Rowan or mountain ash trees are part of the Sorbus genus that also includes service trees and whitebeam. In common with one another, they produce mostly red/orange and often vibrant berries. Many are used to make jams, jellies and liqueurs. Personally I most enjoy the birds that seem to like them so much! Where I live, it seems to be starlings and thrushes that come out for these berries.
However, it isn’t uncommon for rowan trees to have seasons when they produce very few berries or none at all! I became interested in this quirk.
Rowan or mountain ash trees don’t produce berries until they are around 10 years old, depending on the propagation method. Pruning at the wrong time can inhibit berry production, as can spring frosts, biennial bearing patterns, disease, inadequate sunlight, and lack of soil macronutrients.
Having a) grown up around mountain ashes, b) a serious tree passion and c) spent a lot of time researching this problem in all kinds of trees, I hope I’ll be able to unpack this problem in a straightforward and methodical way.
By the way… mountain ash trees aren’t closely related to ‘ash’ trees (genus Fraxinus). Just wanted to clear that up as it often seems to cause confusion!
If your tree isn’t producing either flowers or berries, check out this post.
When do Rowan/Mountain Ash trees usually make their berries?
The time of year
In the northern hemisphere, mountain ashes usually flower in the northern hemisphere in May. Shortly afterward young berries appear – they grow over the summer and are usually ripe by late summer. The berries of Sorbus aucuparia (the European rowan) and Sorbus Americana (the American rowan) are orange to red, but there are numerous ornamental Sorbus cultivars. Sorbus pseudohupehensis ‘Pink Pagoda’ has pink berries, and Sorbus ‘Joseph Rock’ has yellow fruit.
The typical age
As stated above, most mountain ash species are around 10 years old when they start making berries. This corresponds to reproductive maturity, where flowers also start to appear for the first time. If your tree is around this age and it looks otherwise healthy, it might simply be maturing. It’s been known for some rowans to take even longer to do this – even 20 years!
However, these timescales apply to rowan trees that are grown from seed, and many nursery-bought mountain ashes will be grafted trees.
Grafting is a faster method of propagating (making new trees). An upper part of one rowan is attached to the rootstock) of another and it’s grown from there. These trees get a huge headstart towards maturity, already having an established root system.
You might be able to tell if your tree was grafted by looking for a ‘join’ – like a scar – at the base of the trunk. If you can call the nursery that you bought it from, ask them if their rowans are grafted and when they usually flower. It might be several years fewer than 10.
PRO TIP – if there are any other mountain ashes near you, inspect them. Is the flower or berry situation different, or are they looking the same? This can help you work out if there’s a problem with your tree or not.
Are rowan trees self-fertile?
Rowan trees are monoecious – meaning the flowers on each tree aren’t either male or female. They’re hermaphrodites, having both male and female reproductive parts on the same flower. With the help of insect pollination, this makes them essentially self-fertile, and therefore more reliable at producing fruit compared to some other fruiting trees, such as mulberry and persimmon, where trees producing male flowers needed to be planted near enough to ensure pollen makes it to the female flowers on another.
However, there are some exceptions and a common phenomenon in many monoecious trees applies here. Mountain ashes usually produce many more healthy berries if they do have other mountain ashes nearby so that can cross-pollinate with each other and introduce new genetic variations. This reduces the risk of parthenocarpy, where, sometimes due to certain genetic quirks that occur with self-pollination, berries develop without seed and can fall off early.
If you have some berries, cut some open to make sure that they contain seed. If not, planting another rowan is likely to help (what’s more, they’ll pollinate each other, making more than double the berries!). Speak to a local nursery about compatible varieties.
How does pruning affect rowan tree berries?
If you haven’t pruned your rowan in the past year, you could scroll on. down. But if you’re interested: pruning can affect the production of berries considerably. Rowans are frequently planted in small gardens, and although an ideal tree for many small yards, they can still easily outgrow their space, so they often need pruning and shaping.
The general rule for most trees is to prune in the winter during dormancy (or any time of the year for dead or dying branches). However… this tree produces its flowers (and therefore its berries) on last year’s growth, so with pruning in winter you’ll inevitably chop off some of the flower buds.
For this reason, some people recommend pruning mountain ashes during the summer instead.
It’s often possible to tell apart flowering buds from leaf buds during winter pruning though. Leaf buds are thinner and pointer, while flowering buds are shorter and fatter. Pay attention to the location of the flowering buds when you make your cuts.
Biennial bearing and mountain ashes
A common phenomenon in many fruit, berry and nut trees is biennial bearing – the tendency for trees for focus on flower production one year, and berry production the next. It seems to be reasonably common in rowan trees too. If you notice a drop in berry production this year (but not a complete wipeout), particularly if there were a lot of flowers, and the tree looks otherwise healthy, this could be the cause.
This tendency disappears in many trees as they age.
Rowan/mountain ash berries and temperature changes
Rowan trees are hardy – they grow natively over the whole of the UK right up to the north of Scotland (not the warmest spot at the best of times) and have become naturalized in Canada and Alaska. So they’re considered ‘hardy to USDA zone 2’ in the states (here’s a map showing your hardiness zone if you’re in the US).
I use an app on my phone and keep a close eye on any frost alerts in spring so I can protect my trees overnight using hessian or horticultural fleece. Even wrapping some plastic sheeting around trees at this time of year, if they aren’t yet that big, can give a degree of frost protection.
Here’s a video that shows how simple it really is. It’s worth it when you get to enjoy seeing flowers, berries and many hungry birds appearing on your tree over the coming months.
Sun, soil, water and rowan tree berries
These environmental factors will have an effect on mountain ash’s overall growth and vigor. Since it takes a huge amount of energy for trees to produce fruit, anything that causes the tree to grow poorly will cause a poor show of berries as well.
Is your rowan tree in a spot where it doesn’t get much sun? These trees are able to tolerate shade, but only partial shade. They really prefer full sun, especially when it comes to berries, which usually require some decent summer sunshine during the ripening period. Compare your tree to other rowans on more sun-facing sites and see if there’s a difference.
Soil isn’t usually too much of an issue as long as it’s well-drained – rowans don’t grow well on saturated land. They seem to manage to root on thin soil and are particularly happy growing on slopes. They tolerate a range of pH levels, but like most trees, they seem to like it slightly acidic (pH 6 or so).
The macronutrient content of the soil can have an impact on berries, particularly if there is too little potassium, which is important in flower production, or if it’s too high in nitrogen, which usually stimulates the tree to put its energies into growth and foliage instead.
Berry production also requires a lot of water, so even though these trees will usually survive through brief periods of drought, if you’re hoping for a good berry yield, you should consider watering your tree. This is most crucial for recently planted trees that don’t have hugely established root systems. You may get a clue to the tree’s parched state if the leaves look wilted or if some are yellowing prematurely.
Usually, 20-30 minutes of watering with a hose once a week during dry spells in the summer will do. Alternatively, check the soil around the tree base (an inch or so into the ground) with your finger and see if it feels moist or not. If the soil slides off your finger, it needs some water.
Will fertilizing the tree help berry production?
Yes, potentially. A well-balanced fertilizer with equal portions of nitrogen, phosphate and potassium (N, P and K, usually expressed as 10:10:10 or in a similar ratio), can be used – just be sure to use it in accordance with the manufacturer’s instructions, as over-fertilizing can be damaging. You’ll find online a number of fertilizers that are specific for fruit or vegetable production, often with higher potassium content which arguably might be more specifically helpful.
It’s quite easy to find out if your soil’s high or low in N, P or K using a home testing kit! I use the Rapitest kit, available to buy from Amazon.
Diseases that can affect mountain ash berries
Suspect disease if you see:
- dead or dying branches, or parts of branches
- spots on leaves, flowers or berries
- poor growth, particularly when compared to other specimens – appearing generally ‘unhealthy’
- Spots on the trunk and branches (cankers) that appear dar or wet, or that ooze
In the UK, silver leaf disease is a particular issue for rowans. It causes a silvery discoloration on the leaves which can spread and cause dieback of whole branches. Like most fungal infections, the goal is control rather than cure. The simplest way to try to achieve this is to prune off branches that appear infected (take at least 6 inches extra of ‘good’ branch as well to make sure you get it all). The secateurs should be sterilized with alcohol gel in between cuts, and diseased branches need to be removed and burned or binned to reduce spore release later on.
In the USA, fire blight is a danger to the mountain ashes, as it is for all of the rose (Rosaceae) family. It’s a bacterial infection that hides in cankers in the tree bark and can infect and rapidly kill entire branches. The leaves of infected branches go totally brown and wither as if they’ve been scorched in a fire – hence the name.
As with silver leaf disease above, careful hygiene and removal of all pruned parts is essential. Trained arborists can apply antibiotics and other sprays. Fire blight does occur in the UK, but it isn’t nearly as common as in warmer parts of the world.
Rowan tree, no berries?
In summary, if your mountain ash tree’s low on berries, have a look at others in the area if you can first, for comparison, and ask yourself these questions
- Has it ever produced flowers or berries? It might not have reached maturity yet
- If it’s been pruned recently, were a lot of the flower buds cut off before they could open?
- Were there any frosts in spring when the delicate buds were opening?
- Could it be down to biennial bearing?
- Is my tree getting enough sun exposure, and water in summer dry spells?
- Is my soil nutritionally balanced, and could I add some fertilizer to it?
- Are there any signs of disease when I look at it up close?
I really hope you found this article informative. I certainly enjoyed writing it. Please have a look at this other post on rowan trees!