I had to write this post as I’ve been asked twice now WHY mulberries trees sometimes seem to disappoint in the fruit department, while others have branches laden with ripe pink, red or black fruits, attracting many birds – and staining your pavement! It’s a question that, in my opinion, hasn’t (so far) been clearly answered anywhere online. I hope to make it clear in this post.
Mulberry trees grown from seed do not usually produce fruit until they are 8 or 9 years old. Only some are self-fruiting; many are dioecious (having only either male or female flowers). Male mulberries will not produce at all, while female trees must be pollinated by a male tree to produce.
That’s right. Your mulberry tree may be taking its time to bear fruit, or it may be that it will never produce. BUT there are some interesting caveats. Including the fact that mulberry trees, in certain circumstances, can change gender! Furthermore, there are a range of other factors that will increase or decrease mulberry yield. In the article below I’ll help you diagnose the issue for your mulberry tree (or trees), and where possible, remedy it.
How old are mulberry trees when they start producing fruit?
As a general rule, mulberry trees that come from a seed (or any solitary trees that are found growing natively) will be in their ninth or tenth year before the first mulberries are seen.
It’s often said that they take several years to produce the very best fruit though. Mature mulberry trees will keep on producing throughout their lifespan, which is usually said to be similar to that of a human – around 75 years (then again, there are mulberries that are over 200 years old. Why? Check out my post do trees die of old age, or can they live forever!).
BUT if you bought a mulberry tree online or from a nursery, there’s a good chance they are a grafted mulberry – meaning they were started off when a scion (cutting from one mulberry) was transplanted on to the rootstock of a more mature mulberry tree. Benefitting from the maturity of the roots, these may produce fruits after only a year or so. Excellent!
In fact, many mulberry cultivars have been developed and grown for certain characteristics, including early and vigorous fruiting. Even if they aren’t grafted, you may get mulberries after 5 years or so.
What time of the year should mulberry fruits appear?
Mulberry trees typically display their catkins (flowers) in spring, and their fruit should be visibly growing by late spring or early summer. The fruits can be harvested between June and August, with the black mulberry tree tending to ripen later than the red and white mulberries.
How to tell if your mulberry tree is male, female… or both
Both male and female mulberry trees produce flowers when they are mature. They both take the form of catkins – a dangling cluster of green – but aren’t too difficult to tell apart.
Male catkins are about longer – up to 2 inches – and take a more dangling appearance. Female catkins are shorter – about 1 inch at most.
If you can find both male and female catkins, you’re on to a winner. Pollen from the male catkins will travel by wind and fertilize the females, which should quickly ripen and you’ll have a mulberry harvest.
If you can only find female flowers, it’ll be a little more complicated. In this case, you might get lucky if there’s another mulberry tree nearby that can pollinate it by wind. But as with many fruit trees, the best option may be to plant another mulberry tree – one with male flowers – on your property. Fruit growers often do this. Check your prevailing wind direction and consider planting it upwind. Here’s a useful wind map for the USA.
Many fruit growers find that even if there are both female and male flowers on your tree, you may get an even better yield if you plant a second mulberry with male flowers.
If you’re going to buy a second mulberry as a pollinator, definitely ask for a grafted one, so you’ll get flowers (and therefore pollen) in a year or two, not 8 years down the line!
The most vigorous pollinator of all the mulberries is the white mulberry, Morus alba. In fact, for this reason it’s regarded as one of the worst trees for allergy sufferers. It’s regarded as an invasive species, particularly as it prolific pollination causes it to hybridize with the red mulberry (the only true American native) so that over successive generations, the red mulberry is becoming scarce. So I recommend to avoid it.
Very cool fact though – the catapulting of pollen from stamens of the white mulberry currently holds the record for fastest motion in the plant kingdom – over half the speed of sound. The scientists who discovered this argued it’s the fastest motion ever observed in biology. Link for any science geeks reading. Here’s the video.
If you’re buying a mulberry tree from a nursery or online, there’s a very good chance it will be self-fruiting. This either means it will produce both male and female catkins, or it might even be able to produce seedless fruit.
Finally, grafting a male twig onto a female mulberry tree (or vice versa) can be attempted, instead of planting a second one. Fruit growers sometimes do this – it saves space for more fruiting trees.
Here’s a how-to video, involving the less common Pakistan mulberry, Morus macroura.
Can mulberry trees change gender?
Yes, mulberries can change from male to female or vice versa, and may also at times bear flowers of both gender. This phenomenon is often thought to be stimulated by tree stress, in which the tree makes a final effort to reproduce before death.
Other trees that can change gender include yew trees and certain maples.
While I wouldn’t hold my breath waiting for it to happen to your mulberry, through this method male-flowered trees can become fruit-bearing.
Could temperature changes affect mulberry yield?
Mulberry trees have limits to their cold tolerance. They will grow in regions as cold as USDA hardiness zone 5, meaning areas where there is an average annual minimum winter temperature of -20 to -15 °F (-28.9 to -26.1 °C). Frost, particularly when the tree is emerging from dormancy and has tender new growth, is a known factor in diminishing mulberry yield. They’re more likely to grow well in warmer regions.
The black mulberry in particular may struggle if you’re growing in zone 5 or 6, and do better from zone 7 (where the average annual minimum winter temperature is 0 to 5 °F/-17.8 to -15 °C) and upwards.
Sun, soil, and water requirements for mulberry production
Just as they prefer it when they’re putting on height, they like full sun when it comes to producing fruit. Mulberry trees will tolerate light shade, but won’t normally do well at all if grown in a very shady spot.
They’re famously tolerant when it comes to different soil types and pH levels, but slightly sandy, well-drained soils will be best for a good yield.
Could a lack of mulberries be caused by disease?
Mulberries are relatively disease-resistant, but they do get mulberry blight. It’s a bacterial infection caused by the pathogen Pseudomonas syringae. It causes irregular dark green, brown or black spots on the leaves and fruits.
Take a look at your mulberry tree’s leaves. If you see any discolored spots, I’d probably give a call to a local arborist. They’ll prune off the diseased areas (and can do so without spreading the infection to other tree parts). The chemical control agents used depend on whether you’re planning to eat any mulberries, so I would defer to the experts on this one.
Pruning a mulberry tree to increase fruit yield
Mulberry trees are well-known for being able to endure even harsh pruning. Pruning for growth or shape will need a different approach to pruning for fruit production.
When mulberries are grown commercially, new shoots and leaves that don’t have any developing fruit on them are often snipped off weekly during the growing season, so the plant puts its energy into ripening its mulberries.
I found a video of this online. This type of pruning is very easy to do yourself. You’ll be making mulberry jam in no time!
I really hope you enjoyed this article and found it useful
For even more helpful advice on mulberry trees, check out this post!
Nickispeaki, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons
DS28, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons