Problems
Many problems encountered in growing tropical ferns are practical, concerning maintenance of a suitable environment. Other problems relate to the extravagant growth of some ferns and the need to control this for ease of access, watering and so on. However, there are a few problems that relate to infestation with pests, infection with fungi and viruses, and overgrowth of other flora.
Pests
Whilst ferns grown outdoors are relatively free of pests, indoor conditions can support far greater populations of predatory animals that can damage ferns of all ages and sizes. Ferns that are semi-tropical often benefit from a period outside in the summer and this can greatly reduce insect populations, though it may expose them to damage by slugs, snails and weevils which may be brought back into the glasshouse or home.

Earthworms
In the garden, earthworms are regarded as very beneficial. They help to aerate the soil and draw organic matter from the surface to deeper levels. This matter is ingested and excreted, helping to produce a rich soil. However, in pots, they repeatedly ingest the same soil and break down the organic matter into finer and finer particles. Eventually, the compost becomes homogeneous and putty-like. This retains water and prevents free drainage. For those ferns that require a light, free-draining compost, this process is not beneficial and can lead to rotting off. The presence of an earthworm in a pot is often made evident when a plant is repotted. The earthworm itself may be seen or it may be difficult to tap out the plant from the pot as the compost is sticky. However, the earthworm should be removed and placed somewhere more suitable.
Ambigolimax valentianus, Greenhouse Slug (© Peter Blake)
Oxychilus alliarius, Garlic Snail (© Peter Blake)
Slugs and snails
Slugs are common predators of glasshouse plants. Indeed, the most frequent, Ambigolimax valentianus, has the common name of the ‘Greenhouse Slug’. Its other name, the ‘Iberian Threeband Slug’ describes both its appearance and its origin. It was recorded in the UK in the first half of the 20th century and its population has exploded to become not only common indoors but also outside. It is easily recognisable as it is usually only a few centimetres long with obvious longitudinal stripes on a pale body. It will eat any greenery.
Slugs enter the greenhouse through gaps in windows and doors, but also as eggs in compost. Keeping compost in a slug and snail free area is advisable. Populations can be controlled by regular inspection of the base of pots and damp corners of the greenhouse, or by night-time torchlight searches. They can also hide beneath moss or other low-growing. Pieces of fruit, vegetables or ‘beer’ traps placed in infested areas can be used to snare the slugs.

Another method of control are poisonous metaldehyde slug pellets, used sparingly and away from any children, animals or birds. The poison is delivered in pellets made from bran or grain that the slug or snail will be attracted to. Unfortunately, other animals also find it palatable. As a result, metaldehyde pellets are now banned in the UK. These worked by causing the slugs to stop producing their natural mucus causing them to dehydrate. These have been replaced by ferric phosphate pellets, these are considered organic, with less harm to the environment. They work differently to metaldehyde, taking a longer time to affect the slug, meaning that the slug might be hiding in the compost when it eventually succumbs to the poison. The pellets are unsightly and in humid condition will quickly go mouldy.

A biological nematode control is available, Phasmarhabditis hermaphrodita. To be active, the nematodes need the compost to be above 12 °C and to be moist. Also, they need the slug to spend some time under the compost surface so that they can enter the slug through the respiratory opening and release a bacterium which destroys the slug. So, nematodes are not very effective against slugs that live on the surface of pots and nor are they effective in controlling snails. Research from the RHS found that the use of mulches reduced the effectiveness of the nematodes.
Large snails are usually obvious and can be picked off plants and pots. Smaller snails can be controlled with slug pellets. However, the ‘Garlic Snail’, Oxychilus alliarius, can damage small plants and make lacework of large plants without being seen. This snail is usually only 5 mm across and ventures out at night. It is not particularly attracted to slug pellets and visual searches should be carried out. They gain their name from the slight smell of garlic emitted when they are squashed. They have gained popularity in the last few years as microfauna candidates for bioactive terrariums along with springtails and isopods. In certain set-ups O. alliarius may indeed serve as interesting custodians or as ‘clean-up crew’ for terrariums. But they can quickly cause extensive damage to many ferns and their appetite seems quite indiscriminate.
Idiopterus nephrolepidis, Black Fern Aphid (© Peter Blake)
Greenfly (© Peter Blake)
Aphids
Black Fern Aphids, Idiopterus nephrolepidis, are a particular problem, but green and white species can also infest plants, usually on the underside of the frond where the protective cuticle is thinnest. The consequences of aphid attack range from a diminution in size of new fronds and a general lack of vigour, to distortion and death of fronds. Aphid damage may cause a secondary fungal or viral infection that might not present fully until later in the growing season. Aphids produce excreta, fancifully called ‘honeydew’, which attracts sooty mould growth and ants.

Aphids can be controlled in a number of ways. The first is physical removal by brushing or washing. However, not all ferns are suitable for such rough treatment, particularly on emerging croziers. Another method is to spray the plant with an organic soap mix which washes off the aphids’ waterproofing and causes them to dry out and die. Not all ferns will tolerate this spray and some genera seem particularly intolerant.

A third method is to try biological control with a number of different parasitic wasps, gall midges, ladybirds and so on. If these are to be used, then other chemical methods of control have to be avoided and the environmental conditions for the parasites to thrive have to be met. Some biological control animals are only available in certain seasons.

If chemical control is to be used, then dust susceptible plants with a proprietary insecticidal powder, usually sold as wasp or ant killer. These products usually contain a pyrethrin compound which is effective within 24 hours. The powder can be applied with a puffer to produce an insecticidal cloud or sprinkled onto the affected fronds with a fine mesh strainer. The latter method reduces the amount in the atmosphere that is subsequently breathed in, but only treats the areas that are sprinkled. These powders are very effective and can be used in a light concentration on a regular basis to avoid new colony formation.
Finally, chemical insecticides marketed in pressurised cans should be avoided as both the propellant which provides the pressure, and the diluent in which the insecticide is dissolved, can damage fern fronds. A systemic insecticide, containing acetamiprid, is a better option if other methods of control fail.
Scale Insect (© Peter Blake)
Pinnaspis aspidistrae, Fern Scale (© Peter Blake)
Scale insects
There are many species of scale insects that adversely affect the health of large plants and can kill small ferns. They can make a plant unattractive with ugly lumps on stipes and rachises, usually on the under surface of a frond, and they can also produce copious ‘honeydew’ which then gets infected by sooty mould. The scale insects have a thick waterproof, waxy scale or shell, like a limpet, that makes it difficult to dislodge or kill them. Some species of scale insect are hermaphroditic and self-fertilising, whilst others reproduce sexually. The females lay their eggs under the scale and, when the eggs hatch, the young ‘crawlers’ emerge and move to new parts of the plant or migrate to new plants. Once they settle and produce the scale the females do not move again, though the males may develop into the winged form and are capable of short-distance flight.
Eradicating scale insects is not an easy task and requires vigilance so that infestations can be dealt with promptly. If a frond is very badly infested, remove it, and throw it away. If the infestation is less, then a systemic poison can be effective. This can be applied both to the fronds and to the roots. Acetamiprid, a neonicotinoid, is available as a control of vine weevils both in the underground larval form and the adult weevil. This compound is also effective against scale insects although the dead scales may remain attached to the fern and have to be rubbed off. However, it is important not to drench young, growing foliage as this can be damaged by chemical treatment resulting in distorted, brittle fronds with brown edges.
Special mention should be made of the ‘Fern Scale’ or ‘Coconut Scale’, Pinnaspis aspidistrae. Usually, it is not the adult tan-coloured female scales that are seen but the white, 1–2 mm immature male insects. These can be present in their thousands and give the fern the appearance of having white, finely ground, desiccated coconut sprinkled over it. They are also very likely to settle on the lamina and produce pale ‘bleached’ spots where they suck the sap from the frond. The appearance of the plant is ruined, and heavy infestations can prove lethal. Prompt systemic treatment should be used.
Mealybug (© Peter Blake)
Mealybug
These insects are called ‘mealy’ because they produce a covering of soft, white, flour-like, wax. This protects them from water and from predators. There are several species of mealybug which usually live in colonies in the difficult to reach parts of ferns. Ferns with dense scale cover on the rhizome or stipe are also vulnerable. Topical treatment is not very effective because of the waterproofing of the ‘meal’. However, small infestations can be dealt with by touching them with a cotton bud soaked in methylated spirit. This dissolves the ‘meal’ and reveals the insect, which then dehydrates and dies. Another effective organic treatment that can be applied to the whole plant is the use of dissolved soaps that remove the waxy protection from the insect, leading to its death. There are also two biological controls, parasitoid wasps (Leptomastix dactylopii) and a beetle (Cryptolaemus montrouzieri), both of which naturally kill mealybugs. However, severe infestations are best dealt with by using a systemic poison such as acetamiprid.
Thrips (© Peter Blake)
Thrips
There are hundreds of species of thrips but the most common in greenhouses is Heliothrips haemarrhoidalis. The insect can reduce areas of a frond to paper-thin brown patches, ruining the appearance and viability of the frond. Thrips thrive in warm, moist conditions and can reproduce very quickly. They are small (1.5 mm) and capable of flight. They eat during the day but can hide at night in the surface of compost where they reproduce and undergo their larval stages. There are a number of insects and mites that prey on thrips and which may be used for biological control. Otherwise, a drench is needed to treat both the plant and the compost. This may be an organic soap solution or a chemical agent, such as acetamiprid.
Red spider mite
These tiny creatures are too small to see with the naked eye and may even be difficult to spot with a magnifying glass. There are several species but they are not spiders and nor are they necessarily red. They are arachnids with eight legs that occur in hot dry conditions and suck sap from the underside of fronds within a fine web which protects them. Affected fronds become papery and dry and the plant suffers from a lack of vigour. They may be controlled biologically by other parasitic mites or by proprietary miticides, although raising the humidity and spraying with water or an organic soap solution may also be an effective way of eradicating them.
Periplaneta australasiae, Australian Cockroach (© Peter Blake)
Cockroaches
These insects are unlikely to be a problem in most amateur collections of indoor and tropical ferns but can be a problem in large collections of tree ferns and in commercial greenhouses. Cockroaches can eat almost anything, including cellulose. So, whilst living plant material may not be top of their list of preferred foods, they can cause damage when no alternative is available. The Australian Cockroach, Periplaneta australasiae, has unfortunately been introduced to the UK and can eat emerging tree-fern fronds and also burrow inside the stipes. This can severely damage or kill tree ferns, both large and small.
Cockroaches are notoriously difficult to control in large greenhouses where there are ducts for heating pipes, hidden recesses and so on. In smaller collections thorough cleaning of the cracks and corners of the greenhouse is a good start, especially damp areas, and commercial DIY poisoned baiting systems may be effective. However, cockroach eggs can last up to three months, nymphs for up to a year, and adults for up to two years. Not surprisingly, the infestation can recur. Professional pest control help may be needed to have access to poisons not available to the general public.
Vine weevil larva (© Peter Blake)
Fern Weevil larva (© Peter Blake)
Weevils
Black Vine Weevils, Otiorhynchus sulcatus, have become a scourge of many potted plants, including ferns, but generally are more of a problem outdoors than indoors. However, infestation can occur, particularly if potted ferns are put in the garden for the summer. The larvae do the most damage in eating off the roots of the fern to the point where it may just be lifted off the soil. However, they can also live in the crown of a large fern. The adult weevils eat notches out of the edges of the fronds at night. Picking the adults off the fronds is difficult as they drop to the ground at the first disturbance and are well-camouflaged. They are less likely to thrive in heavier loam-based substrates and can be deterred from laying their eggs by using sharp gravel mulches. If plants are outdoors keep the pots tidy and avoid any build up of leaf litter that may create hiding places for the adults during the day.
Another weevil to worry about is the Fern Weevil, Syagrius fulvitarsis. The larva of this weevil hatches from an egg inserted into the base of a stipe where it grows and kills the frond. After pupation, the adult then emerges from a tiny hole in the stipe, which can sometimes be seen. In the UK, Polypodium species are particularly affected, but larger ferns and tree ferns may also be infested.
Whilst biological control of vine weevil larvae can be tried using several parasitic organisms, control of fern weevil is more difficult as the larva is well encased in the stipe. Systemic chemical treatment with acetamiprid may help for both types of weevils.
Fungus gnats
Fungus gnats are tiny, midge-like flies that run around on, or fly very close to, the surface of moist compost in pots. They have an underground larval form which can damage the rhizoids and rootlets of prothalli and sporelings. Larger ferns may be reduced in vigour but are seldom killed. To avoid these insect larvae damaging small plants, fresh sterilised compost should be used for spore sowing and for raising sporelings. To eradicate established infestations in the pots of larger ferns, one biological control method is to use Bacillus thuringiensis var. israelensis. This bacterium is applied to the compost and infests the larvae. It produces a highly specific toxin resulting in death of the gnat larvae. It is important that the conditions of use as regards temperature and humidity are observed or the bacterium will not live long enough to affect the larvae. Alternative organic control methods are predatory mites, Hypoaspis miles, nematodes, or a type of beetle. Adult populations may be controlled by the use of sticky flypaper held close to the compost.
Filamentous Mould (© Peter Blake)
Grey Mould (© Peter Blake)
Diseases
Virtually all fern diseases are caused by fungi. There are a great many fungi that will thrive in warm, still, humid air. Generally, ferns are fairly resistant to fungi, but this resistance is diminished if the fern is not thriving due to an infestation by a pest or from overwatering, or lack of nutrition or light. Sporelings are more susceptible than mature plants.
Fungi
Three fungi are worthy of mention, although there are many others, some specific to individual species or genera: Rhizoctonia species, Botrytis cinerea and Septoria asplenii.
Rhizoctonia is a common cause of ‘damping off’ in sporelings, causing them to fall over and then rot. In mature ferns it can cause rot in the bases of the stipes which then collapse and leave a wet, soggy area. It can also cause black rot in Platycerium shield fronds if they are kept too wet, especially when it is cold. It is a fungus that thrives in over-watered conditions and the spores can remain in the compost. The best solution is to repot mature plants that are afflicted, to only use new, sterilised compost for sporelings and for repotting, and to limit watering to the minimum necessary for the health of the plant.
Botrytis cinerea is commonly known as grey mould. It forms very fine mycelial webs over the surface of plants and pots in very wet conditions, such as those suitable for filmy ferns. It can even exist on the surface of still water. The grey appearance occurs when the mycelium is fruiting and spores are being released. It has a distinctive musty smell and can destroy some fern species. Chemical control is possible but the circumstances in which the fungus occurs are those necessary for delicate ferns to grow and these ferns may not tolerate antifungal chemicals. The solution has to be maintaining some airflow, removing rotten plant material and keeping the growing environment as clean as possible. Avoid overcrowding and ensure that the plants are in loose, free-draining compost even if the plants need to stand in water. Consider using an element like grit, perlite or pumice in the mix, to enable a degree of airflow through the substrate. With regular changing of the water, this may help control the infection. In terrariums, tiny arthropods called springtails are sometimes introduced to eat fungi and control disease
Asplenium can be susceptible to Septoria asplenii which can cause brown patches on the fronds, which subsequently die. As with the other fungal infections, good air movement and avoidance of overwatering can help reduce fungal attack.
Viruses
Only recently has it been found that there is at least one virus that can infect and damage ferns. This is the Japanese Holly Fern Mosaic Virus (JHMoV) which has been found to cause pale circular spots, line stains and distortion in the pinnae of Cyrtomium falcatum. This problem was first seen in the early 2000s in large-scale horticultural glasshouses where transmission from plant to plant was attributed to thrips. However, it is now apparent that the virus can also be transmitted on tools and containers. It can also become incorporated into the spores of infected plants and travel widely. Whilst this infection makes the plant unattractive, it is unlikely to kill it. It appears that JHMoV, or a very similar virus, can also infect Rumohra adiantiformis, which is of economic consequence in the floristry trade where Leather Leaf Fern is widely used in flower arrangements. There are no antiviral treatments for infected plants and any outbreak must be managed by strict phytosanitary control, meaning that infected fronds, or whole plants, should be destroyed and strict hygiene used to avoid contamination of uninfected stock. It is likely that there are other mosaic viruses that can infect ferns in industrial horticulture, one being Cucumber Mosaic Virus, although neither this nor JHMoV has been found in ferns growing in the wild.
Weeds
Weeds can be a problem in the greenhouse and hothouse and it is important to keep them in check and not allow them to get out of control. Many of these will be familiar annual or perennial garden weeds. There are also some which are more specific to fern collections.
Volunteers
Several months after starting a fern collection in the greenhouse or hothouse, it is quite common for new ferns to appear in pots, on capillary matting, or on the ground. These are known as volunteers. Usually, they are ferns that have grown from spores that have travelled in on the air current or have arisen from spores present in the substrate. They may be the progeny of ferns in your own collection.

If you have a natural display it is an option to leave these volunteers to develop. This could add to the overall lushness in the display. However, be selective, as ferns that volunteer can quickly take over and crowd out other species. As the ferns grow, root systems will get entangled making them surprisingly hard to remove. From experience, it is best to remove these when they are small sporelings. If you want to keep them to add to your collection, pot them up and grow them on, and then replant in a good spot once they are bigger.
If your collection is in separate pots and containers, it is best to remove these volunteers at the earliest opportunity. Either discard the sporelings or pot them up to see how they develop. You will quickly develop an eye for identifying interesting species, even in their juvenile state.
Moss
Mosses are often attractive and they can also be a useful substrate for many ferns, as discussed earlier in this chapter. However, the hothouse and greenhouse can create optimal conditions for certain mosses to take over and potentially crowd out smaller species of ferns. Moss can create too much humidity around the rhizome or crown and this can cause fungal infection and rot. Pests can also take refuge in moss and be difficult to spot and remove. Whether you have a natural display or a collection in pots, it is a good idea to remove moss from around the crown or rhizome from time to time, particularly when growing small ferns.
Liverworts
Another common weed in the damp hothouse and greenhouse are liverworts. Again, many of these are attractive and unusual in their own right but, like moss, they can gradually form a hard impregnable crust on top of the pot. They can grow over and into the crowns and rhizomes of ferns. They can be surprisingly adherent and difficult to remove and can make pots difficult to water. So enjoy them for a short while and then it is sensible to remove them by lifting them off with the tip of a blunt knife or with tweezers.
Algae
Unless your conditions are very wet you are unlikely to get problems with algae. If you get algae on the top of pots it may be a sign that you are overwatering or that the substrate is waterlogged, but that will ultimately depend on the watering needs of the fern you are growing. It is not uncommon for various algae to grow on the top of benches, attaching themselves to capillary matting or to LECA balls, or even to the sides of terracotta pots. Algae can be all kinds of colours from light green, through blues to dark browns. This is not a problem but can look unsightly depending on the aesthetic you are going for.
However, algae can be a problem when growing spores. Again, it is a sign that the conditions are too wet, so it is worth trying to remove some of the excess water. Remove patches of algae as they can take over the container, it is likely that they will reappear unless the conditions are made drier.
In aquariums certain algae can smoother plants and look very unsightly. Filamentous algae is the main problem. Occasionally you may get algal blooms suspended in the water. Beard algae can be attractive and less of a problem but will attach themselves and grow on the surface of slower growing aquatic plants like aquatic ferns.