Beach Plum

Nitrogen as the leaves emerge, split on sandy ground

As foliage emerges in spring, give a young transplant 0.6-1.0 oz of actual nitrogen per plant and an established plant 0.1-0.2 lb of nitrogen spread under the drip line. On sandy soil, split that into two or three applications 2-3 weeks apart rather than one dose, and set the rate from a soil test. You are aiming for at least 1.5 feet of shoot growth for the year. Beach plum fruits on the current season's shoots, so the nitrogen that drives spring shoot extension is what builds next year's crop - and on the sand this plant is native to, a single large dose leaches past the roots before it is taken up.

Fall potassium and magnesium after the fruit is off

After harvest, apply potassium magnesium sulfate (0-0-22-11) at the rate your soil test calls for. Hold the spring nitrogen for green tip - fall nitrogen is not the goal here. Fruiting pulls potassium out of the plant, and the sandy, low-exchange soils beach plum favors hold little of it; potassium magnesium sulfate replaces both potassium and magnesium without pushing the late soft growth that nitrogen would.

Dormant prune: take out about a quarter of the fruiting wood

In late winter to early spring, while the plant is fully dormant and before buds break, remove 25-30% of the fruiting wood along with crossed, shaded, cracked and downward-pointing branches. Take out low limbs that would let fruit touch the ground, and keep an open center so light reaches into the canopy. Beach plum bears its fruit toward the base of the previous season's shoots and rarely forms long-lived spurs, so removing a quarter to a third of the fruiting wood does two things at once: it decreases the crop load and it stimulates the new shoot growth that will carry next year's fruit, which is what evens out a biennial swing into a steadier yield.

Black knot: cut out every gall while the wood is frozen

While the plant is dormant and temperatures are still below freezing, cut out each black, hardened swelling at least 4 inches below the gall, back into clean wood. Burn, bury or otherwise remove the prunings from the planting instead of leaving them on the ground. The black knot fungus grows inside the branch well past the visible gall, so a short cut leaves live fungus behind, and two-year-old galls release wind-blown spores every wet spell in spring - including from cut branches lying under the plant.

⚠ Pruning in freezing weather matters: cuts made in mild, wet conditions can be infected by the spores you just released.

Strip the mummified plums before bud break

Before buds break in early spring, walk the plants and pull off every shriveled, mummified fruit still hanging in the canopy, and rake up dropped fruit from underneath. Also remove dead twigs and branches carrying cankers. Brown rot overwinters in mummified fruit and infected wood, and those mummies are the spore source that infects the flowers a few weeks later - removing them cuts the season's starting inoculum before it can do anything.

Plum curculio: spray once the last petals are down

Wait until bloom is completely finished, then spray carbaryl or phosmet for plum curculio, and repeat about 10 days later. In a cool spring a third application may be needed. Surround kaolin clay, reapplied weekly into early July, is the non-synthetic option. Female curculios move in after bloom and cut a crescent-shaped slit in each young fruit to lay an egg, and the grub feeding inside makes the fruit drop in early summer - the beetles are exposed on the fruit surface only in that window between petal fall and mid-July.

⚠ Never apply an insecticide while any bloom remains - carbaryl and phosmet are highly toxic to the bees and native flies this self-sterile plant depends on for pollination. Follow the label's pre-harvest interval for plums.

Brown rot: protect the fruit as it colors up

Begin fungicide applications 2-3 weeks before you expect to pick, as the fruit starts to color and soften, and repeat on the label's interval until harvest. Captan, myclobutanil, propiconazole or sulfur are the options labeled for fruit rot on stone fruit. Ripening fruit is sugary and thin-skinned, and brown rot infects during warm wet spells - UMN notes a blossom needs only about 3 hours of wetness at 70F to be infected, and ripening fruit is likewise taken in short warm wet periods - so a single warm rain close to picking can turn a crop into mummies.

⚠ Chlorothalonil is labeled on stone fruit for blossom blight only, not for fruit rot - do not use it here. Check the pre-harvest interval on your product's label and stop spraying that many days before you pick; the label is the law.

Pick over the plants repeatedly from late summer on

Expect fruit from the third year after planting, starting in late summer and running as late as October. Pick over the planting several times rather than once - seedling beach plums are genetically diverse and ripen on their own schedules, so fruit on neighboring plants can be weeks apart. Pick ripe fruit promptly rather than letting it hang. Most beach plum plantings are seedlings, not clones, so there is no single ripening date to work from; and overripe fruit left on the plant is where brown rot builds the mummies that infect next spring's flowers.