Japanese holly (Ilex crenata) is becoming more and more popular because of the decline of the mighty boxwood. Boxwood blight makes Buxus an unreliable choice, and Japanese holly’s ready to step in – it’s a really close alternative in terms of appearance, and it’s more hardy to cold temperatures.
However, I’ve been seeing more and more issues cropping up with Japanese holly. It’s not without its own problems. Stunted growth, sudden leaf drop, leaves turning yellow or brown, insect and pest woes… if you have any of these hollies you should know how to see off these issues – ideally BEFORE you have a sick or dying shrub. But if you’re too late, you should know how to revive it.
Japanese hollies that are stressed usually display prematurely yellow or brown leaves. Most frequently, this is related to lack of water, heat stress, winter desiccation, alkaline soil, or poor iron uptake. The lethal ‘black root rot’ is the most common and important Japanese holly disease.
Over time I’ve developed an extensive knowledge of Japanese holly. So I’m delighted to bring you a straightforward guide, that I’m going to try to keep clear of jargon.
You may have one of the popular Japanese holly cultivars – ‘Soft touch’, ‘Hoogendorn’, ‘Steeds’, ‘Snowflake’, ‘Compacta’, ‘Convexa’, ‘Sky pencil’, ‘Helleri’ or ‘Hetzii’, for example – there are many, but they share almost all the same issues, so read on.
My Japanese holly has yellow leaves
Is it normal for Japanese holly trees to have yellow leaves, or is it a sign of sickness? Yes, they’re ‘evergreen’, but all evergreens lose leaves – it’s just that they do it slowly and gradually, keeping them for a lot longer.
In normal circumstances, a few of the older leaves on a Japanese holly will turn yellow each year in spring or early summer, before falling off. These tend to be near the base of the plant and closer to the trunk, where they’re shaded out by outer foliage.
So if you only have a few yellow leaves and they’re not part of the outer foliage (particularly if they’re lower down) it’s likely there isn’t a big problem. But if you have
(a) a lot of yellow leaves (either a lot more than last year, or more on one holly than on another), or
(b) you have outer leaves that are yellow, OR
(c) if it isn’t happening in spring or early summer
Causes of yellow leaves on Japanese holly
1. Holly stress
Stress in Japanese hollies is most frequently caused by recent transplanting, lack of water, excess water or soil that’s too alkaline. Read on for more detail on how to identify and remedy these.
2. Iron deficiency
Much is made about holly trees’ tendency to exhibit yellow leaves when the soil’s low in iron, and Japanese holly’s no exception. Chlorosis is the term given when the leaves lack the green pigment chlorophyll and instead look a sickly yellow.
The amount of available iron in the soil is closely related to the pH level i.e. how acidic or alkaline the soil is. In neutral or alkaline soil (ph 7 or above) the holly roots struggle to take up iron. They do better in acidic soil (4.5-6.5).
So if you’ve got numerous leaves that are yellow, particularly if they’re on the outer plant or you’re seeing them outside of the normal spring/early summer, the first port of call is to test your soil’s pH to check if it’s acidic or not.
Every gardener should really know what their soil pH is, because it’s one of the biggest factors in knowing what will grow and what won’t. I use the Rapitest soil test kit (which will also give me nutrient levels) and, for quicker checks around the garden, the Rapitest Soil pH meter. There are lots of other meters on Amazon, but Rapitest uses the correct logarithmic scale, so it’s much more accurate and not too expensive either.
If the soil pH comes up neutral or alkaline (7 or above), you can try to acidify it by adding elemental sulfur to it – you can get it from nurseries or online. Lowering the pH makes the roots more able to take up iron, which may sort out your chlorosis problem.
If it’s acidic (pH <7), add iron itself to the soil. You can buy chelated iron for this job. Just make sure you follow all the manufacturer’s instructions and don’t overdo it.
My Japanese holly has brown leaves
In general, brown Japanese holly leaves (particularly brown tips) indicate a different set of issues than yellow leaves.
Drought and brown leaves
Japanese hollies are moderately drought-tolerant when they’re established for 2 or 3 years – young plants with developing root systems, particularly if very recently planted, will succumb readily to drought. The most common cause of browning foliage on Japanese holly is lack of water.
When these shrubs are turning brown, the first thing I’ll check is how frequently they’re watered – sometimes the answer is ‘never’! Hedges should not be expected to ‘go on’ by themselves, they really do need to be watered.
In general, for a young Japanese holly, you should water it weekly during the summer and once monthly during the colder months.
Yep, you should water in the winter as well – because Japanese holly is susceptible to winter injury.
Winter injury causing brown leaves
The second most common cause of brown leaves is winter injury, aka winter desiccation – which could also be termed windburn. This is closely related to drought.
Being evergreen, Japanese holly continues to lose water by evaporation (more accurately called transpiration) from its leaves all winter. This is particularly true in strong winter winds. The roots can struggle in winter to take up enough water to replace (especially if the ground freezes), and you get dried, shriveled, brown foliage – particularly at the tips and edges of leaves.
If the brown leaves appeared in late winter or early spring, winter injury is the likely cause.
Winter injury is typically asymmetrical – so the brown foliage is focused on the wind-exposed side.
The risk of winter injury is greatly increased by exposure to salt. Japanese hollies can look burned or scorched on the side that gets sprayed by road salt from a road or driveway
Preventing drought, winter injury and wind burn – best practices
If your Japanese holly is turning brown on the wind-exposed side, it’s important to help it recover and to protect it.
You can do this by:
- Making sure the holly is adequately mulched. This means spread a 3-4 inch thick layer of, for example, organic bark chippings, over the roots. The mulch insulates the roots against temperature changes, suppresses competition from grass and weeds, and helps rainwater permeate downwards. Try to prevent the mulch actually touching the main trunk through (to discourage rot).
- Watering your Japanese holly all year round. A deep soak once a week in summer and once monthly for the rest of the year (including winter) – unless the soil already seems very wet. Roots dry out surprisingly easily if your soil is reasonably free-draining, even in winter.
- Erecting some basic wind protection. Driving in some basic posts on the side of your holly that gets the wind, and nailing on some windbreak netting is a simple way to prevent your shrubs drying on that side.
- Spraying on anti-desiccant. Useful if you only have one or two plants. It coats the leaves, reducing water loss.
All of these three are particularly important in young Japanese hollies!
My Japanese holly is dropping leaves
Leaf drop in Japanese hollies usually results from severe stress or disease. The most common cause is recent transplanting, termed ‘transplant shock’, but it may be caused by a disease or pest.
If your shrub has dropped significant foliage in the first year after being planted, it’s probably transplant shock. This occurs because the roots, which are often damaged or severed in the process, are flung into a new environment. They take some time to adapt so that they’re able to take up water and nutrients normally.
It’s pretty common for Japanese hollies to die in the first year they’re planted!
The holly can react to this stress by shedding leaves – focusing on root establishment instead.
Hollies (and practically all other shrubs and trees) should be planted in autumn or winter, when the soil is cool and moist and the leaves and shoots aren’t actively growing. This gives the roots time to establish before the next growth season begins.
However, lots of Japanese hollies are planted in the summer when we’re all taking an interest in our yards and landscapers are most active. These shrubs are most susceptible to transplant shock.
Fortunately, hollies have significant powers of regeneration when they’ve lost their leaves (particularly the English holly), so your Japanese holly should survive if you do two things:
- Give it lots of water. Probably a large bucket or two a day in summer, reduced to weekly in the cooler months (yep, even in midwinter, as long as the ground isn’t frozen).
- Make sure there’s a good layer of mulch (organic bark chippings are great) under your shrubs. This helps water get down to the roots (instead of evaporating or running off) and prevents competition from weeds and grass.
Mulch is often neglected, but so important for healthy hollies. As with all trees and shrubs, just leave a 1-2 inch gap around the trunk so it isn’t touching it, as mulch piled up here can cause trunk rot.
Japanese holly and heat stress
Japanese holly tends not to thrive in USDA hardiness zones 8 and above, which includes the southern US states and the west coast. This type of stress can cause brown or yellow leaves, or foliage that looks brittle.
It might do better in partial shade in these areas, but if it’s in full sun, it may never make it. Your best bet is to water very frequently and applying plenty of mulch to the roots (at the risk of sounding like a broken record, this is soo important!).
Diseases and pests affecting Japanese holly
There are three types of infestation or infection that you need to know about if you have, or are going to plant, Japanese holly. They’re all fairly common.
Known as bot canker for short, this fungal disease is a cause of the foliage on entire branches turning yellow or brown and prematurely dropping off. Symptoms of another fungal canker disease, Phomopsis, are very similar.
If you see a large section of your holly turning brown or yellow all at once while other areas are spared, inspect along that branch closely all the way to the trunk, looking for an oozing break or swellings somewhere along it – this is the ‘canker’ that gives away the diagnosis.
Fungicides aren’t usually very effective – you’ve got to use cultural control i.e. prune off the affected branch at least a couple of inches closer to the trunk than the canker is. If cutting more than one branch, sterilize your pruners with something like rubbing alcohol or alcoholic hand sanitizer in between cuts, so you don’t spread the infection around.
Make sure you completely remove and dispose of anything you prune off – if left on the ground, spores can travel up to the living shoots again.
Black root rot
Black root rot, caused by the fungus Thielaviopsis, affects all holly species, but Japanese holly is the one you’ll most often see getting infected. It’s particularly susceptible.
Black root rot is the most important disease affecting Ilex crenata.
Root rot infections are tricky because by the time you get symptoms on the upper part of the plant (dieback, yellowing of a large portion of the tree, slowed growth, leaf drop) the roots are already completely infested.
Root rot diseases (including Phytophthora root rot, which is the best-known one – read more about it here) are much more likely to occur when the plant’s growing in soil that’s constantly saturated – typically at the bottom of a slope or in a gulley. Is there always a puddle of water around the roots?
There’s actually a very useful video on YouTube about black root rot in Japanese Holly. It’s a short but informative watch.
Unfortunately once the holly’s showing signs above ground, it usually has to be removed completely.
Nematodes are a type of worm that lives in soil. They’re so tiny that they’re only visible through microscopes. Japanese holly is one of the holly species that is susceptible to damage from nematodes (others like yaupon holly are resistant).
This is another one that’s hard to diagnose. You usually need to pull up a plant to look at the roots – they’ll have knobbly swellings (galls) along them.
Unlike black root rot, it affects hollies growing in free-draining soil the most (typically soil that’s a bit sandy). It doesn’t usually affect Japanese hollies growing in containers.
Nematode infestation causes dieback, yellowing leaves and poor growth, but sometimes minor infestation causes few symptoms and the tree can still thrive.
If you suspect nematodes are damaging your Japanese holly, this will require some specialist input – I’d contact a local arborist or your local extension office and ask for a call-out. Much of the management is likely to involve mulching, frequent watering and fertilizing.
Spots on Japanese holly leaves
If you have spots on Japanese holly leaves that are already yellow, this is simply an opportunistic fungus feeding on foliage that’s already weakened and dying. It’s of no consequence in itself as long as it isn’t spreading throughout the rest of the plant.
If you’ve got spots on leaves that are otherwise green, it’s probably Native holly leaf miner, which affects American, English and Japanese hollies. The spots look like irregular yellow, brown or purplish blotches, and you can occasionally appreciate linear ‘burrows’. They’re caused by tiny larvae (grubs) burrowing along the leaf. The spots are best seen in winter.
Holly leaf miner is hard to eradicate, and doesn’t cause much damage, so it’s best just to tolerate it. It’s good to rake up and dispose of fallen leaves that have these spots though, as it might reduce the amount of infestation the following year.
Could it be a soil problem?
Japanese holly’s really rather tolerant to most soil types, whether fertile or infertile, sand or clay – that’s one of the reasons it’s so popular. The only thing they don’t like is alkaline soil (pH >7), particularly as this makes it difficult for the roots to take up iron from the soil, leading to yellow leaves (chlorosis). See the section above – ‘iron deficiency’, which includes advice on how to test and alter your soil’s pH.
However, in clay soil, they’re more likely to get stress due to poor drainage and be predisposed to the dreaded black root rot.
In sandy soil, they’re more likely to suffer drought issues and suffer from nematode infestation of the roots.
Japanese holly – how much water?
Japanese holly’s more tolerant to drought than most similar plants, but it really does need regular watering for a few years after planting.
I usually recommend a once-weekly soak with a hose during the summer (20 minutes or so), then from autumn to spring, a monthly water of the same duration.
In hot, dry weather, increase it to twice weekly (especially if you’re in USDA hardiness zones 8 or higher, where Japanese holly starts to experience heat stress – here’s a link to the US map again and one for the UK.
Alternatively, you can go by the soil itself- just use your finger (or a trowel) to dig down about 2 inches into the soil under your holly. If it feels moist and sticks to your finger, the plants don’t need water right now.
You’re overdoing the watering if you’re seeing puddles of water persisting under your hollies. This matters, because overwatering’s a major cause of stress and prematurely yellowing/dropping leaves in hollies.
Remember the critical importance of applying mulch over the roots, which helps the water get down to the roots (instead of simply running off the soil) and keeps down weeds and grass, which compete for moisture.
Will fertilizer help?
It probably will. Remember that hollies that are stressed for whatever reason – including lack of nutrients – are more likely to pick up other problems and become diseased or infested.
I’d recommend a slow-release ‘complete’ fertilizer – one that contains nitrogen, phosphate and potassium, which are the three most important components. Just make sure you follow the manufacturer’s instructions (too much fertilizer can harm the tree too!).
Miracle-Gro All Purpose continuous-release plant food is the stuff I recommend for Japanese holly.
Frequently-asked questions about Japanese holly
Will Japanese holly grow in shade?
Yes, Japanese holly is fairly shade-tolerant. It will survive in full shade, but is more likely to thrive in partial sunlight. In hotter areas, such as the southern US states, Japanese holly can struggle in the heat, so is more likely to do grow well if it receives at least partial shade.
How and when to prune Japanese holly
Japanese holly can tolerate harsh pruning, though it’s probably not quite as forgiving as the very similar-looking boxwood. The best time to prune is in late winter or early spring before the growing season begins. Dead or dying branches can be pruned off at any time.
If growing a Japanese holly hedge, I’d recommend pruning the sides only until your hedge gets to the desired height. You could use two-handed shears or a motorized hedge trimmer, depending on how much you need to prune!
Pruning is good for Japanese holly, particularly if you’re hoping to have dense tree or hedge eventually – it actually stimulates new branches and shoots. ‘Growth follows the knife’ as the old gardening saying goes.
Does Japanese holly get berries?
Yes, female Japanese holly does get berries, though not the classic red type that we associate with Christmas. The flowers are tiny and the berries that follow are small and black (though there’s some color variation with certain Japanese holly cultivars). They appear in summer and ripen in late autumn.
Are Japanese holly deer resistant?
Yes, happily, Japanese holly are not usually browsed by deer. It’s similar to boxwood in this respect.
Are Japanese holly poisonous to humans or animals?
Japanese holly carries low toxicity to humans and animals, with ingestion likely to cause gastrointestinal upset.
I wasn’t able to find any published cases of animals or humans coming to significant harm from this plant.
Does Japanese holly have invasive roots?
There’s a very low risk that Japanese holly’s roots will cause any disruption to paving, foundations, pipes, septic tanks or anything else. It’s regarded as a good choice for growing near to a wall or along a driveway.
What is Japanese holly’s lifespan
Japanese holly could live 100 years or more in ideal conditions. Most don’t die of old age though – instead they typically succumb after an unusually dry summer, or to disease – particularly black root rot.
Japanese holly vs boxwood – which is best?
Boxwood decline is unfortunately becoming so common that I wouldn’t plant one now. While it’s a little slower-growing, for the long run I’d go for Japanese holly every time.
harum.koh from Kobe city, Japan, CC BY-SA 2.0 https://creativecommons.org/licenses/by-sa/2.0, via Wikimedia Commons
KENPEI, CC BY-SA 3.0 http://creativecommons.org/licenses/by-sa/3.0/, via Wikimedia Commons
Alexis, CC BY 4.0 https://creativecommons.org/licenses/by/4.0, via Wikimedia Commons
Forest and Kim Starr, CC BY 2.0 https://creativecommons.org/licenses/by/2.0, via Wikimedia Commons