide such as glyphosate, which is deactivated by soil contact and harmless to planned transplants or seedlings, followed by residual pre-emergent herbicides to extend the weed-free period, if information on compatible herbicides is available (Elmore 2000); and a combination of mulch and herbicide use during establishment may prove to be the most effective method (Elmore et al. 1997). At the same time, herbicides may need to be applied when the ground cover plants are actively growing, so selectivity of action becomes paramount. Currently there is a relative dearth of information on herbicide compatibility with ground cover plants, which this thesis will be addressing.
Selective herbicides kill weeds without harming the intended planting. Such selectivity may be accomplished through deactivation where the protected plant is able to rapidly metabolise the active ingredient (commonly through dealkylation, deamination, or conjugation of the ingredient molecule with another metabolite such as glutathion) before it accumulates to concentrations high enough to be damaging; whereas the weed lacks this ability (Copping & Hewitt 1998). Selectivity may also be achieved through physical properties. For example, diuron does not harm trees because it does not move deeply enough into the soil profile to reach tree roots.
Herbicides can also be classified based on when they should be applied to weeds. Pre- emergence herbicides refer to those which are applied to the soil before the weeds germinate. To use pre-emergence herbicides effectively, one should have knowledge of which weeds are likely to be a problem at the site and apply suitable pre-emergence herbicides to target that weed (Silvy 2012). Examples include oxyfluorfen and oxadiazon. They should be applied when the soil is moist in spring, but to avoid contact damage with trees and deep-rooting shrubs, it may be necessary to apply in early spring before new foliage appears. Oxyfluorfen and oxadiazon can selectively control weeds around trees and large shrubs due to their low solubility in soil, so very little herbicide moves into the rooting zone of these species.
In contrast, post-emergence herbicides are applied to the weeds after they have germinated. Some of the earliest such herbicides to be developed were 2,4-D and MCPA in the 1940s, and they are still widely used today. These are synthetic hormone herbicides which can selectively remove dicotyledonous weeds from cereal crops (LeBaron et al. 2008). Post-emergence herbicides are further differentiated to those which only damage those parts of the foliage that they contact, and those which translocate throughout the plant within the phloem system after absorption. Contact post-emergence herbicides, such as bentazone and ioxynil, typically do not damage grass species or broad-leaved species with waxy leaves, since the herbicides do not stay on such foliage for long. Translocated herbicides can be absorbed by the plant and be mobilised to kill other parts of the weed such as the root system not in contact with the spray (Monaco et al. 2002). Clopyralid and MCPA are examples of translocated herbicides.
While herbicides are generally safe and cost effective (Prichard et al. 1989; Gianessi & Reigner 2007; Wibawa et al. 2010; Jat et al. 2011) when used within appropriate guidelines, there are detractors who prefer consumption of organically grown products. Organic growers may use natural plant extracts such as clove oil and vinegar (acetic acid) for weed control, but will have to rely on hand weeding or other mechanical means to supplement these methods (Evans & Bellinder 2009). A comparison between herbicides and hand weeding concluded that hand weeding produced
superior yield in rice production (Parmeet et al. 2008) and brinjal (aubergine) production (Meena et al. 2006) but unremarkable results in groundnut fields (Chaudhari et al. 2007). Yet another study showed that a combination of hand weeding and herbicide treatment produced greatest seed yield and protein content in soybeans (Geet et al. 2007). Unfortunately, the high time consumption and labour intensiveness of hand weeding operations mean it is not a feasible option on a large scale (Hussain et al. 2008).
Site conditions need to be assessed so that the right choice in species selection can be made (Hirshfield 2006). This planning includes ensuring that other landscape plants do not share the same rooting depth and foliage height to avoid competition. Planting of ground cover plants require consideration of their mature size so there will be the right allowance in spacing for healthy plants and optimal weed control. Once a decision is made, the plants may be positioned in rows with plant centres staggered at the mid-points of neighbouring rows (MacKenzie 1997).
Ground cover establishment may be assisted by adding fertiliser after planting to encourage leafy growth. To ensure ground cover plants can easily gain the benefits of fertiliser, planting should preferably coincide with the rainy season of the local climate. If dry fertilisers are applied, it would be useful to follow with watering to wash off any fertiliser stuck to foliage to avoid burns, and to promote availability of fertiliser nutrients. The addition of fertiliser is however subject to diminishing returns (Spillman 1923; Chavas et al. 2010) and the bioavailability of the nutrients subject to formulation or origin of added fertiliser (Toole 2004). There may also be concerns of ecological disruption due to leaching (McDowell & Koopmans 2006).
After planting and fertilising ground cover plants, mulch can be added over the exposed soil surfaces to discourage germination of weed seedlings (Thomas 1977; Brown & Tworkoski 2004;
Jodaugienė et al. 2006). Should opportunistic weeds still occur during the establishment phase of the ground cover plant, post-emergent herbicides may be judiciously applied. If weed numbers are not high, hand weeding may be a feasible option.