We repot most amorphophallus in the spring just as their new growth begins to emerge. This way they start out with fresh soil, new fertilizer and are ready to go. This ensures that all of last year’s roots have rotted away. We hate to repot once a tuber has gotten established in its pot, unless it is about to split its container with its vigorous growth. The only exceptions are the tropical species like titanum and gigas. They do not appear to have a defined growing season when grown in tropical areas. They sprout or go dormant at almost any time of the year. So we pot them up when they look too big for their pots. On these giant species we like to keep repotting them often as seedlings, sometimes 3-4 times for the first few years. Then you can add more and more fertilizer.
We now use a commercial mix. Fafford #3B. It is a peat moss based soil with pine soil conditioner and perlite. Most important is that it drain well. We used to mix our own soil with 6 cu. ft. compressed peat, 3 cu. ft. perlite, 2 bags pine soil conditioner. We liked it better as there was less pine soil conditioner, but haven’t the time to mix our own anymore.
Size depends on the species and tubers size, just make sure it has very good drainage. I will often over pot if I know the plant is stable to give the tuber plenty of space to expand. The tubers can often expand and multiply very quickly. For newly acquired plants I pot small in soft nursery containers and repot again in a few months if the plant has grown very quickly. Once a species like the A. titanum gets large it is difficult to find large enough pots. For a giant species you will need a pot that is at least twice the diameter of the tuber about half way down the pot. When actively growing a young tuber might need even more space.
This is a very important item as most amorphophallus species are very heavy feeders. We use a combination of fertilizers. We add soil to the middle of the container and add generous quantities of a fertilizer called “Roots 2 dry”. (We order it from Hummert Catalog, they are on the web) In a 3-gallon pot add up to 1 cup for vigorous species such as the Titan. We also add generous amounts (1/4 cup) of bone meal. Recently the famous Mr. Stinky was planted in a 42” planter, adding about 5 gallons of the granular Roots2, and a 10lb bag of bone meal. Mix into the soil well. Then add a shallow layer of soil with out the fertilizer. Then place the tuber and fill with more soil. Put the tuber in the upper part of the pot, several inches under the soil surface. Then I use generous quantities of Florikan – blend 12 month Foliage & Ornamental 15-4-9 + Mg. (a Nutricote time release mixture with trace elements) Top dress with a little more soil and water it in.
We tend to plant high because the larger species will put out thick roots from the growing point that quickly grow to the bottom of the pot and begin to shorten. The soil level will often drop quickly when growth begins and the soil is compressed under the tuber. Add more soil to the surface as that happens. Then the tuber produces many smaller feeding roots that fill the container and make use of all that fertilizer. Try not to place much fertilizer over the top center of the tuber, though we have never had a problem even when it was accidentally done.
During the dormant period, let an Amorphophallus titanum rest in the moist soil. If you have to worry about winter chills, keep the soil just barely moist, but titanum and most of the tropical species don’t like to be kept dry. Other species vary in their need to rest in soil. A number like to be kept very dry. A note about young titan tubers… When they are growing vigorously as a young tuber they can skip dormancy for a year or even more, adding a new leaf before the loss of the older one. Keep them very well fed, as this is the one time in their life that they often produce multiple offsets.
My final suggestion is to keep them warm. In their native habitat they are never exposed to anything but sultry humid warmth, and they love it.
Courtesy Craig Allen