RHS members' top gardening queries, 2021

The RHS Gardening Advice team responded to more than 100,000 gardening enquiries in 2021 – here are the 10 most frequently asked questions, to help you get ahead of potential problems this year.

1. My houseplant is yellow – what’s the problem?

Unfortunately plants can respond to stress only in a limited number of ways – so yellowing can be due to cold or draughty conditions, but more commonly, insufficient nutrients is to blame.

Yellowing foliage indicates a lack of nutrients, notably nitrogen and iron. Occasionally it can be other nutrients, magnesium for example, if the lower leaves yellow first. To confuse matters poor root function can prevent plants acquiring nutrients: 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 very congested take the opportunity to repot into a larger pot with fresh potting compost or replace in the same pot after removing 25% 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, over-watering is to blame. The plant can often be saved by removing dead roots and associated compost and repotting, perhaps in a smaller pot, 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. This inhibits uptake of nutrients especially iron. Ideally water with rainwater to avoid this, but where this is not possible treat plants with chelated iron fertiliser which can be taken up by plants despite the alkaline root zone.

The RHS gardening advice team find over-watering kills more plants than any other cause!

See also: Leaf damage on houseplants

*Mostly London and South East England. The presence of limescale in your kettle is a good indicator of hard water.

2. My tomato plants died suddenly last year. What can I do to protect my crop this time round?

2021 saw a five-fold increase in tomato blight – a fungal infection that appears as dark marks on stems, brown blotches on fruits and leaf rots. Once it takes hold a plant rarely survives beyond a week. 

Tomato blight appears and spreads in warm, wet late summer conditions and with last summer notably wetter than in previous years, reports of the problem peaked between July and October. 

Unfortunately no fungicides are available to gardeners to control blight. When weather conditions favour blight gardeners can only save what fruits they can. 

For subsequent years gardeners can try rotating their crops to reduce the risk of potential infection from resting spores, avoiding growing potatoes and tomatoes in areas where blighted plants occurred the previous summer. 

Infected material should be deeply buried (below the depth of cultivation), consigned to the local council green waste collection (if allowed), recycling centre or burned, rather than composted. 

Potatoes left in the ground or dumped can regrow in spring and are a potent source of blight infection. Destroy any unwanted or unusable potatoes by deep burial, burning or via the council’s recycling facility or green waste bins. 

And it’s important to clean any garden equipment or plant supports that have previously come into contact with blight with a disinfectant such as Jeyes Fluid before re-use, to make absolutely sure that there is no disease transfer. Hard surfaces and the glass in greenhouses can be cleaned in the same way.

Gardeners can also try growing tomato cultivars that claim resistance to the disease (see seed catalogues or packets for details). Although seldom immune, these can offer enough protection for a worthwhile crop to be gathered.

3. I hear a lot about mulching in spring – what is it and what are the benefits?

Mulching means covering the soil with a lightproof material that suppresses weeds, slows evaporation and in some cases improves the soil. It is commonly done in spring before weed seed germination peaks.

Bulky organic mulches of woodchip or bark degrade slowly and suppress weeds well, especially around trees and shrubs. 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 as well as improve the soil. However, as they are finely textured they are less effective at preventing weeds and degrade quite quickly. They are especially useful around vegetables, fruit and roses as these need high levels of nutrients.

Confusingly, some opaque sheet materials such as black plastic [ideally biodegradable], or special paper, are called mulches – they are very effective at weed control but do not improve soil.

4. My shrubs need cutting back, when is the best time to do this?

Shrubs can grow fast, particularly in years with good rainfall – such as in 2021 in the south – blocking paths, obscuring windows, spilling over lawns and smothering other plants. 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 (camellias and choisyas for example), or in spring for winter-flowering types such as mahonias. Bear in mind that pruning must retain some flowered shoots if you want berries – 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.

Prune spring and early summer deciduous flowering shrubs, philadelphus for example, after flowering by removing one stem in four to near ground level. Choose the biggest oldest shoots for the chop. For particularly large plants cut out one in three (or even one in two) to reduce size.

See also: RHS guide to pruning shrubs

5. I'm renting and I want to grow big plants in pots: any suggestions?

Trees, shrubs and climbers are long term container plants that renters can take with them when they re-locate. Shrubs, including dwarf conifers, are the easiest, being very robust plants. Some of the most popular types such as camellias and rhododendrons also like acidic soil. Acid soil is easier to provide in pots (by using ericaceous compost) than attempting to change garden soil. Some shrubs resent dry soils and hate waterlogged 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.

Ideally use soil based potting compost such as John Innes No3 or peat-free material with extra loam or John Innes compost added. Pots over 45cm in diameter are very heavy and can be hard to move especially if filled with soil based potting compost.

Roses and climbers take less well to life in pots than other shrubs, but smaller patio types are reliable choices. On the other hand certain less usual subjects such as phormiums, hardy yuccas and, if winter protection is available, succulents such as agaves make very satisfactory larger container specimens. 

Trees often do very well in pots – the restricted root zone stunts them to some degree and a stunted tree 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% of the potting media with fresh material, is very effective at keeping long term plants healthy.

Word of warning – vine weevil can be a menace in pots. Counter these by a late summer drench with special nematodes (biocontrol).

See also: How to garden when you rent

6. My rose’s leaves had unsightly black markings on them – what should I do?

Rose black spot is a common fungal complaint that disfigures many 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. The estimated 3 million people new to gardening however are often understandably alarmed, however. Collect and destroy or bury fallen affected leaves and prune out stems with lesions in spring. 

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 rose varieties are bred for black spot resistance and only lightly affected. The new introductions however tend to fall victim after some years, so respite is only temporary. Certain older roses, the 1949 climber ‘Aloha’ for example, species such as Rosa rugosa, and some groundcover roses, notably the ‘Flower Carpet Series’,  are also less susceptible.

See also: RHS find a plant

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 leaving mere skeletons. Box plants often can survive extensive damage a regrow, but plants can die if the bark is eaten or defoliation occurs over several years. 

First reported in private gardens in 2011, box tree moth continues to extend its range from southern England. Many gardeners have been encountering this moth caterpillar for the first time as it spreads – so alternatives to box could be a sensible precaution in your garden and include holly, pittosporum and berberis.

See also: RHS podcast – The A-mazing guide to hedges

8. How can I minimise plastic use when gardening?

Plastic offers inexpensive and lightweight but not particularly durable garden equipment and materials. Disposing of garden plastic is not always easy, due to contamination by soil which can prevent recycling.

Sundries such as labels and string are particularly important. String at least has many natural forms but replacing labels is trickier – some wooden labels may remain legible for a useful period.

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 and consign to the recycling system, unlike black pots. Transparent trays can also be recycled. Reasonably durable pots and seed trays 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 very many years of service.

See also: How to go plastic-free in your garden

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 tell-tale shiny mucus trail – although the culprits are usually well hidden by day. Larvae of many insects can feast on leaves including caterpillars (moths and butterflies) and sawfly and some beetle larvae. Very often 'frass' (insect faeces) are present even though the larvae are camouflaged and hard to spot. 

These larvae tend to grow slowly at first so are often overlooked. As they mature they can devour leaves voraciously, although by the time the gardener notices they will often be long gone. Woodlice and millipedes might be found near the damage caused by other invertebrates, however these valuable recyclers are usually only taking advantage of already damaged and rotting plant material.

Ripped leaf edges can indicate bird damage, and if larger stems severed inspect the ground for the cloven hoof marks of deer or 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 keep the plant munching bug numbers under control.

See also: Bring wildlife into your garden

10. The fruit on my trees appeared brown and shrivelled last year. What was the problem?

Brown rot is a particularly distressing fungal disease of apples, pears, plums, cherries and some other fruit and ornamental trees. It causes a brown, spreading rot in fruit, 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. While frequent frosts in April 2021 reduced the amount of blossom, thereby further reducing the incidence of blossom wilt, subsequent moist summer weather conditions were conducive to development of brown rot in fruit. 

As with management of most plant diseases in home gardens, the most practical strategy is to minimise the carry-over of the pathogen to the following year. With trees this includes pruning out 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.

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