When you browse our bamboos and ornamental grasses you will see the hardiness zone for individual plants in the bio data. The information relates to the graph on the left and is the lowest mean temperature “the average annual minimum temperature”. Owing to the moderating effect of the Gulf Stream on the British temperate maritime climate, Britain, and Ireland to greater extent, have milder winters than their northerly position suggests. This means that the hardiness zones relevant to Britain and Ireland are quite high, from 7 to 10,

A hardiness zone is a subcategory of Vertical Zonation which is a geographically-defined area in which a specific category of plant is capable of growing, as defined by climatic conditions, including its ability to withstand the minimum temperatures of the zone . For example, a plant that is described as "hardy to zone 10" means that the plant can withstand a minimum temperature of -1°C. A more resilient plant that is "hardy to zone 9" can tolerate a minimum temperature of -7°C.

The problem is that the hardiness zones do not take into account the reliability of the snow cover. Snow acts as an insulator against extreme cold, protecting the root system of hibernating plants. If the snow cover is reliable, and generally present through winter, the actual temperature to which the roots are exposed will not be as low as the hardiness zone number would indicate Other factors that affect plant survival, though not considered in hardiness zones, are: soil moisture, humidity, the number of days of frost, and the risk of a rare catastrophic cold snap. Some risk evaluation – the probability of getting a particularly severe low temperature – often would be more useful than just the average conditions.

Lastly, many plants will survive in a locality but will not flower if the day length is insufficient or if they require vernalization (a particular duration of low temperature such a Stipa barbata for instance). With annuals, the time of planting can often be adjusted to allow growth beyond their normal geographical range