Rowan trees are also known as mountain ash, but confusingly they aren’t closely related to ‘ash’ trees! I’ve grown up around these trees. The European rowan (Sorbus aucuparia) that grew in my front yard some years was covered in white blooms in late spring – but there were years when there were few flowers, or barely any – any subsequently no striking red berries, or flocks of visiting birds to feast on them. The same happened last year to a tree near my home – a smart-looking European rowan cultivar known for its yellow berries – ‘Joseph Rock’. But why?
Rowan or mountain ash trees do not begin to flower until they are around 10 years old, depending on propagation method. In established trees, flowering can be inhibited by poorly-timed pruning, late frosts, disease, drought and lack of adequate sunlight.
Having quite the love for rowan trees and a tree obsession in general, became interested in understanding the ins and outs of this problem. A number of gardeners I’ve spoken to feel concerned that when their tree doesn’t flower, it’s indicative of a more serious underlying problem.
In this article, I hope to unpack all the causes so you can easily identify what’s going on with your tree.
Hint: if there are any other rowan trees in your area, take a look and see if they’re flowering better than yours, or have a healthier looking shape or foliage. This will give you a clue as to whether it’s a problem with your particular tree or not.
When do Rowan/Mountain Ash trees usually bloom?
So just to establish some brief facts, these trees grow natively in the UK and USA. They’re also sometimes called quickbeam. The name ‘mountain ash’ is more commonly used in the states, where the name rowan tree is sometimes reserved for the European species Sorbus aucuparia, which became naturalized throughout North America as far north as Alaska.
Rowan tree flowers typically appear during May, though sometimes as late as June. The flowers are usually in dense cream-white collections and contain five petals each.
If your tree has never bloomed but looks otherwise healthy, it probably just hasn’t reached maturity yet. Rowan trees that are grown from seed typically take around 10 years to begin displaying flowers, though some may take a lot longer – 15 or even 20 years has been reported.
However, if you bought your tree from a nursery, it might be worth giving them a call to ask about how their rowans trees are usually propagated, because there’s a good chance they’re grafted – meaning an upper part (called a scion) has been artificially attached to the rootstock of a more mature rowan tree. This is a more reliable and quicker method of producing new saleable trees compared with growing trees from seed, so commonly used by tree nurseries.
If it has been grafted, this young tree has a huge head start, as it’s already growing on a mature root system, and probably will take a much shorter time to flower – possibly as little as one or two years.
How does pruning affect rowan tree flowering?
If your tree’s had any pruning in the past year, read this section carefully! If not, scroll on to the next section.
Mountain ash trees usually grow in a compact, upright oval habit and don’t usually need that much pruning, but it’s common to see these being cut back quite a bit when they’re grown as a border tree or in a small space.
The timing’s important. You’ll usually read that you should prune trees when they’re dormant in winter. But as this tree develops its flower buds on the previous year’s growth, pruning during the winter will inevitably result in cutting off many of these buds before they get to become flowers.
So, if you want to see lots of flowers, prune and shape your rowan tree in mid-to-late summer, after all the flowers have lost their petals. Dead or dying branches can and should be removed as soon as possible though, regardless of the time of year.
This also means that the tree has plenty of time to heal the pruning cuts before the next flowering season, and will by that stage no longer be expending its energy doing so – leaving more energy for flower production.
By the way, with close attention, it’s possible to tell flowering buds from leaf buds on mountain ashes. Flowering buds tend to be round and fat compared with leaf buds. Leaf buds on the rowan are usually reddish-purple and more pointy.
Biennial bearing and mountain ash
Is it possible that there’s just natural variation in the tree’s flowering pattern? Yes. While it’s not anywhere near as common as in fruit trees such as apple, pear and avocado, many growers have observed a ‘biennial bearing’ pattern in their rowan trees as well.
This relates to a genetic tendency for certain trees to focus on flower production one year, and fruit production the next.
Did your tree have a particularly abundant crop of flowers last year? Or on the other hand, were there few berries last year? Either might explain the lack of flowers this year.
Rowan/mountain ash flowering and temperature changes
Sorbus trees such as mountain ash are generally regarded as hardy – particularly the European rowan commonly found in the UK. They’ll grow well anywhere in northern Europe, even in the coldest parts of Scotland, where it’s regarded as one of the national trees. It has also become naturalized throughout the USA as far north as southern Alaska. So it’s regarded as ‘hardy to USDA zone 3 (or even zone 2)’. Here’s a map to hardiness zones if you’re in the US.
However – just because the tree will grow well in cold regions doesn’t mean it’ll always flower well. Frosty weather that occurs at the wrong time can wipe out a good deal of flowering potential, particularly when a rowan tree is young.
This occurs especially if there are frosts in late spring, just when new buds are beginning to open and they’re at their most delicate. The frost won’t kill the tree, but it can kill the buds. There’s not much you can do about this once it’s already happened, but some do go to the effort to protect their trees from frost for this reason.
It’s not difficult to wrap or cover a young tree in hessian or horticultural fleece if a deep frost is expected, and you won’t regret it when you see flower and berries appear later on.
Here’s a simple video demonstrating one method for fruit trees.
I use an app on my phone to alert me to any frost or snow warnings in spring time. I don’t wrap up my trees to the degree shown in the video, but even a little shelter seems to be enough to keep them from freezing.
This is even easier if you’re growing your rowan in a container – just move it into a sheltered porch or garage until the cold snap that endangers the flower buds has passed.
Rowan trees don’t do well in hot weather. In the southern states of the USA, they don’t grow well and if the winter is particularly mild, they can be prevented from entering dormancy – meaning their natural cycle is completely interrupted. This means they won’t produce flowers the following growing season.
Sun, soil, water and rowan tree blossoms
The same environmental factors that affect the growth and vigor of mountain ash will affect the flowers as well, since producing the blooms requires enormous energy expenditure on the part of the tree.
Rowan trees need some sunlight – they can tolerate partial shade, but don’t do well in very shady spots. If your tree’s constantly in the shade, it’s very likely to suffer from poor blossoming and stunted growth. If you’ve recently planted it, it may be worthwhile digging it up and transplanting it to a sunnier spot.
Your soil is unlikely to be the issue, provided it drains fairly well and isn’t waterlogged. These trees are known to be able to grow on slopes and on thin soil, and they tolerate a range of pH levels from mildly acidic to alkaline.
Rowan trees have moderate tolerance to drought. This is more of an issue when trees are young and just entering maturity. If a lack of water is contributing to a poor bloom, you may also see browning in the leaf edges and tips and wilting of the branches.
It’s probably enough to just make sure the soil remains moist near the base of the tree. Check this by digging your finger about an inch into the soil. The soil should feel damp and stick to your finger. If it doesn’t, it’s time to water. Give it a soak for at least 30 mins around the area beneath the tree’s canopy.
If your tree doesn’t have a good layer of organic mulch around the base, that’s one thing you should fix. Often the tree’s trunk is surrounded by grass, which outcompetes the tree for available rainwater. A surround of mulch such as bark chippings that’s 3-4 inches thick is ideal for stopping the roots from dying out, and leaves more available water for your tree by suppressing grass and other plant life. Just don’t pile the mulch up against the trunk – make the mulch shallow here, to prevent trunk-rot.
Diseases that affect rowan tree flowering
Have a look closely at your tree. Consider this cause in particular if you see:
- spots on the leaves or flowers
- dead or dying branches or leaf clusters
- Lack of leaf growth
- Dark or discolored spots on the bark
Rowan trees are susceptible to a number of diseases that can inhibit their blooming potential. In the UK, the biggest issue is silver leaf disease, a fungal infection that causes a silvery unhealthy color to develop over the leaves. It can cause dieback of entire branches and all the flowers therein. Infected tree parts need to be pruned off and completely removed (burned or binned). It’s important to prune away an extra 6 inches of healthy branch as well, just to make sure you’ve got all the infected parts, and to sterilize your secateurs between cuts to avoid spreading it in the process.
In the US, fire blight is a bigger issue. It causes all leaves on a branch or part of a branch to shrivel and go brown – as if they’ve been burned by fire. There are accompanying ‘cankers’ on the bark – dark, wet-looking ditches or spots, in which the infection can linger. Again it’s important to prune and remove, using sterile shears, but you would normally take up to a foot of healthy branch beyond the infected part. This disease is lethal to trees. For a more in-depth explanation of diagnosing and managing fire blight, see this post.
Will fertilizing the tree help it bloom?
Possibly. Sometimes it’s the wrong fertilizer, so much as the lack of it. Nitrogen-rich fertilizers tend to stimulate leafy growth at the expense of flowers. Potassium-rich ones stimulate flowers and fruit.
Fertilizers are usually labelled as something like ’10-10-10′, meaning equal proportions of the three main macronutrients involved in plant growth – N, P and K (nitrogen, potash and potassium). ‘Bloom-booster’ fertilizers are commercially available – these will have a high ‘middle’ number.
Just make sure you follow the manufacturer’s instructions and don’t apply any more frequently than what’s directed.
Wood ash is a natural high-potash alternative to commercial fertilizer.
How do you know if your soil might be low in potassium, or have too much nitrogen? It’s actually really to find out. I use the Rapitest soil kit at home. It’s available on Amazon.
So if your rowan tree isn’t blooming…
- Look at other mountain ash trees nearby and compare
- If it never bloomed before, consider if it’s just too young
- If it’s been pruned lately, consider if some of the flower buds have been cut off
- Think – were there significant frosts when the buds were starting to open in spring?
- If the tree’s looking the worse for wear generally, consider if it’s getting enough sunlight and water, and check for any signs of disease
- If all else looks good, consider a high-potash fertilizer.
I hope that’s helpful! I’ll go further into the problem with my post about rowan trees that aren’t producing berries!