This review for the Special Issue “Challenges for sustainable plant cultivation practices and produce supply” comes at the time of the EU single-use plastics ban, affecting items such as drinking straws and plastic shopping bags; presently, there is also the dispute about small plastic fruit packs under 1.5 kg, e.g., in France and elsewhere, with a focus on SDG 12 (“responsible consumption and production”). Plastic plays a dominant role in agriculture and horticulture, and it answers the following question [1,2]^[1][2]: “Can horticulture exist and provide year round supply of local fruit by a horticultural process called forcing (from strawberry, cherry to white asparagus), ensure fruit quality, save water and herbicides and protect the crops from insects and climate change (hail)?”.

2. Alternatives (Plastics) to Black Mulch-PE versus PLA

A wide range of papers (e.g., [7,14,15,16,17,18,19,20]^{[3][4][5][6][7][8][9][10]}) have dealt with alternative biodegradable plastics on bare soil for short cultivation cycles, but they are unsuitable for long-term covers on a rough soil surface. Their main practical and ecological advantages are that they can be left in the field (Figure 1) and/or buried in the soil to be degraded by microorganisms. Fungi, bacteria, and algae can transform these residues into carbon dioxide, methane, water, and biomass [7,16,18]^[3][6][8]. The biodegradable mulches currently in use are mainly based on starch and cellulose, polyhydroxybutyrate/valerate copolymers (PHB), and polylactic acid polymers (PLA) (Cozzolino et al.) [19]^[9], which are molecules that are susceptible to UV- and visible light-facilitated photo-oxidation or that are thermo-oxidized at high temperatures [5,6,7]^[3][11][12]. While PLA and PBB are “compostable under industrial conditions and completely degrade in the soil” (EN 13432) [21]^[13], PHB so far lacks physical strength for field use, which is an objective for the current research project ENSURE. PLA polylactic/copolyester blends, which is currently the most likely alternative, originate from a corn starch fermentation process. They are chemically synthesized and are hence not of fossil origin. But is PLA more sustainable than LD-PE, e.g., when used as black mulch in strawberry cultivation in Spain or elsewhere? A limitation of biodegradable films is that, similarly to PE films, they are subjected to weathering and the chemical substances that are used on crops, as well as to soil microorganisms; hence, there is a risk of incomplete soil coverage for the entire crop cycle. Dark or transparent PE mulch used in agriculture fulfill certain criteria [19]^[9], such as the tensile strength at break or the tensile elongation at break of at least 16 MPa and 180–250%, respectively. Biodegradable films have both tensile stress and tensile elongation at a break that is lower than those of PE (Scarascia-Mugnozza et al., 2011) [7]^[3]. However, their mechanical properties fall within the range required to ensure an adequate soil coverage during the entire strawberry growing cycle. Moreover, on the basis of ecotoxicological tests, the authors demonstrated the absence of soil ecotoxicity at the end of the crop cycle after burying the material. To balance the biodegradation and physical–mechanical properties of a mulch plastic, a biodegradable mulch needs to endure until the end of the crop cycle with adequate weed control, similarly to PE mulch. In Portugal, starch-based biodegradable mulch provided such adequate ground cover and weed suppression during a strawberry autumn–winter cycle (Andrade et al. 2014) [22]^[14]. Giordano et al. (2020) [23]^[15] favoured three (out of two PEs) (Ecoflex, FKur GmbH, Willich, Germany) and eight PLA (20–40 µm/m²) biodegradable mulches in a strawberry field under the high light/UV/temperature med climate in Huelva, Spain.

2.1. PLA–Biodegradable Plastic (Mulch)—A More Sustainable Option?: “Plough the Plastic”

In practice i.e., the field, a thinner version of PLA (typically 10 µm PLA/m²) compared with PE (30–50 µm) is often used to enable faster biodegradation on and in the soil. This thinner PLA is firstly exposed to light/UV on the ground and then after ploughing to enable intimate contact with soil microbes. Two contrasting independent sustainability studies, both under a comparable Western European climate, exist in the literature. The UMSICHT study (Bertling et al., 2019) [24]^[16] is based on modeling, and many assumptions and conclusions severely contrast with the “Imulch” (“intelligent mulch” study 2021) [25]^[17]. Thise study is based on laboratory and sub-commissioned studies and developed new methodologies for applications such as microplastic detection (Table 1). The 3-year “Imulch” project (2021) [25]^[17] showed, contrary to general expectations, a longer longevity of PLA/PBAT films in the field than previously found in the laboratory, where the simulated long-term exposure was speeded up in a short time. While the Bertling study (2019) [24]^[16] used 50 µm/m² PE, the thin 10 µm PLA/PBAT in the “Imulch” study started to biodegrade in Western European fields after 6 weeks but without complete degradation after 3 months the 30 µm PE showed no signs of change. Table 1 shows that (a) microplastic residues were below detection limits and (b) were independent of the land use, contrary to the expectations and common belief. This is in line with the 2–3% residual LD-PE after field use as assumed (not measured) by Bertling et al. (2019) [24]^[16] and which is rated by experts as an overestimate.

2.2. LCA (Life Cycle Assessment) and GHG of Alternative (Biodegradable) Plastics

There are two very different approaches in the literature. In the first one, GHG emissions of ca. 1600 kg CO_2e/ha of mulch were similar for both LD-PE (30 µm) and PLA/PBAT (Imulch, 2021) [25]^[17] with the energy-expensive precursor adipic acid. The second one (Chen et al., 2023) [26]^[18]) also ends up at ca. 1600 kgCO_2e/ha for LD-PE, but 2380 kg CO_2e/ha for biodegradable PLA/PBAT mulch. An analysis of the employed LCA shows that it includes nitrite oxide and CO₂ emissions during the cultivation phase, but it uses the thinner 10 µm LD-PE mulch. Whereas the first LCA benefits from the thermal digestion of the thicker PE, which is absent from the second study, the latter ends up in a contrary result.

2.3. Hormonal and Endocrinal Activity—Beyond Sustainability

The “Imulch” study (2021) [25]^[17] investigated hormone action that was associated with the adsorption of pesticides. The bioassays showed no endocrinal (hormonal) activity of either type of black plastic mulch, no uptake into the plant, and no adsorption and resorption of copper (used as a fungicide in organic growing) or herbicides used in conventional growing.