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Pecans carry both sexes on every tree. The male flowers hang as long green catkins, three to a cluster, from the base of last year's growth. The female flowers are small, upright and easy to miss, sitting in clusters at the tips of the current season's shoots. Wind carries the pollen; bees play no part, so nothing you do for insects affects the crop.
The catch is timing. On any given tree, the catkins shed pollen and the female flowers become receptive at different times, and the gap is usually a week to ten days. This is called dichogamy, and it is not a fault or a weather accident. It is the tree's own mechanism for avoiding self-pollination, and it is why a lone pecan flowers heavily every year and drops most of its tiny nutlets in early summer.
A single tree does set a few nuts, because the gap is not always complete and because late catkins overlap early female flowers within a large crown. That is why owners of lone trees report a small crop rather than none at all, and then wrongly conclude that pollination is not their problem.
Type I and Type II: the two opposite groups
Every pecan variety falls into one of two groups according to which sex goes first. Type I, or protandrous, trees shed their pollen first and become receptive on the female side afterwards. Type II, or protogynous, trees do the reverse: the female flowers are receptive first, and the catkins shed later.
Put like that, the answer is obvious. A Type I tree is shedding pollen exactly when a Type II tree is waiting to receive it, and later in the season the Type II tree sheds onto the now-receptive Type I. The two groups pollinate each other in both directions. Two trees from the same group are close to useless as partners, because they are doing the same thing on the same days.
So the rule for a home planting is one of each. Two trees is the minimum that works properly; three or four with at least one of each group is better, because a late frost or a wet spell during one shedding period no longer wipes out the whole season.
| Group | Also called | Sheds pollen | Female receptive | Partner needed |
|---|---|---|---|---|
| Type I | Protandrous | Early in the bloom period | Later, after shedding ends | A Type II tree |
| Type II | Protogynous | Late in the bloom period | Early, before shedding starts | A Type I tree |
| Two Type I trees | Same group | Both early | Both late | Still need a Type II |
| Two Type II trees | Same group | Both late | Both early | Still need a Type I |
| Lone tree of either group | Self only | One window | A different window | Sets a few nuts at best |
If you do not know which group your trees belong to, watch them yourself for one season. Note the day catkins first shed visible pollen when you flick them, and the day the little female flowers at the shoot tips turn sticky and open. Whichever comes first tells you the group.
Distance, wind and the rest of the neighborhood
Pecan pollen is light and travels a long way on moving air, so partner trees do not need to be close. Anything within 150 feet (45 m) works reliably, and pollen carries several hundred feet downwind. If you have a partner tree on a neighboring property or a wild pecan along a watercourse, your tree may already be covered.
Because it is wind pollination, weather during the shedding window matters more than anything else. Days of heavy rain wash pollen out of the air and off the female flowers, and dead-still humid weather leaves it hanging in the crown that produced it. A dry breezy week during bloom is what you want, and there is nothing to be done about the years when you do not get it.
Prevailing wind is worth using. If the wind at bloom time comes reliably from one direction, place the pollen partner upwind of the tree you most want to crop. On a small property this is a matter of a few tens of feet and costs nothing to get right at planting.
Reading a poor set correctly
When a mature pecan crops badly, the cause is usually one of four things, and they look different if you check at the right moment. Walk the tree at bloom and again a month later rather than waiting until autumn to draw conclusions.
Missing pollination shows as female flowers that shrivel and drop within two or three weeks of opening, with almost nothing left on the shoot tips by early summer. Alternate bearing shows as a tree that set an enormous crop last year and produced almost no female flowers at all this year. Late frost shows as blackened shoot tips and dead flower clusters immediately after the cold night. Kernel failure from a short season shows as a full set of nuts that stay on the tree and turn out empty at cracking.
Keeping two lines in a notebook each spring settles this permanently: the date catkins shed, the date female flowers open, and roughly how many clusters you can see. After three seasons the pattern is unmistakable, and you will know whether you have a partner problem or a climate one.