The reasons your roses are failing, and what to do about it

Yahoo

08-02-2025

There are many reasons why your garden roses can get sick and die. The National Trust discovered this when it asked top rosarian Michael Marriott to investigate the ailing rose beds at five of its top properties: Mottisfont in Hampshire, Sissinghurst Castle Garden in Sussex, Polesden Lacey in Surrey, Nymans in Sussex and Morden Hall Park in south London.
Marriott, an internationally recognised specialist, diagnosed common causes that afflict roses, whether they are planted in private or public gardens. A major issue is impoverished soil from a build-up of fertilisers applied over many years, or over-zealously. This is particularly true of animal manures, which we are told roses need but which lead to an imbalance of nutrients in the soil.
Marriott also found that soil was inadequately mulched, leaving it exposed to adverse weather and to harmful compaction from being walked on when gardeners were pruning roses. Other major factors include rose varieties simply not being robust enough to cope with pests and diseases and climate change, and plants that are too old. Traditional monoculture beds, planted only with roses, as at National Trust gardens, also lack the diversity needed to fight off pests and diseases.
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The symptoms
The reasons
The solutions
The following conditions indicate that your roses are struggling.
For example, plants that produce few blooms that are over too quickly, or repeat bloomers flowering only once.
Polesden Lacey has 28 monoculture rose beds dating from the 1920s. The beds have suffered between 40 to 50 per cent losses, with the rest in poor health.
'When I started here in February 2023 the roses were really poorly,' says head gardener Natan Cointet. 'If you have a healthy rose it'll send out new shoots, but there wasn't much of that going on. The growth that was left was quite old and there weren't many flowers. I dug one up and the roots were rotting.'
At Nymans, the original rose garden dates from the 1920s. It was expanded in the early 1990s, and up until two years ago there were more than 600 rose plants including ramblers, damask, climbers, and hybrid musks, says Joe Whelan, head gardener. In 2005 a large quantity of David Austin repeat flowering roses were added.
Whelan says: 'Roses are tough work at the best of times and you always expect to lose and replace a few each year. However, since about 2021 we were starting to lose quite a lot each year to the point a plant would be so unhealthy there'd be no new growth. They were so unhealthy they weren't doing anything, they had no vigour and they weren't flowering.'
The tips of the stems shrivel and turn black.
Leaves falling off of rose bushes can be caused by a number of things including fungus, heat and a lack of water.
This includes attacks from aphids and rose sawfly, and rampant black spot, rust and powdery mildew.
These were the main issues that Marriott unearthed at the National Trust gardens.
Compacted soil: 'If the soil isn't right, that's your starting point,' says Marriott. He always digs a hole of a spit's [spade's] depth for inspection. For example, a rotten eggs smell might be poor drainage; a lack of worms suggests infertility or, as at the National Trust gardens, compacted soil forming hardpans (dense layers).
'With a hardpan, when you plant a rose, if the roots can't go down they grow sideways,' he explains. 'This caused major problems with plants in the hot, dry summers of 2022 and 2023 because their roots hadn't gone down enough to get water, nutrients and anchorage.'
Over-fertilised soil: Tests at the gardens revealed high levels of phosphate and potash [phosphorus and potassium], preventing plants imbibing essential micronutrients including iron, magnesium and zinc. The imbalance is caused by an excess of chemical fertilisers and animal manures, applied over many years or used excessively. Animal manures, traditionally recommended for roses, are high in phosphate and potash.
Exposure: Inadequate mulching and interplanting between roses leaves bare soil which compacts from being walked on and suffers weather-induced hard crusts and erosion.
Many varieties at the National Trust gardens are not tough enough to cope with our changing weather, or with pests and diseases without chemical help.
'A lot of roses introduced during the 20th century don't have a good level of disease resistance because they were bred at a time when fungicides and pesticides were used,' says Marriott. Some varieties are unable to cope with increasingly hot, dry summers and excessive rainfall. Roses like sun and a moist but well-drained soil, but at Mottisfont, for example, the walled rose garden gets too hot for some varieties and the garden also suffers from waterlogging due to inadequate drainage.
Species roses – original wild roses – can live for a hundred years. But many, especially hybrid perpetuals and bourbons, deteriorate after 15 to 20 years, and once-flowering varieties after 20 to 30 years, says Marriott. 'I've seen roses in the ground which in human terms are 150 to 200 years old, well past their sell-by date.'
The following are Marriott's suggestions for improving rose performance.
'Go round and score the roses for flowers, vigour, health, etc. If one has produced half a dozen flowers and dropped its leaves, think why: is it too old, overcrowded, or a poor variety? You've got to assess the whole thing.'
Phosphates and potash linger for years, and soil naturally contains both, so if you have an excess, leave them to dissipate naturally. 'If the [soil analysis] index for phosphate and potash is two or three that's fine, you can carry on fertilising. If it goes up to five or six, you shouldn't be putting in phosphate or potash. Nitrogen is important so apply an organic, slow-release version such as dried blood fertiliser,' says Marriott.
Polesden Lacey is implementing a cover-cropping programme – adding plants to cover the soil – with sweet clover in rose beds. Clover's long roots break up pans and absorb phosphorous, so when it is dug up it removes the excess.
Cointet says: 'Hopefully by autumn 2025 we'll notice that the soil health is better. It's a succession of taking out older roses that are sick and dying, cover cropping, then replanting with newer types of roses.'
This will protect soil, boost biodiversity and add nutrients. Marriott recommends phacelia. Other options include geraniums, Salvia nemerosa, nepeta and lavender.
This will help improve resilience, but avoid planting too close to the rose base, says Marriott: 'For most shrub roses 4ft tall by 4ft across, if you have the other plant about 30 to 40cm from the base, the roots won't interfere with the rose.'
Cushion it against compaction and retain moisture. Partly decomposed bark is good because it does not break down too quickly. A mulch needs to be at least two to three inches thick to be effective.
Marriott suggests applying foliar feeds, such as organic seaweed-based liquid feed, which boost the immune system of leaves: 'You're only putting it on the leaves, so even though the foliate feed might contain phosphate and potash, nothing reaches the soil.'
'If you start controlling pests it breaks up the eco system and beneficial insects that help to control them will die of starvation,' says Marriott. 'I have Rosa 'Adélaïde d'Orléans' in my own garden which is one of my favourites. One day I noticed there were bad outbreaks of aphids on the young stems. I didn't do anything about it and the next day I walked past and they'd been eaten and were gone. Just hold your nerve.'
Breeders are producing tough varieties now, says Marriott. For example, David Austin; Dickson; Kordes; Noack; Tantau; Velle-Boudolf (Louis Lens nursery); and Warners. Select varieties to suit your needs.
Polesden Lacey has already removed Rosa 'Frensham'; 'Lady Marmalade'; 'Freedom'; and 'Lovely Lady' and started replanting with Rosa 'Ice Cream' and 'Bride & Groom', chosen for good disease resistance and compact shape. It has also selected 'Queen of Sweden' and 'Desdemona'.
Cointet says: 'Modern shrub roses are a lot more robust, and easier to look after, so we shouldn't have the same problems again. It's a 1920s rose garden so it's not easy to have to say that 1920s roses aren't going to cut it any more, but we have to make it future-proof.'
Michael Marriott's report will be published in early March 2025.