It’s not just the heat, it’s the humidity – for plants too | (2024)

So many gardeners who live in the subtropics tell me they must have brown thumbs because they keep killing lavender. My plea to ‘subtropicallians’ is this – your ability to grow lavender has nothing to do with the colour of your thumbs and everything to do with your climate!

Modern western gardening has its roots in a cool temperate European climate, extending through to warm temperate and Mediterranean climates. This has taught us much about temperature tolerance of plants, with the impact of frosts and the severity of frosts still being the one of the most used climate factors when assessing what to grow where. For gardeners in those climates this remains very useful information. For those of us in the subtropical regions, it is almost useless. For us, temperatures matter mainly in that we have (almost) no frosts at all so there is no cold spell that many temperate plants need. And yet so many plants which are tolerant of high temperatures do not necessarily grow in the subtropics. Why? – Humidity!

In the subtropics, humidity is a very significant climate factor. Our winters are mild and, although this is our dry season, humidity levels will often still be in the 50-60% range. Our hot wet summers are when the humidity becomes more noticeable, often averaging between 60 -70% throughout the entire wet season. In general warm temperatures + high humidity = lots of plant growth and this is certainly true in the subtropics as well as the tropics. Things seem to grow before your eyes.

The extra moisture in the air means that plants have a higher available moisture, transpire less and are able to keep their stomata (the tiny holes in their leaves which allow gas exchange, including water vapour) open longer which in turn means more photosynthesis and more growth. So it would seem that higher humidity levels would be good for all plants, right? Wrong!

Just as many people not used to a subtropicalclimate struggle to breathe comfortably in thehumid summers, so do some plants.

Some plants simply cannot cope with the humidity at all. It all relates to the way in which plants transpire. Plants which are adapted to dry environments will have many adaptations to reduce water loss through transpiration. One of these is to have fewer stomata or to keep their stomata closed during the day, both of which save water but result in less gas exchange. High humidity tends to slightly reduce available gas concentrations in the air, so that the combined effect is that these plants are breathing less efficiently than they need to be.

Plants adapted for high humidity, or at least not adapted for low humidity, will keep their stomata open for longer during the day and therefore have a higher gas exchange. As we know, it is the gas exchange which drives the process of photosynthesis which in turn fuels plant growth.

For those gardeners who struggle with humidity, it is more than just how well we are breathing. Humidity has little effect on how we feel at lower temperatures, but when higher temperatures are combined with high humidity they can have a compound effect. For example Brisbane experienced a heat wave this past summer– temperatures of 35 and 36ºC which, when combined with humidity of 65 – 75% which we have also experienced, “feels like” 44 – 49ºC to our bodies. This is called the Heat Index, and you can be sure that our plants felt the heat as well.

Another adaptation of plants to their climate relates to the shape of their leaves. Plants adapted to areas of high humidity often have leaves with pointed ends called ‘drip tips’. Drip tips are designed to allow excess water to flow off the leaf, preventing excess moisture sitting on the leaf. Plants without this adaptation will be more likely to have moisture retained on the leaf after rain and in high humidity events, which in turn will make them more prone to fungal diseases.

These are broad generalisations of course and all rules are made to be broken. There will of course be exceptions to these generalisations. Rosemary, a very typical Mediterranean plant, tends to do quite well in the subtropics when provided with good drainage and yet lavender which is also a Mediterranean plant tends to be very susceptible.

So – which are the plants we can, and can’t, grow in the subtropics? There is no secret formula and the answer lies largely in the origins of the plant. Typically those plants that come from areas with winter rainfall and free draining soils will have much lower humidity tolerance. This tends to include the Mediterranean plants we all love like lavender, thyme and olives, or grey-leafed plants like Artemisia (wormwood), santolina or globe artickokes, and the majority of the fabulous western Australian natives – including the hybridised ones. Even some of the southern natives originating in the Sydney region have too low a humidity tolerance to grow up here in subtropical Queensland.

Many grey-leafed plants also feature hairy leaves, an adaptation to trap water vapour close to the leaf surface and reduce transpiration. This is a wonderful adaptation for a dry climate, but in areas of high humidity it is like us putting on a polyester shirt – it doesn’t breathe and we are soaked to the skin with sweat in no time. We can take the shirt off, but our grey, hairy-leafed plants can’t and are highly prone to fungal diseases, if they live long enough.

There are other plants with hairy leaves which are adapted to high humidity such as the African violets and many of the gesneriads (the plant family the African violets belong to). These in general have evolved in places where humidity is high, but rainfall is often low, or available soil moisture is low, such as cloud forests. In these cases the leaves are able to trap additional moisture allowing them to ‘breathe in’ water to supplement low water to the roots. This delicate balance of humidity and semi-dry roots is one of the reasons these plants can be tricky to grow.

Some of the plants more commonly expected to grow well in cooler climates, such as the hydrangea pictured above, cope quite well with the heat and humidity in the subtropics, but prefer to be kept in the shade here. Although the most commonly known hydrangea, Hydrangeamacrophylla, originates in Japan in a temperate climate, there are many hydrangea species native to subtropical regions.

By all means grow lavender in the subtropics – and enjoy it as an annual. If you are lucky and get a spot with good airflow and less humidity build-up you may get a fewyears out of your lavender, but if you only get one year, at least you will not be disappointed. Either way you will be lucky to get more than onegood year out of lavender, after that it may survive but struggle to look any good.

The same applies to any plant grown outside its preferred climate – simply expect a shorter life span and poorer growth and enjoy it while you have it. There are always micro climates within any broad climate zone. These are small areas where the climate varies a little and can often be exploited. In the subtropics, you may find a western facing hot spot which is exposed and therefore has good airflow, can work for those plants with low humidity tolerance.

As a very rough guide – the more dry-adapted a plant is, they greater the chance it won’t like humidity. This of course poses something of a problem with the recent trend towards drought tolerant gardening, and why this might need to be rethought for the subtropics, but more of that coming soon.

Specialist plants such as many orchids and tillandsias (including this Old Man’s Beard below) are so efficient at drawing their water from the air that their roots are adapted for anchorage only and high humidity is essential to them.