Kniphofia in the UK are generally untroubled by pests. Where problems do arise however, typically slug/snails and aphids are the cause, with both able to damage both flowers/inflorescence and newly emerging shoots.
Aphids typically cause damage earlier in the year, where it is not uncommon to see aphids infesting flowers causing them to deform in an unsightly manner. Later in the year aphids may also infest the leaves of some cultivars and species – potentially reducing their vigour.
While irritating for the gardener, in truth there is often little need to worry about aphids on Kniphofia, with damage usually only cosmetic. In the most extreme infestations, simple measures such as rubbing the aphids off with your fingers may be necessary but as a rule aphid infestations of Kniphofia are fairly transient.
Slugs and Snails
Slug and snail associated damage is often more severe than aphid damage but is primarily problematic in the same areas, chiefly the flowers. Most commonly, slug/snails chew through emerging flower stems, causing marking of the flower stem of partial destruction of individual flowers. At its most extreme this damage may cause the flower stem to break in the wind.
More problematic however is slug damage in the spring. In cold winters some Kniphofia species and cultivars may die down entirely before reemerging with new rosettes from the crown. These new rosettes are something of a delicacy for slugs and snails therefore vigilence may be required to protect new growth.
Perhaps the most significant challenge to growing Kniphofia in the UK is disease – chiefly crown/root rot. To understand the potential scale of this problem, one need only refer to the RHS Kniphofia trial of 1971-1973 where the entire trial had to be abandoned due to most plants succumbing to root rot. Similarly a previous national collection, held at Bridgemere nursery, Cheshire, was also lost to root rot in 1993.
Crown/root rot is often talked about as a singular entity. In truth however these terms refer simply to the symptoms which present themselves following Kniphofia infection by a range of different oppurtunisitc bacteria, fungi or oomycetes – with Fusarium, Rhizoctonia, and Phytophthora the most implicated agents.
While crown/root rot therefore stems from an infection, I tend to think of crown/root rots instead as a symptom of a stressed plant – where it has been grown in a place or way that favours the opportunistic infection. As with all diseases, the best way to control the problem is therefore by preventing it in the first place.
The key factors to consider for crown/root rot are rainfall, soil structure and how the Kniphofia is planted. Of these factors, perhaps the least important is rainfall. So long as your garden doesn’t flood and your Kniphofia is planted correctly, neither an abundance nor dearth of rain are likely to worry it.
Far more important is soil structure/drainage. As a general rule pH doesn’t really affect Kniphofia, with most varieties being happy to grow in acidic or alkaline soils. It is the drainage and water holding capacity of the soil which is key. Ideally Kniphofia should be planted into a well drained loam – with additional organic matter and/or grit being useful to improve sandy or clay soils closer to this loamy ideal.
As for how to plant your Kniphofia to prevent crown/root rot, I give my recommendations on the Kniphofia planting tips page.