Thompson & Morgan Gardening Blog

Follow our tips to keep your geraniums healthy
Image: Geranium ‘Jackpot Mixed’ F1 Hybrid from Thompson & Morgan

Geraniums are extremely tolerant plants and known for being reliable, sturdy growers that perform well with very little attention, which is ideal! However, from time to time problems can crop up and we have put together the following ‘troubleshooting’ section to help you get the very best from your plants. Thankfully there aren’t many pests that are attracted to geraniums and most problems are easily treatable so don’t panic if you encounter any problems – you don’t have to throw your lovely plants away!

Click on the topic below which best describes the problem:

1. Holes in geranium leaves
2. Small white flies on pelargoniums
3. Small green flies on geraniums
4. Small black flies on the soil
5. Brown patches on the leaves of zonal geraniums
6. Brown marks on the backs Ivy Leaved geraniums
7. Grey mould on geranium leaves and/or stems
8. Geraniums not flowering
9. Geraniums not thriving
10. Rotted geranium stem
11. Lower geranium leaves turning yellow
12. Cauliflower-like growth on geranium stem
13. Geranium Plant collapses and dies

Holes in geranium or pelargonium leaves

This is usually caused by caterpillars. There is a moth that can appear about August or September that will chew the leaves of the zonals which needs catching in the evenings or eradicating with a systemic insecticide. Geraniums are rarely affected by slugs and snails.

Whitefly on pelargoniums

Whitefly can be a problem with the regal and sometimes the scented pelargoniums, though they do not actually damage the plant. Garden Centres are loaded with insecticides to combat this pest, but it is a case of persevering during the warm weather, as they breed very rapidly. We spend a huge amount of money to keep this pest under control – and it still pops up again! Try using “Provado”, it could help here!

Greenfly on geraniums

Greenfly are more of a problem than Whitefly, as these DO damage the plant as they can distort the leaves and spread quickly. Obtain a ‘systemic spray’ from the hardware shop (systemic just means the spray gets into the plant system and the fly eat the leaves and get the insecticide into them). Spray the whole plant, particularly under the leaves and the compost too. Best to isolate the plant if possible to stop them spreading or spray all plants so they are all protected.

Sciarid flies

These are small black flies which you will see on the surface of the compost, and their larvae can damage the roots. They can thrive in peat composts, but are not normally so active that they kill the plants. Once their life cycle moves on, they disappear, so are only a nuisance for about two months in the year. Drenching with a weak solution of Jeyes Fluid will usually put an end to them. Correct watering – keeping the soil moist but never wet – will help to keep them away.

Pelargonium rust

Pelargonium rust can affect many varieties of geranium and pelargonium.
Image: PlantpathfindCC BY-SA 3.0 via Wikimedia Commons

Pelargonium rust can affect the zonal varieties, and it is getting everywhere nowadays. It first came into the country in 1964 and has gradually spread from Eastbourne, where it was first detected. It only affects the zonal types, and particularly thrives during a damp summer or autumn. However, it is not “life threatening” to the plants, and luckily it does not seem to infect the plants very rapidly, so simply removing the affected leaves will be a good control. We also advise spraying with a suitable fungicide usually available at Garden Centres.

Make sure you spray the underside of the leaves and the compost too so that all spores are treated. Within a day or two take off the affected leaves and either burn or put into the dustbin – do not put them on the compost heap. We do not recommend destroying your plants, as pelargonium rust is only a fungus, much like grey mould or botrytis, and is now endemic in this country, so any new plants you get will most likely suffer from it sooner or later. Ivy leaf geraniums never have rust, only zonals are ever infected.

Oedema

If you see brown marks on the backs of the leaves of your ivy leaved geraniums, you’re probably seeing a physiological disorder called Oedema, rather than a disease. This often affects the older leaves of ivy or hybrid ivy types and is caused by erratic watering. If the plants have got rather dry and are then watered the stomata on the back of the leaves cannot always cope, and they burst. Afterwards they callous over, so what you see is like a scar.

We suggest removing any leaves that look unsightly – the new leaves that grow will not have it. Be careful to keep the roots of the plants moist at all times, especially at the times of the year when they are growing rapidly and are transpiring a great deal. Moist, but never waterlogged, is the golden rule.

Grey Mould

Grey mould, or botrytis, to give it its proper name, is a nuisance once autumn arrives. Damaged leaves or dying flowers will begin to rot once the cold, damp days arrive, and petals falling onto leaves can cause damage. The answer is threefold. Step one is to make sure there are no damaged leaves or flowers on the plants, the second is to supply adequate ventilation so that there is movement of air, and the third is to visit the Garden Centre to buy a fungicide designed to combat grey mould. Smokes are preferable in a greenhouse, because they do not increase the humidity, but are not practical in a porch or conservatory.

Geranium plants not flowering

If plants are not flowering, check the following:

1. What type of geranium is it? Regal and Angels naturally flower for a shorter period than other types.
2. Is the plant growing well – bushy, healthy and happy looking?
3. What feed is it getting? The best feed to boost flowering is high potash – tomato feed is good for encouraging flowering.
4. Light – the better the light, the more geraniums will flower.
5. Watering – is the plant getting enough to drink? The soil should moist at all times but never leave the plant sitting in puddles of water!
6. What size pot is it in? – if the plant is in a huge pot it will be filling the pot with roots and not be concentrating on flowering. Try reducing the pot size – this restricts the roots so plant put its effort into flowering.

Geranium plants not thriving

If your geraniums and pelargoniums are not thriving, check the following:

1. Have a look at the roots – Take the plant out of the pot and have a look at the roots. If they’re white then all is healthy and fine. If browning then the roots are dying.
2. What compost is it growing in? It needs to be in general purpose compost – not bark based or coir as that will hold too much moisture.
3. Is the compost stale and compacted? Try replanting into fresh compost.
4. What feed is it getting? The best feed for foliage growth is a balanced fertiliser – our geranium fertiliser is good for foliage and flowering.
5. Light – the better the light, the more geraniums will thrive.
6. Watering – is the plant getting too much water? If the plant is too wet it will ‘drown’ as the roots need to have air around them for the oxygen.
7. Look for sciarid fly – if there are small black flies on the surface of the compost, their larvae may be damaging the roots. They can thrive in peat composts, but are not normally so active that they kill the plants. Once their life cycle moves on, they disappear, so are only a nuisance for about two months in the year. Drenching with a weak solution of Jeyes Fluid will usually put an end to them. Correct watering – keeping the soil moist but never wet – will help to keep them away.

Stem rot

Geraniums are prone to stem rot, and we find that it is more likely to happen in very hot weather. It’s caused by a soil-borne fungus, and if the plant’s container gets hot, it seems to give rise to the trouble. From our experience, it also seems to occur if the plant has dried out too much, and is then copiously watered.

Yellowing geraniums

If you spot yellowing of the bottom leaves of your geranium plants this can occur for any of several reasons:

1. Insufficient light is reaching the lower part of the plant. This is probably the cause of the problem if the plants are too close together, or are too far from a good source of light. If you use a photographer’s light meter, you will discover that being three feet in from a window will reduce the light level by 50%!
2. The plants are receiving insufficient water at the roots. Although all of the pelargonium family will rot in a humid atmosphere, it is a mistake to think that they need to be kept dry at the roots. They are never dormant, so require moisture all the year round to transpire, but less, of course, in winter and in dull weather. When bone dry the stems go hard and woody, and the plant never grows as well – it is always best to renew the plant with fresh cuttings when this has occurred.
3. The plants are drowning! Too much water will exclude the oxygen from the roots, causing them to die. It is said that 90% of house plants are killed from over watering. Never be afraid of taking a plant out of it’s pot to see what is happening to the roots. Sometimes it is possible to take a cutting off the top of a plant if it is only rotting at the bottom.
4. The plants have been moved recently and are adjusting to their new environment.

Leafy gall

This is a strange, cauliflower-like growth that occurs where the stem enters the soil, and can occur on any of the pelargonium family. This problem is a complete mystery, as nobody has yet found the cause, so therefore there is no cure. It occurs completely indiscriminately – the first time we found it, in our early days of growing pelargoniums, we rushed off to a nurseryman and said “Look what we’ve found!” “Oh yes,” he said, “I just break that off and throw it away.” And this is still the only thing one can do. The plants continue to grow quite normally once it is removed, and cuttings taken from those plants do not necessarily have it – it just occurs as and when it feels like it!

Geranium plant collapse

Sometimes geranium plants suddenly collapse and die. This is know an ‘Plant collapse’ and has two main causes:

1. Vine weevil. This is a pest which seems to be on the increase and is difficult to eradicate. We know that fuchsia growers also are very concerned about it. A garden centre is the place to go to for advice on suitable chemicals to combat the pest. We think they will probably recommend a product from PBI called “Provado”, but they might have other suggestions. We know of one nurseryman who recommends letting bantam chickens loose in the greenhouse! Levingtons do now produce a compost that will kill vine weevil, but that would entail washing the roots and repotting everything. We are sorry there is no magic remedy to this problem.
2. Mice! We once had a letter from a frantic customer who said her plants in the conservatory were keeling over from above the soil level. In fear and trepidation, in case we upset her, we phoned and suggested she might have mice in the conservatory. “Do you know! I think you might be right!” she said. Phew!

We hope this advice and guidance is helpful. For even more information about geranium growing and care, visit our geraniums hub page for a wealth of helpful advice.