Cleistocactus strausii
The cactus prefers free draining soils, strong sunlight, but not high temperatures — in fact it can withstand hard frosts down to -10 °C. In its natural habitat it receives plenty of water during the summer, but almost none over the winter. In cultivation, watering too much in winter often leads to root rot.
Aquatic weed harvester
Water is an important resource and in many countries, waterways are increasingly clogged by aquatic plant growth. This is particularly so in tropical countries where warmer water means the plants grow more quickly, and increasing run-off of fertilisers and effluent has exacerbated the problem. Irrigation ditches and pumps can become overgrown with vegetation, power station and factory water intakes can get blocked, boats can get hindered, fish stocks can be disrupted, and water moves more slowly, resulting in greater evapotranspiration and a greater risk of flooding. In some large irrigation projects in India, canals have become so overgrown with vegetation that water flow has been reduced to a fifth of its previous amount. In Bangladesh, floodwater has washed mats of water hyacinth onto paddy fields, overwhelming the emerging rice crops. Small fish can become entangled in excessive algal growth.
Rice is the main aquatic plant grown for human food, but smaller areas of watercress and water chestnut are also cultivated. In their native environments, aquatic weeds are part of a balanced ecosystem, and it is mainly introduced species of water plant that become invasive and cause problems by congesting water bodies. The worst culprits, found in both temperate and tropical waterways, are floating plants such as water hyacinth, water lettuce and "Salvinia", fully submerged rooting plants such as "Hydrilla" and water milfoil and rooting plants that reach the surface such as cattail, papyrus, bulrush and reed.