A Partially Parasitic Paintbrush Problem
- Kelly Arnold, Greenhouse manager
- Jul 21, 2016
- 5 min read
In every greenhouse there are plants that will present horticulturalist with a challenge. Here is a story about a pretty little thieving forb and the limitations of a pot. The experiment is to take a product that now goes to restoration sites and make a product for your garden.
Castilleja is a genus of plant commonly referred to as paintbrush; there are over 200 species of plants that make up this genus. The species of paintbrush native the West are beautiful forbs –that could be a desirable addition to many commercial landscapes and home gardens.
Paintbrush grown in a container presents an interesting challenge. It is a semi or hemi parasitic plant. This means that the plant can be propagated alone from seeds without a host. The plant can even grow and establish root systems without a host, but the plant never really flourishes. The paintbrush’s roots will search for a host’s roots then rob this other plant's nutrients. Once the plant has attached to the host plant it produces the large bloom that the majority of Montana outdoor enthusiasts know from sight.
At Great Bear Restoration we have grown several Castilleja plants for many years. Every plant at Great Bear gets a custom soil that contains slow release fertilizer and a mycorrhiazal application. This allows us to produce a plant that is root tight (hold the form of the container with its root system, even when removed from the container) in a 10 cubic inch tube that can then be successfully planted in the field. Of course, if you plant these 10 cubic inch plants, the installation will often be designed to have its victim nearby. The Castilleja roots will seek out its neighbor for its nutrient boost.
For a container grower to have a Castilleja plant in a quart or gallon container for landscape and gardens, a special addition is needed in the pot so that we can deliver a flourishing, attractive plant. The special addition will allow the plant to perform without the grower knowing the advanced biology of the plant. In other words, I designed a custom plant knowing that the plant would have a high likelihood of surviving without depending on proximity to other plants. The special ingredient - a host plant within the containerized paintbrush.
My goal was to produce a healthy, flourishing paintbrush in a quart container and it was to be coupled with a host plant. The question was, what plant would be a good partner for the paintbrush? The native Castilleja typically pillages nutrients from sagebrush (Artemisia) or from bunch grasses like Idaho Fescue (Festuca idahoensis). Castilleja are not picky, so almost any grass, forb or shrub species would theoretically work. The paintbrush’s preference was not my limiting factor in selecting a coupling species; the host would have to work in a small container.
In my limited trials thus far, I have tried growing paintbrush with its suggested victim, which was native bunch grasses. The issues that were encountered were caused by the pot’s size. After transplant into a quart container the bunchgrasses quickly over grew the pot and smothered the mooching paintbrush. In theory a greenhouse manager could keep the host plant pruned back but the space proximity would be such an issue even after transplanting that the paintbrushes survival would seem doubtful. So, the host plant must have a basal area that doesn’t threaten the paintbrush. This limits the selection of host plants down to small wispier forbs. It is a pleasant coincidence that Great Bear produce’s a large selection of these forb species. This leads me to my current trials…..
The Castilleja plant that I happened to have sitting around the greenhouse is Castilleja integra, commonly known as Whole Leaf Paintbrush. This species grows mostly in the southwest United States; it will grow to be one and a half feet at the most and has a large red flower. It likes arid areas that are very well drained. There are other Castilleja species available that are native to the Montana area but the thing that swayed my decision in using Castilleja integra was convenience; I had a bunch sitting next to my desk. Next I choose array of species to try as hosts, once again the heaviest factor in my decision making process was availability. Luckily at Great Bear we have great availability! The host species included:
Yucca glauca – Soap Weed Yucca
I choose this plant because it grows natively with Castilleja integra. It likes well-drained, dry soil. It is a smaller yucca leaving room for the paintbrush to grow around and with it. This plants maybe a sub shrub but why not give it a try.
This plant was chosen because it is smaller, only growing to be 2-3 feet tall and 1-2 feet wide. Hopefully, this small size should give the paintbrush a fighting chance.
This forb is just cute! It is small and low growing it usually never gets more than 6 inches tall. It grows all over the central United States, from Montana to Texas and was my first choice for a solution to the paintbrush problem.
This was choice two for the paintbrush problem. Clovers are legumes so they should fixate lots of nitrogen to feed the greedy paintbrush and the plant itself is a wispy leafed plant. Go clover!!
This plant almost didn’t make the experiment. While harebells flowers are small and delicate (1-2 feet high) the plant has a hearty width and may prove an aggressive adversary for space.
This for is a nice 1-2 feet tall with a red flower that may look nice with paintbrush.
This forb likes well drained soil and does grow to be very tall, 6-12 inches. Its base area is very large though; mostly it was sitting there begging to be included in the experiment.
This forb is a lot like the above buckwheat, likes well-drained soils but may be too competitive for the paintbrush. Again, why not?
I have high hopes for this forb. It is an attractive plant that is in high demand. It can get fairly large but I have a good gut feeling.
This arid loving form has the same soil likes as the Castilla integra. It is gets fairly large, up to 3 and half feet tall but in my experience it isn’t very leafy.
I also added a lupine species although I have little hopes that this host will survive or provide any results.
The trial was set up with 5 quarts of every combination and also 5 quarts of just one plug of Castilla integra. The quarts were “potted up” with the 7 cubic inch plants in opposing corners. The quarts were potted up with the trials over the course of a month so there is a little variation in sizing because of growth.
Stay tuned – I’ll keep you up to date.
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