Horticulturist, gardening consultant, and designer Jonathan Garner loves this time of year. Autumn is one of his favourite seasons.
It won’t be long before the unsavoury periods of our various climate patterns begin to settle. The heat and dry of the Mediterranean regions begin to cool and receive rain. The humid tropics begin to cool and dry up. The inescapable and draining humidity starts to reduce in my coastal Sydney zone while the autumn colours start showing their beauty in the cooler temperate zones where I prefer to work.
I know very few landscape professionals that enjoy humidity.
Through all of the climate zones, autumn is a transformative season for the plants.
This is most noticeable at the extreme ends of our climate types – by ‘extreme ends’, I mean the tropics, the cool temperate and the mediterranean climates.
It’s a time when plants (and insects) prepare for winter dormancy in the temperate and tropical zones, or adapt to the changes in environmental conditions in other climate areas. It’s the time when breathtaking autumn foliage colours can adorn the landscape.
What is autumn colour all about?
The spectacular phenomenon where the leaves of many deciduous trees and shrubs change from greens to vibrant shades of yellows, oranges, reds and purples is the result of complex biological and chemical processes going on in the leaf. The colour change is triggered by shorter daylight hours, cold night temperatures, and to a slightly lesser extent, dry conditions. Most autumn-coloured plants pretty well stick to the same colour or pigment every year. This is due to the inherited colour or genes of the particular cultivar or clone. The intensity of the colour each year is generally due to the environmental factors mentioned earlier.
Chlorophyll. The green stuff
Most plants are green and all plants have a green pigment called chlorophyll. Even burgundy- or purple-coloured plants have chlorophyll. It’s just hiding beneath another couple of pigments which I’ll touch on in a tick.
Chlorophyll is the green pigment responsible for photosynthesis – the process which allows plants to convert water, sunlight and nutrients into something solid like plant tissue or wood. So, during the growing season, chlorophyll is constantly produced and broken down. In autumn, as the days shorten and temperatures drop, chlorophyll production in deciduous plants slows down and eventually stops. During this time the plant breaks down the chlorophyll to recover valuable nutrients such as nitrogen, phosphorous and magnesium, which are transported back into the branches, stems and roots for re-use in spring.
Carotenoids
The name kind of gives it away.
These yellow and orange pigments are always present in leaves, but are usually masked by the dominant green chlorophyll. When chlorophyll breaks down, carotenoids become visible, giving leaves yellow and orange hues. When a plant gets low on nitrogen (the fuel for chlorophyll) the yellows of carotenoids start to show.
The yellows of variegated plants happen for other reasons. Jump on the net if you want to learn about this. We only have limited space here.
Anthocyanins
Anthocyanins give the reds, crimsons and purples produced in the leaf during autumn, and are generally sugars left over after the plant has recovered the valuable nutrients.
Anthocyanins are also responsible for the deep burgundy colours of plants like New Zealand Flax and Cordylines, so the plants that don’t look green actually are. There’s just a heap of anthocyanins sitting above the chlorophyll.
Tannins
This is the boring brown pigment that’s left behind.
It’s also the frustrating, staining pigment that leaches from hardwood timbers like Merbau and Blackbutt.
Tannins play an important role in preserving plant tissue from decay. Oak and Plane tree leaves are full of tannins. This is the main reason why you see these leaves still blowing around the yard over the following year or longer.
Choosing autumn-colour trees
Something to keep in mind with autumn colour is that it’s important to buy stock propagated by clones.
If you’re buying autumn-colour plants grown from seed such as Liquid Amber and Japanese Maple, then choose your stock in autumn. If you’re wondering which trees are the best for colour in your region, visit local gardens. If you’re wondering where to position plants in the landscape for autumn colour, avoid shade and areas exposed to high wind. The shade reduces exposure to cold and necessary sun while windy areas tend to relocate the foliage from said plant to elsewhere.
Autumn planting
Planting in autumn is the best. Establishing plants in the landscape is so much easier than any other time of the year. The soil is warm and plants are preparing themselves for rest. They’re reabsorbing sugars and starches. They’re focussing energies on root growth.
With evergreen plants, the warm soil encourages root growth while the cool nights and the shorter days reduce the above-ground growth. Every leaf is concentrating on creating sugars and starches for roots to grow, for storage areas to fatten and stems to bulk up. This current healthy root growth will make for strong above-ground growth through the next spring and summer.
Root pruning for transplanting
Generally speaking, autumn is the best time to either transplant or prepare for transplanting.
Root pruning during early autumn essentially works the same as pruning branches. Cutting a healthy root encourages it to branch just behind the incision. Around shrubs, I use a sharpened spade. Around trees, I use a trenching machine or highpressure water jet to sever the roots close to the base. The roots branch quite rapidly and serve to create a denser root ball close to the trunk. This makes lifting the plant much easier during the cold of winter. On old plants and more sensitive one’s like Camellia, I’ll root prune one or two years prior to transplanting.
Natives…I don’t even attempt to shift them.
Stay puts
Other varieties of plants I don’t shift in autumn are citrus, palms, bamboo and warm-season ornamental grasses.
I’ll definitely root prune citrus and palms in early autumn, but will wait until the warmth of spring arrives before actually lifting the plant from the ground. The roots of citrus and warmseason ornamental grasses such as bamboo (yes, it’s actually a variety of grass) won’t grow through winter. Transplanting these creatures through winter usually condemns them to a slow decline towards death.
Keeping records
Autumn is a great time to reflect on the previous growing season and which pest and diseases may have been challenging. Recording this information and setting reminders on your project-management app will improve efficiencies (and revenue). Having a record of when pest and disease broke out will enable applying preventative measures rather than reactive actions next growing season.
Late summer and autumn is when some diseases – such as honey fungus (armillaria) – show their fruiting bodies (mushroom). Jump on the net if you’re unsure what these look like. Lawn army worm is another challenge that loves late summer and autumn. I think we’ll delve deeper into pest and disease in a future conversation.
