The RHS advisory team responded to more than 100,000 gardening inquiries in 2021, spanning everything from warm-weather watering advice to disease management. Here are their answers to the ten most asked questions.
1. My houseplant is yellow — what’s the problem?
Overwatering kills more plants than any other cause, but yellowing can be due to cold or draughty conditions, or more commonly, a lack of nutrients, notably nitrogen and iron, or magnesium. To confuse matters, poor root function can prevent plants acquiring nutrients and this is usually related to overwatering and subsequent root death.
To investigate, tip the plant from its pot and examine the roots. If the roots are pale and firm, repot and feed with a balanced liquid fertiliser. If the roots are in good health but congested, repot into a larger pot with fresh potting compost or replace in the same pot after removing a quarter of the potting compost and replacing with fresh material.
If the roots are dark, brittle and rotted, and the potting media excessively damp and perhaps with a sour smell, overwatering is to blame. Again, repot with fresh potting compost. Foliar feeds (dilute fertiliser sprayed onto the leaves) can help the plant to grow new roots.
In hard water regions, alkaline water leads to excess calcium in the root zone that inhibits uptake of nutrients, especially iron. Ideally water with rainwater or deionised water to avoid this, but where this is not possible, treat plants with chelated iron fertiliser.
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2. My tomato plants died last year. What can I do to protect my crop this time round?
Last year there was a reported five-fold increase in tomato blight — a fungal infection that appears as dark marks on stems and brown blotches on fruits with leaf rots. Once it takes hold a plant rarely survives beyond a week. Tomato blight appears and spreads in warm, wet late summer conditions. No fungicides are available to gardeners to control blight — gardeners can only save what fruits they can.
For subsequent years try rotating crops to reduce the risk of potential infection from resting spores, and avoid growing potatoes and tomatoes in areas where blighted plants occurred the previous summer. Potatoes left in the ground or dumped can regrow in spring and are a potent source of blight infection. Infected material should either be deeply buried (below the depth of cultivation), consigned to the local council green waste collection (if allowed) or recycling centre, or burnt rather than composted.
Clean any garden equipment or plant supports that have previously come into contact with blight with a disinfectant. Hard surfaces and the glass in greenhouses can be cleaned in the same way.
3. I hear a lot about mulching in spring — what is it and what are the benefits?
Mulching is covering the soil with a light-proof material that suppresses weeds, slows evaporation and in some cases improves the soil. It is common in spring before weed seed germination peaks. Bulky organic mulches of wood chip or bark suppress weeds well, especially around trees and shrubs, and degrade slowly. As they rot, worms and other soil organisms move the organic matter into the soil, improving its texture.
Other materials, such as composted manure or mushroom compost, are rich in plant nutrients and feed the plants, but being finely textured they are less effective at preventing weeds and degrade quite quickly. They are especially useful around vegetables and roses that need high levels of nutrients.
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Confusingly, some opaque sheets, such as black plastic sheets or special paper, are called mulches — they are very effective at weed control but do not improve soil.
4. Some of the shrubs in my garden need cutting back as they’ve more than doubled in size. When is the best time to do this?
Shrubs are some of the most robust plants and can recover from pruning in any season in most cases. However some pruning seasons are better for flower production than others.
Evergreen shrubs in need of hard cutting-back are best treated in late winter or early spring. Otherwise pruning after flowering is usually best. Bear in mind that pruning must retain some flowered shoots if berries are expected — such as with pyracantha, for example.
Late summer-flowering deciduous shrubs bear flowers on new shoots. Shortening all shoots by two-thirds in late winter and early spring promotes plentiful strong new shoots.
Spring and early summer deciduous flowering shrubs — Philadelphus, for example — are pruned after flowering by removing one stem in four to near ground level, choosing the biggest, oldest shoots to chop. For particularly large plants cut out one in three or even one in two to reduce size.
5. I live in a rental property and want to grow larger plants in pots. What does well in a container and what should be avoided?
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Trees and shrubs, including climbers, are long-term container plants that renters might like to take with them when they move. Shrubs, including dwarf conifers, are the easiest, being very robust plants. Some of the best like acidic soil, such as camellia and rhododendron — acid soil is easier to provide in pots than attempting to change garden soil. Some shrubs resent dry soils and hate water-logged ones even more: Japanese maples and daphnes for example. These conditions can be hard to avoid in pots so these plants are perhaps best avoided.
Roses and climbers take less well to life in pots than other shrubs, but smaller patio types are reliable choices. However, less usual plants, such as Phormium, hardy yuccas and, if winter protection is available, succulents such as agave, make very satisfactory larger container specimens.
Trees often do very well in pots — the restricted root zone stunts them to some degree and is characterful, like a larger form of bonsai. Grow as for shrubs — birches and hardy palms are good subjects.
Repotting every two or three years in late winter, either into a bigger pot or back into the same one after replacing 25 per cent of the potting media with fresh material, is very effective at keeping long-term plants healthy.
A word of warning — vine weevil can be a menace in pots. Counter these with a late summer drench with specialised nematodes.
6. My rose’s leaves had unsightly black markings on them last year – what can I do to ensure they remain green and healthy?
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Rose black spot is a common fungal complaint, disfiguring but seldom killing most roses — including hybrid teas, floribundas, climbers and patio types.
Many gardeners tolerate a certain level of black spot and practice good plant hygiene to limit its spread and return the following year — collecting and destroying or burying fallen affected leaves, and pruning out stems with lesions in spring. The estimated three million people new to gardening, however, are often understandably alarmed.
Where the problem is widespread and cultural management is not effective, fungicides are available. These should be used exactly as directed by the manufacturers and used only in a minimal and highly targeted manner.
Many new introductions are bred for resistance — however they tend to fall victim after some years, so respite is only temporary. Certain older roses: the 1949 climber ‘Aloha’; species such as Rosa rugosa; and some groundcover roses, notably the ‘Flower Carpet’ series, are also less susceptible.
7. My box plants have been stripped of their leaves – what should I do?
Box tree moth caterpillars, the larvae of box tree moths, start feeding within the canopy of box trees, bushes and hedges and can strip the remaining foliage. Box plants can often survive extensive damage and regrow, but plants can die if the bark is eaten or defoliation occurs over several years. A relatively new introduction to Britain, first reported in private gardens in 2011, the caterpillars continue to extend their range from southern England. Alternatives to box could be a sensible precaution in your garden — try holly, Pittosporum or Berberis.
8. How can I minimise plastic use when gardening?
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Plastic garden equipment and materials are inexpensive, lightweight and easy to maintain but not particularly durable. Disposing of garden plastics is not always easy, particularly due to contamination by soil, which can prevent recycling.
Pots are perhaps the commonest plastic waste, but some pots and cell trays are recyclable. Look for plants in grey pots called taupe, that more and more recycling services can separate, unlike black pots. Transparent trays can also be recycled. When buying pots and seed trays, reasonably durable ones made of bamboo and other biodegradable options are available.
Using more durable products that have many seasons of potential use is less problematic than single-use products. Glass cloches and metal watering cans, for example, have a higher initial cost but potentially many years of service.
9. What’s eating my plant?
Holes in leaves, fruit and stems and ragged leaf edges have many causes. Slugs and snails are perhaps the most frequent grazers, sometimes leaving behind a telltale shiny mucous trail. Larvae of many insects can feast on leaves, including caterpillars, sawfly and some beetle larvae. Very often “frass” (insect faeces) is present even though the larvae are camouflaged and hard to spot in the day. As the larvae mature, they can devour leaves voraciously, but by the time the gardener notices they will often be long gone. Ripped leaf edges can indicate bird damage, and if larger stems are severed, inspect the ground for the cloven hoof marks of deer or the footprints of rabbits.
The best advice in a healthy garden is to turn a blind eye to low-level damage, remove visible signs of problem nibbles by hand and make your garden an attractive place for larger insects, birds and mammals, who will delight in helping to keep numbers under control.
10. The fruit on my trees appeared brown and shrivelled last year. What was the problem and will it happen again?
Brown rot is a particularly distressing fungal disease of apples, pears, plums, cherries and some other fruit and ornamental trees, causing a brown, spreading rot as the fruit approaches maturity or later in storage, cruelly crushing gardeners’ expectation of a good harvest. It is caused by the same fungi that cause blossom wilt of the flowers and fruit spurs.
The most practical strategy is to minimise the carry-over of the pathogen to the following year. This includes pruning-out trees and disposing of infected spurs and blossoms to reduce the amount of fungus available to infect fruit; removing and disposing of all brown, rotted fruit promptly; and considering replacing persistently affected plants with less susceptible cultivars.
