Gadenin日本語Get the appThe flower is built for carrion flies, and it is bad at it
Pawpaw opens dull maroon, purple-brown flowers before the leaves, hanging downward on the previous season's wood. They are not bee flowers. The color, the faint fermenting or slightly meaty smell and the downward pose all advertise to blowflies, sap beetles and other small carrion and fungus insects. Honeybees ignore them almost completely, and no amount of hive keeping will improve a pawpaw crop.
The insects that do visit are not efficient. They wander, they are few in number in a tidy garden, and they are active on far fewer days than a bee. The usual result is a tree that flowers spectacularly and sets a handful of fruit, or none. Growers who hand pollinate typically go from a scattering of fruit to loaded branches, and it is the one intervention that transforms the crop.
There is a second obstacle built into the flower itself. Each flower is protogynous: the female stigma is receptive first, and by the time that same flower sheds its own pollen the stigma is already past. A flower therefore cannot pollinate itself even in theory, and neither can another flower on the same tree, because pawpaw is self-incompatible. Pollen has to come from a genetically different tree.
Reading the flower: which one is giving pollen and which one is receiving
Everything depends on picking the right flower for each job, and the two stages are easy to tell apart once you have looked at a few. Open the flower gently with a fingertip and look into the center. Work through the tree over two or three weeks; flowers open in waves, so on any given day you will find both stages somewhere on the same branch.
| Stage | Petal color | What the center looks like | Use it for |
|---|---|---|---|
| Female stage | Green fading to pale maroon | Stigmas glossy, green and sticky; anthers still tight and pale | Receiving pollen |
| Male stage | Deep maroon, petals fully reflexed | Anther ball brown and fuzzy, sheds visible pollen when tapped | Collecting pollen |
| Too early | Hard green bud, petals closed | Nothing accessible | Leave it |
| Too late | Petals limp or browning at the edge | Stigmas dry and dull, anthers spent | Leave it |
How to hand pollinate
You need a small soft artist's brush, about size 2 to 4, and a container to carry pollen in: a film canister, a small jar or a folded paper cone. Work in the middle of a mild day when the flowers are dry. The whole operation takes ten to fifteen minutes a day for the two or three weeks the tree is in flower.
- Find a male-stage flower on tree A Look for a flower whose anther ball has turned brown and fuzzy. Tap it over your palm; if a faint yellow-brown dust falls, it is ready. Petals fully reflexed and dark maroon is the outward sign.
- Load the brush Twirl the brush gently against the anther ball until the bristles carry a visible dusting of pollen. Do not scrub. If you cannot see pollen on the bristles against a dark background, the flower is not ready and nothing will be transferred.
- Move to tree B and find a female-stage flower The stigmas must be glossy and green and feel tacky to a light touch. A dull, dry stigma has passed and will not take. This flower must be on the other tree: pollen from the same tree, even from a different branch, sets nothing.
- Paint the stigmas Dab the loaded brush directly onto the cluster of stigmas in the flower center several times. Be thorough. A pawpaw flower has many separate pistils and each one you miss is a fruit you do not get; each fertilized pistil becomes one fruit in the cluster.
- Reload often and work both directions Recharge the brush every two or three flowers. Then reverse the direction, collecting from tree B and painting tree A, so both trees carry a crop. Aim to treat 20 to 50 flowers per tree over the flowering period.
- Mark and record what you did Tie a loop of colored thread on each branch you pollinated and note the date and which tree gave the pollen. In midsummer you can see exactly which branches and which pollen direction actually set fruit, and next spring you know where to concentrate.
Do not wash the brush between trees during a session, but do start each new session with a clean dry brush. Damp bristles clump the pollen and it will not release onto the stigma.
Why the fruit still drops after a good set
A successful pollination gives a small cluster of fruit at each flower, and pawpaw routinely drops some of these in early summer. A cluster of six on a slender branch is more than the tree can carry, and self-thinning is normal rather than a failure. Thin deliberately to two or three fruit per cluster once they reach the size of a walnut; the survivors size up far better and the branch does not break under the weight.
Total drop, where every fruit goes in the first few weeks, is a different problem and usually means water stress or incomplete pollination. Only a few of the many pistils being fertilized gives a lopsided, undersized fruit that the tree often abandons. That is another argument for painting the stigmas thoroughly rather than lightly.
Improving the natural pollinators
Hand work is more reliable, but you can help the insects at the same time. Growers traditionally hang something odorous near the flowering tree, a scrap of meat or a piece of well-rotted fruit in an open jar, to draw in blowflies and sap beetles during the two or three weeks of bloom. It is crude, it smells, and it works to a degree.
Less unpleasant and more durable: leave a rough corner near the trees, keep a compost pile within reach, and do not spray anything during flowering. Broad insecticide applied to a pawpaw in bloom removes the only pollinators it has. Shelter also matters, since these are weak fliers and an exposed windy site loses them.