What you need to know about tree ferns for a cool climate
Tree ferns look exotic but they can withstand some frost and snow.
There’s a a ribbon of original rainforest in a gully by a friend’s house in Southern Australia. Tree ferns grow here in their natural habitat. It’s shaded, cool and has even been covered with snow.
(Although not for too long!) But the temperatures frequently range from a few degrees Celsius up to 35 degrees or more. That’s 30-100 degrees Fahrenheit – within the ranges of many British, Northern European or North American gardens.
So if you want an exotic garden in a cool climate, what do you need to know about tree ferns? I asked Stephen Ryan, broadcaster and rare plant specialist, whose nursery is called Dicksonia Rare Plants near Melbourne.
How to choose a tree fern for a cool climate
Stephen says that the most important thing is to buy from a reliable and experienced stockist of tree ferns. In a cool climate, you should buy Dicksonia tree ferns, which are frost hardy down to around minus 5 Celsius.
There are many types of tree ferns, but only Dicksonias and Cyatheas will survive in a temperate climate. And Cyathea tree ferns are not reliably frost hardy so you would need to keep them in a greenhouse or conservatory in the winter.
Dicksonia tree ferns are often sold as a log, with the ferns and roots cut off. This is because the whole trunk is a root system and it can be simply planted in the ground. But, of course, you can’t tell how healthy the plant is until the fronds emerge. That’s why you need to buy from a reputable dealer.
How to plant tree ferns
Stephen says that you plant a tree fern just as you would any other plant. Give the soil a good mulch, dig a hole and put the tree fern in. Backfill with soil and compost and water well.
‘But if you have just bought a trunk,’ he says, ‘you’ll need to plant it deeply enough into the ground so that it won’t topple over. This means losing about 12″ of height. So remember that when you are ordering your tree fern. If it comes with a rootball base, that’s less important.
You also need to support it until it establishes. Unusually, however, you can place rocks or stones in the planting holes to help hold it up. Because the whole trunk is a root system you won’t cause problems with the roots the way you would if you did this with an ordinary tree.
Where to plant a tree fern
Tree ferns like light, but they can burn in direct sunlight. They are happiest under a light canopy of other trees.
If you have a sheltered spot in a courtyard that gets some filtered light, that’s perfect. They’ll also handle shade well.
Keep away from harsh winds or anywhere exposed.
How to water a tree fern
Tree ferns like a fairly damp climate. You should water them in a drought. Stephen runs a hose over the whole trunk (because it’s a root system). He then returns a few minutes later and repeats it, because if a tree fern trunk has got very dry it may not absorb much water. He sometimes repeats that three or four times.
Stephen says there is some debate about whether you can water tree ferns from the top. Some people believe it will cause the crown to rot. ‘However, I’ve watered my tree ferns from the top and have never lost any that way,’ he says.
He often perches the hose on the top, wedged between the stems, and lets it drip down for a few hours. ‘Although be careful not to damage the young croziers by poking too hard down into the crown,’ he adds.
‘And don’t water them from the top in winter because if water freezes in the crown, that will damage them.’
Do you have to feed tree ferns?
In the wild, tree ferns benefit from falling leaves on the forest floor. These rot down into mulch, and Stephen says that a once a year mulch with garden compost or rotted animal manure is all a tree fern needs in a garden. ‘I’ve never given my tree ferns any extra fertiliser,’ he says.
It’s different if your tree fern is in a pot because it can’t take nourishment up from the soil through its roots. You will need to fertilise it.
How to protect tree ferns in winter?
Stephen lives in a mountain area near Melbourne, so the winters do get cold. But they don’t have sustained frost and snow for weeks on end.
The wild tree ferns in the gully near where he lives have been covered in snow several times. But not for long.
So tree ferns can cope with a bit of frost and snow. ‘But if you get sustained sub-zero temperatures so that the trunks freeze, then you’re in an area where tree ferns probably won’t survive,’ he says.
Stephen stuffs some straw gently into the crown of the tree fern to protect the young croziers (fern leaves) from frost. Other people wrap their tree ferns in horticultural fleece or bubble wrap if sustained cold weather is forecast.
If your tree fern is in a pot, you may be able to move it closer to the house. The best protection is to plant tree ferns in a sheltered spot, away from frost pockets or nasty winds. Generally a house wall will be a warmer place for a plant in winter.
How to prune a tree fern?
Tree ferns don’t actually need pruning, says Stephen. When the unfurled croziers go old and brown, you can cut them away.
However he warns against over-zealous trimming. ‘If you take away all the old growth, you expose the new growth to wind and weather,’ he says. ‘And you also get a shuttlecock effect. I think tree ferns look better with some natural curve to the leaves.’
Can you move a tree fern?
Stephen says that you can move a Dicksonia tree fern in two ways. Firstly, you can simply cut the trunk off wherever you like and plant it in the ground. ‘But don’t forget that you’ll lose the height and tree ferns are very slow growing.’ The rootball you leave in the ground won’t grow into a tree fern again. However the trunk is a root system and can be planted straight into the ground.
Or you can dig up a tree fern with its root ball and re-plant it. This is hard work but will maintain the height. And you don’t need to dig up the entire root ball – you can cut around the roots in the ground, although you will need to support it.
There is a video here showing how to move a tree fern:
Why isn’t my tree fern growing?
If the fronds are going brown, Stephen says it is nearly always a water issue or it’s being burned by being in too bright a spot.
‘Either you’ve placed your tree where there’s too much direct sun or you haven’t been watering the trunk enough,’ he says.
If you’ve bought your tree fern recently, it may not have been looked after properly. The fronds may be damaged or they may be small. It could take several years for them to grow well.
If you want to check whether the tree fern is dead, Stephen says put your fingers down into the crown very gently. If you can feel the croziers and they are soft and supple, they’re alive.
‘But if they feel dry and crispy, you’ve lost your tree fern,’ he says.
But don’t throw away a dead tree fern trunk. It’s a wonderful habitat for other plants and for insects. It rots down very slowly so it can sit in your garden covered in moss, alpines or ferns for years.
See Stephen talking about tree ferns for your garden in this video:
More about Stephen Ryan
You can find Stephen Ryan’s Dicksonia Rare Plants in Mount Macedon, Victoria. Or book him for talks, discover his garden tours or visit his garden by visiting StephenRyan.com.
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