Plant Hardiness Zones

For many years we've found the United States Department of Agriculture's Plant Hardiness Zones useful in assessing a plant's suitability for our climate. Liddle Wonder is now in the process of adopting this system through our labels and public information.

In its simplest terms the USDA system has provided a classification of average minimum temperatures and horticulturalists from around the world have assigned plants to these zones. The classification relates to a plant's ability to handle cold temperatures. Thus, a plant that is able to tolerate a light frost is assigned to zone 9, where average minimum temperatures range from -1°C to -5°C.

Plants adopt a range of mechanisms to help them handle cold temperatures. For example many perennials can handle very cold winter conditions by becoming dormant and letting their top growth die off and "retreating" to the warmer soil conditions. They wait for spring warm and shoot away again. Others just lose their leaves, the woody parts of the shrub or tree being better able to withstand cold conditions than the leaves.

We use the phrase "simplest terms" above cautiously - there are many factors that will influence a plant's ability to handle cold conditions - age, shelter, aspect, terrain, soil types, waterlogging among others. Thus, Hardiness Zones provide a guide only, which must be applied with local experience and knowledge

Liddle Wonder contracted NIWA to analyze weather station data to develop New Zealand Zones. We have modified zone boundaries to ensure that there is a difference between the North Island's central plateau and Wanganui for example. In the USA zone 9 goes down to -7°C. If we applied this in NZ, Turangi would be Zone 9, the same as Wanganui and Tauranga!

Many international horticultural reference books contain Zone assignments. We've used these and some "local" knowledge to assign a Zone to plants in our range.

As with all plant and growing circumstances, we cannot over-emphasize the importance of local conditions. Many areas have sites with very specific microclimates, north or south facing aspects, freely drained or cloggy soils. These and many other factors - cold together with wet soil for example - affect a plant's ability to handle harsher or more generous conditions. Above all we encourage gardeners to experiment. This way they'll soon learn the practical boundaries that their garden and site offer, and quickly establish a feel for whether they can garden up a zone, perhaps "half" a zone, and enjoy an even more diverse range of garden plants.