Biological gnat control — Pinguicula gigantea

Pinguicula gigantea, giant butterwort.  At upper left, a cluster of smaller Mexican butterworts are producing their winter rosettes while retaining a few larger summer leaves.

Butterworts, genus Pinguicula, are carnivorous plants that trap and digest small insects with slimy secretions produced by glands covering the surface of their leaves.  The plants are capable of absorbing nitrogen from the digested prey, allowing them to grow under nutrient-poor conditions such as peat bogs or, as in the case of the species illustrated above, seasonally wet seeps on exposed, bare rock (habitat photos here).

The Pinguicula species native to North Carolina are relatively small plants with leaf rosettes only a couple of inches across, but P. gigantea can grow to the size of a large dinner plate.  Its native habitat is in the Mexican state of Oaxaca, but the plants adapt well to cultivation.  They’re particularly useful for controlling the fungus gnats that can infest the soil of potted house plants, and in my greenhouse they have also trapped flies and small cockroaches.

fungus gnats trapped on P. gigantea leaves

In addition to their usefulness as pest control, P. gigantea plants also produce lovely flowers, pale purple with a darker edge to each petal.

 

Mexico is home to many beautiful species of butterworts which can be quite tricky to grow.  Most of the species require a definite dry dormancy, during which they produce smaller, more succulent non-carnivorous leaves in a tighter rosette (see the smaller plants in upper left of the first photograph, above).  If watered too much (or at all) while dormant, they are likely to rot.  During the summer, they like to remain moist, but many of the rock-growing species require an inorganic mix and will rot if grown a soil- or peat-based mix.  I have grown many Mexican Pinguicula species at various times, but P. gigantea is one of only a few species that I have been able to maintain for more than a couple of years.

P. gigantea does have a winter dormancy, but it is less extreme than some of the other species.  Leaf size decreases only slightly, and the plants are tolerant of year-round moisture.  Instead of a mineral-based mix, I grow my P. gigantea in pure long-fiber sphagnum moss.  Or, more accurately, I grow them on sphagnum.  The thin, rather feeble-looking white roots spread across the surface of the moss and among the mat of dead leaves that slowly builds up, rather than penetrating deeply into the pot.

Propagation of this and other butterwort species is easy.  The somewhat brittle leaves detach easily from the rosette, and if placed on moist sphagnum, perhaps in a plastic bag to keep humidity high, will reliably produce a few plantlets at the broken base of the leaf.

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Six on Saturday #12: Nepenthes

While watering plants in my greenhouse today, I noticed that my Nepenthes burbidgeae x platychila has produced its first upper pitcher.  I snapped a picture and then decided I’d stick to a single theme for today’s Six on Saturday.  This isn’t the best time of year for Nepenthes, but I was able to find five more pitchers in reasonable shape.

For those who are unfamiliar with the genus, Nepenthes are carnivorous plants, large scrambling vines native to a broad region extending from Madagascar to Australia.  The greatest diversity of species is in southeast Asia: the Philippines, Indonesia, and Malaysia.  Nepenthes leaves are modified into ornate pitcher traps that capture and digest insects (and occasionally small vertebrates).  This prey provides nitrogen that is lacking in the nutrient-poor soil in which Nepenthes grow.

With the exception of a few finicky, species, most Nepenthes are reasonably easy to grow in well-drained, acidic potting mixes that are low in nitrogen.  Most growers use combinations of long-fiber sphagnum moss, peat, perlite, and fir bark.  I also include stalite in my mix, because most of the perlite available in local garden centers is contaminated with slow-release fertilizer (WHY do manufacturers insist on adding fertilizer to a product whose main selling point is that it adds aeration without affecting other soil parameters???!!!???).

To pitcher well, Nepenthes need high humidity, so they are usually grown in greenhouses or large terrariums.  However, I successfully grew Nepenthes graciliflora and Nepenthes x Ventrata on a windowsill in my student apartment twenty years ago.  Temperature requirements vary depending on the altitude at which the plants originate.  Most of my plants originate from moderate altitudes and look best in spring, when the greenhouse is around 86-88 F (30 C) during the day and 60 F (15.5 C) at night.  Lowlanders suffer in winter unless I hang them high in the greenhouse, and highlanders tend to die after a couple of summers.

In the greenhouse, the plants catch their own prey, primarily cockroaches, wasps, and ants, that I supplement with crushed snails.  I learned from sad experience that slugs can glide right out of the pitchers and settle down in the pot.  I never fertilize the roots, although some growers swear by cold coffee.

Anyway, here are six Nepenthes.  For more of Six on Saturday, see the Propagator’s blog.

1.  Nepenthes burbidgeae x platychila (upper pitcher)

This is a primary hybrid of two species renowned for their colorful peristomes (the slippery collar around the mouth of the pitcher).  Most Nepenthes produce two types of pitchers.  Squat lower pitchers sit on or near the ground and often have wings that crawling insects can climb.  Upper pitchers, as seen here, are usually more funnel shaped and hang high in the branches to catch flying insects.

One of the odd quirks of Nepenthes is that male and female flowers appear on different plants, and at least in cultivation, males outnumber females.  Consequently, it is difficult to propagate species by seed, and many artificially propagated plants are hybrids.  Species are usually propagated by tissue culture, and some species are represented in cultivation by only a handful of clones.

2. Nepenthes maxima, Gunung Lumut, Sulawesi (lower pitcher)

N. maxima is a very variable species.  I have one clone that produces pitchers about the length of my index finger, but this one has lower pitchers almost a foot long.

3. Nepenthes maxima x aristolochioides (lower pitcher)

One parent of this plant is the large N. maxima clone shown above.  The other parent is a dwarf highland species that requires cool temperatures.  The hybrid seems much more tolerant and has done well in my greenhouse for several years.

Here is an upper pitcher that I photographed last spring:

I have since pruned the vine, so it will be a year or so before I see more uppers.

4.  Nepenthes graciliflora

N. graciliflora is a very vigorous and easy to grow species from the Philippines.  This was one of the first Nepenthes that I grew, and it has traveled with me for about twenty years.  Every few years, it starts to get woody and overgrown, so I root a couple of fresh cuttings from the tip of the vine and establish a new plant.

5.  Nepenthes rafflesiana (lower pitcher)

A large lowland species from Borneo, Sumatra, and penisular Malaysia.  As seen here, the squat lowers have wide wings.  Where the peristome rises to the lid, it has sharp teeth.  To keep my N. rafflesiana warm in winter, I hang it up near the roof, at the end closest to the heater.

6. Nepenthes ampullaria (red form)

N. ampullaria grows in the same habitat as N. rafflesiana, with which it produces the natural hybrid N. x hookeriana.  This species has squat little pitchers with small lids and wide open mouths.  Analysis of the “necromass” in N. ampullaria pitchers has shown that it is primarily plant debris, so this species may be vegetarian or “detritivorous” instead of carnivorous (see: Bohn and Federle (2004), Insect aquaplaning: Nepenthes pitcher plants capture prey with the peristome, a fully wettable water-lubricated anisotropic surface.  Proc. Natl. Acad. Sci. USA 101: 14138-14143).

N. ampullaria rarely produces upper pitchers in cultivation, but in addition to the usual lower pitchers, it also produces rosettes of ground pitchers which appear directly from the soil surrounding the base of the vines.  My young plant has just started growing its first rosette, but the ground pitchers aren’t full size yet.