Making Sense of Plant Labels (PartTwo)
September 29, 2023

By Kate and Len Lucas

The best times for planting garden perennials outdoors are March to April and September to October. So, if you are wandering through a garden centre looking for plants that will go through the winter and look great next year then you may be faced with some words and phrases on the labels which at first sight look harmless enough. In this second article, we want to have a close look at some more of those words and phrases on plant labels. What we don’t want to do is write a gardening dictionary and it is worth bearing in mind that very often the garden centre you are in probably didn’t write the label you are trying to read.

One of the most common words found on plant labels is “Hardy” often in combination with other words like “perennial” or “herbaceous,” or even both as in “Hardy Herbaceous Perennial.” For the sake of completeness that means you can grow it outdoors, will die down in the winter and it will grow again next year. That’s a bit wordy hence the shorter version.

“Hardy” should mean that you can grow it safely outside all the year round down to temperatures well below zero degrees centigrade.

Warmer summers and less cold winters are all very well but that doesn’t mean we can all grow a date palm where we like. Here are a couple of phrases that we think will become more common.

“Frost Hardy”. It’s an odd phrase but believe it or not it is helpful because it tells you that the plant can be left out all year but might struggle if the temperature falls a few degrees below zero. We think it probably means no more than minus 5 degrees centigrade. We have such a plant in our garden and last winter we protected it with bubble wrap for its first year at least.

The closer air at zero degrees centigrade is to the ground the more likely ice will form near the ground or in lower lying parts of the garden. What kills the plant are the ice crystals which occupy more space than the water that gave rise to it meaning the ice bursts the cell walls of the plant. Exactly what happens when you get a burst pipe at home.

So if your new plant is labelled as frost hardy try and place it near the house or sheltered by a fence or other plants rather than exposed all by itself and we would always mulch the crown of the plant before winter sets in.

The Royal Horticultural Society has come to our rescue for plants they consider to be the best in our gardens. They award some plants the “Award of Garden Merit” and give them an” H “number. There is a range of these numbers from H7, the most hardy, to H1 meaning tender and only for a greenhouse.  H7 means that the plant will survive temperatures of minus 20 degrees centigrade. If your plant label has H4 the RHS says ok down to minus 5 centigrade. The RHS don’t use the phrase frost hardy but we suggest it lies somewhere in between H4 and H3 and if your plant is H3 you are in that border line area which brings us nicely to “Requires a Sheltered Spot”.

But sheltered from what exactly? Or does it mean that this plant really ought to be in a conservatory all the time or just brought in for the winter? Like frost hardy when you come across this one you can’t really stick it anywhere you like and it usually means protect it from cold winds and it would be wise to check how low a temperature can it withstand. Japanese maples are a good example.

There is nothing wrong with buying a plant that is frost hardy and needs a sheltered spot. How would you feel in freezing weather with a stiff breeze? Just give it the best you can.

We will finish with a number of label references to daylight. Here goes, “prefers dappled shade”, “best in light shade”, “avoid strong sunlight”. All of these phrases mean that this plant does not like full sun so do yourself a favour and avoid it.

People who run garden centres do try to put the plants on display which give you some guide as to how much daylight is best for them. So if you buy a Hosta in the section marked “Shade Plants” please take that as a sound piece of advice.

If you are unsure about the best conditions for your new purchase, or the label is not clear, then there will be somebody in that garden centre who knows the answer.