Kousa dogwood (also known as Chinese, Japanese, Korean or even the strawberry dogwood) is one of my favorite flowering trees to grow on a property. Hard to beat… if it flowers well! Not to take anything away from the striking fall foliage and the attractive flaking bark, but I’m all about those blossoms. They typically appear later than most other flowering trees and last a lot longer; this is in part because the white or pink ‘flowers’ are actually bracts – modified leaves which, in groups of four, surround the green ‘real’ flowers, that are barely noticeable!
Why do Kousa dogwoods sometimes not bloom as they should??
Kousa dogwoods may not begin blooming until they are 2-8 years old, depending on propagation method. They bloom later than other dogwoods – in late spring or early summer. Blossoming can be impeded by recent transplanting, lack of sunlight, poorly-timing pruning, drought and late spring frosts.
I’m a dogwood fan (yes, there is such a thing!) so I’m very happy to unpack what I’ve written above, and then summarise at the end. It’s frustrating if you’ve planted Cornus kousa and it isn’t blossoming properly, so I hope you find this page helpful.
In my experience many people have expectations about dogwood trees that are based around the flowering dogwood, Cornus Florida. Kousa dogwood has become an increasingly popular alternative in recent years for several reasons, particularly their natural resistance to dogwood anthracnose infection.
What age are kousa dogwoods when they start to bloom?
Trees take time to reach maturity, when they’re ready to bloom and produce fruit. When this happens depends on how the tree was propagated – was it grown from seed, or started, for example, as a cutting from another.
Kousas are actually quite easy to grow from seed, so seedlings are commonly sold. Seedlings will take the longest to flower though – usually 5, 6, 7 or even 8 years. Probably not quite as long as flowering dogwoods on average but it still feels like quite a long time when you’re a homeowner who’s patiently waiting for that display.
Those grown by other methods e.g. by budding, or grafting (which involves attaching a small part of one kousa dogwood to an existing root system) get a head start in life. Having a mature root system already, they can get more nutrients and water and will start blooming much more quickly. This could happen after as little as a couple of years, depending on the growing conditions (see below).
It might be possible to tell if your young tree is grafted – have a look at the base of the trunk and you may see a line in the bark representing the ‘join’ between scion and rootstock. But if you’re not sure, you could certainly call the nursery where you bought the tree and ask them about their usual propagation method, and how long they’d expect flowering to take.
If your young tree was already flowering before you planted it (for example, when it was in a container), but it hasn’t flowered since then, it might be simply the action of planting the tree that’s put a stop to the flowers – this is part of the phenomenon known as ‘transplant shock’. You may get a clue about this if some of the leaves have turned yellow or dropped prematurely. The recovery process can take two or three years, but the tree should eventually flower if you keep it well-watered during dry spells (see below).
What month do kousa dogwoods bloom?
Kousa dogwoods bloom about 2-3 weeks later than Cornus Florida, the ‘flowering dogwood’, native to the US. In the northern hemisphere, this means between the end of March, and at the latest, late June.
One telling difference with the flowering dogwood is that the kousa dogwood’s white bracts appear while the leaves are already out, while the flowering dogwoods appear before the leaves appear.
The blossoms typically last about 6 weeks! So not only are kousas interesting because of their late flowering compared to most other flowering trees.
Pruning kousa dogwood and blossoming
Have you pruned your dogwood in the past year?
You probably already know that the general rule for tree pruning is during dormancy – in the late fall or early winter after deciduous trees like kousa dogwood have lost their leaves.
However, the kousa dogwood needs to be treated differently if you’re interested in seeing plenty of flowers (and why wouldn’t you be?).
This tree sets its flowers on last year’s growth. If you do prune during dormancy, there’s a good chance you’ll cut off many of the flower buds before they ever got the chance to become flowers!
So the best way is to prune just after flowering, in the late summer
But there are some ways to get around this if you still want to prune in dormancy, along with your other trees:
- simply try to avoid cutting off too many flower buds (you may be able to tell the flower buds from the leaf buds – they’re usually shorter and fatter)
- prune alternative limbs each year – so flowering is only impaired the next summer on those branches
Of course, diseased or dead tree parts should be removed when you find them, regardless of the time of year.
Kousa dogwoods and biennial bearing
If your tree flowered well last year but this year isn’t, is it possible that it’s just having an off-year, or taking a rest?
Interestingly, yes, and I’ve found this myself. The most common form of this phenomenon is biennial bearing. This occurs commonly in a number of fruit and nut trees and refers to many trees’ tendency to utilize available energy and nutrients to produce numerous blossoms one year, and numerous fruits the next. See relevant articles on hazelnut, walnut, and mulberry for more on this topic.
If you notice later on in the season that the berry production’s particularly high, this might be a retrospective clue to why your kousa dogwood didn’t flower that well.
Kousa dogwood blossoms and temperature changes
Another reason for the kousa dogwood’s growing popularity is that it’s perhaps slightly more winter-hardy than the flowering dogwood, Cornus florida. It’s similar in that it’ll grow down to around USDA hardiness zone 5 (corresponding regions where there’s an ‘average annual minimum winter temperature’ of -20 to -15 °F (-28.9 to -26.1 °C)), but it’ll be more consistent in producing blossoms in these cooler regions.
However, a cold snap at the wrong time can have a major impact on flowering that summer. If there’s a drop in temperature and frost settles on the tree in late spring, it can kill the tree’s most delicate parts – namely the flowering buds, which are preparing to open soon. This is a common explanation for kousa dogwood (that previously did bloom well) producing a poor show in a given year.
If it’s important to you, it’s not that difficult to wrap a young tree if very cold weather’s expected in spring. You can use plastic or horticultural fleece, or bring it into a garage overnight if it’s still in a container.
The kousa dogwood also doesn’t like the heat as much as the flowering dogwood. While the flowering dogwood can flourish in the warm southern states of the USA, you don’t see so many kousas around there. It’s more likely to consistently flower up to zone 8 (where the average annual minimum winter temperature’s 10 to 15 °F/-12.2 to -9.4 °C).
Here’s a link to the USDA Hardiness Zone map. Canada uses the USDA system as well – here’s their map for reference. Australia’s hardiness zones are equivalent to USDA zone 11 in the north and zone 7b in the south, while areas in the UK range from hardiness zones from 7 to 11 according to the same US scale.
Kousa dogwood blossoms and sunlight?
This is one to consider if your kousa dogwood’s never bloomed properly.
Although it doesn’t tolerate heat as well as the flowering dogwood, it actually is better at tolerating full sunlight.
Flowering dogwoods are known for preferring partial shade, and often grow at the edge of a wooded area. Kousa dogwood’s therefore a good option if you’re planning to plant on an open yard.
Conversely, if your kousa dogwood’s planted on a side of your property that doesn’t get much direct sunlight, it’s unlikely to ever flower (or grow) particularly well.
Soil issues and fertilizer
The kousa dogwood will usually bloom happily in slightly acid-to-neutral soils (i.e. pH of 6 to 7). Many regions have slightly alkaline soil (pH >7); this may be a reason for your kousa dogwood not flowering well. These days it’s hard to know what’s going to take well in your yard without knowing at least approximately what the pH is.
To find out though, you should just perform a simple soil test. I use the Rapitest kit that’s available online. If the pH is slightly alkaline, you may be able to adjust it towards the acidic side using ericaceous plant feed.
In general, kousa dogwood is quite tolerant of the soil type – whether sandy, or clay-like, for example – as long as it’s well drained and deep enough for roots to establish. Most roots for the kousa dogwood will be growing within a foot of the surface.
Will fertilizer help the tree to bloom? As often as the soil’s lacking fertility, I find it’s just as common that fertilizer has been inappropriately applied. Many fertilizers (including lawn feed, which can get into your tree’s root system) contains a lot of nitrogen, which will stimulate green growth at the expense of flowers and berries.
Certainly it’s reasonable, if you haven’t added any fertilizer yet, to give it a try.
The right type of fertilizer for kousa dogwood is Jobe’s Fertilizer Spikes. A ‘complete’ fertilizer meeting the above specifications, it’s incredibly easy to apply in an exactly correct amount – because you just push them into the ground, over the roots. Here’s an affiliate link to check the current price on Amazon.
Will watering the tree help?
As mentioned above, the kousa dogwood doesn’t have a deep root system, so it won’t tolerate periods of drought particularly well. Flowering and fruit production all requires adequate hydration, so if there’s been hot or dry weather and the leaves are looking wilted or curled, by all means you should give it some water – lack of irrigation might be having an impact on its flowering potential.
Established trees usually don’t need to be watered often, except in very dry spells. But younger trees will need a bit every 1-2 weeks. Usually a 20-minute soak around the root area with a hose. I wouldn’t use a sprinkler, just because wet leaves encourage anthracnose infection (although kousa’s quite resistant compared to flowering dogwood).
You can also check the ground manually with your finger – just dig it in an inch or two near the tree’s base, and if the soil doesn’t stick to your finger (or just feels dry to you) give your tree a good soak.
On the other hand, kousa dogwood doesn’t like poorly-drained soil. If it’s been planted in a marshy area, again that might be the reason for a poor show of flowers.
Most arborists these days will recommend using a Tree Gator or a similar watering system. Check the current prices on Amazon here.
Diseases that affect kousa dogwood
As touched on above, kousa dogwood is actually a better choice of dogwood is terms of its minimal susceptibility to anthracnose, a rather common fungal infection (caused by the sinister-sounding Discular destructiva) that has devastated flowering dogwood in many regions. It can cause brown, spotted leaves which lead to major dieback, sometimes of whole limbs – which naturally will lead to much fewer blossoms and a sick-looking dogwood.
Fortunately if kousa dogwoods become infected, it’s usually only to the extent of some leaf spotting and will be unlikely to affect blooming that much, although the spots can be found on the white ‘bracts’ which spoils the look somewhat.
If you find any spotted leaves I’d remove them and dispose of them in the trash (don’t just leave them on the ground, where the spores can reinfect other tree tissue, then or later). Be sure to use clean secateurs, sterilized with rubbing alcohol between cuts, so you don’t spread the infection further.
There are some other infestations that are less common, such as dogwood borer, which can affect the overall vigor of the tree – and anything that affects its vigor and overall health can affect flowering. You can reduce the odds of such infections by being careful not to damage with trunk (often weed trimmers are the culprit here!). Ultimately if your tree’s spotted, or there appear to be wounds (cankers) on the bark, you may need to contact a trained arborist, as most such infections are treated with a combination of skilled pruning and chemical measures.
To put it in good old bullet points, here’s a quick diagnosis guide that I hope will summarise most of what’s above! If your kousa dogwood’s not blossoming well, go through these steps:
- If it’s a young tree, or recently transplanted, but looks otherwise healthy, give it some time
- Remember they flower later than the flowering dogwood – as late as late June
- If you’ve pruned it recently, you may have cut off many of the buds – hopefully by next year it’ll be back to normal. Consider pruning AFTER flowering.
- Did it bloom especially well next year? It may be related to biennial-bearing tendencies.
- If there was a cold snap in late spring, that may have wiped out many of the flowering buds – better luck next year
- Check your soil isn’t too alkaline, and consider some fertilizer that’s proportionally higher in potassium.
- Is your tree planted in the shade, or in waterlogged soil? It may never bloom well.
- Kousa dogwood isn’t great at tolerating drought – keep it well watered during dry spells
- Watch out for leaf spots or cankers on the bark – get an arborist in if so!
Σ64, CC BY 4.0 https://creativecommons.org/licenses/by/4.0, via Wikimedia Commons
AEngelhardt, Public domain, via Wikimedia Commons