Plum

Plum

Fertilize only if last year's shoot growth fell short

In spring, measure last season's new shoot growth before reaching for fertilizer. A young plum should put on 10 to 20 inches of new shoot each year and an established tree 8 to 10 inches. If it is falling short, spread a half cup to a cup of 20-0-0 around the dripline of a young tree, or 1 to 2 cups for a tree five years or older. If growth is on target or excessive, apply nothing. Nitrogen is nearly always the only nutrient a plum lacks, and shoot extension is the tree's own honest report on whether it has enough. Overfeeding buys soft vigorous wood that is more attractive to aphids and slower to harden off for winter, and it costs you fruit.

⚠ Do not fertilize a plum in the year it is transplanted - the roots cannot use it yet.

Plum

Thin young plumsMay – Jun

When fruitlets size up, hand-thin clusters so remaining plums have room to develop without limb breakage. Thinning improves fruit size and reduces broken branches on heavy-setting plum trees.

American Wild Plum

Thin only the trees that overset - most American hybrids do not need it

Once the danger of frost has passed, look at the crop load before you thin anything. American plum and most of its hybrids do not generally need thinning unless you want larger fruit - Alderman in particular carries a full load without help. If the tree is an overbearer like Waneta, or you want bigger plums, break up the clusters and thin to a single plum every 4 to 6 inches of branch, keeping the fruit nearer the trunk end of the limb. Utah State scopes the 4-to-6-inch spacing to European and Japanese plums and says American plums and American hybrids generally do not require it. But some of the hybrids do overset badly: the Wisconsin Carandale trial found Waneta over-fruits to the point that fruit size and quality drop off sharply, and a heavy load on the long branches of these trees breaks wood - while touching fruit in a cluster hands brown rot a bridge from one plum to the next.

Plum

Prune plum for an open canopyMar – Apr

Remove dead damaged and crossing wood after the coldest weather passes; keep the center open for sun and airflow. Open growth reduces brown rot pressure and helps ripen fruit evenly.

⚠ Disinfect tools between diseased cuts.

Plum

Black knot: cut out every gall in late winterFeb – Mar

In February or March, while the tree is dormant and temperatures are below freezing, find the hard black swollen galls on twigs and branches and cut at least 4 inches below each one, back into clean wood. Carry the cut wood out of the orchard and burn, bury, or bag it - do not leave prunings on the ground. The fungus Apiosporina morbosa overwinters alive inside the gall and fires spores at young shoots during spring rains, so a gall left on the ground or in a brush pile keeps infecting the tree it came from. Cutting well below the knot matters because the fungus grows inside the branch further than the visible swelling.

⚠ Cut in freezing weather so spores are not being released onto the fresh pruning wounds you just made.

Plum

Strip the mummies and clear the drops

Pick up fallen fruit through the summer and after harvest, and before bud break next spring strip every shrivelled mummy still hanging in the tree. Cut out twigs killed by brown rot and any cankered branches in February or March. Bury the infected fruit in a compost pile rather than tossing it under the tree. Mummies and cankers are where Monilinia overwinters - each one is a spore cannon aimed at next year's blossoms. Dropped, grub-infested fruit is also how plum curculio larvae reach the soil to pupate, so clearing drops cuts next season's beetle count.

American Wild Plum

Cut out every black knot in late winterFeb – Mar

While the tree is bare and still freezing, find the black, cracked, charcoal-looking swellings on twigs and limbs and cut at least 4 inches below each one, back into clean wood. Carry the prunings off the site and burn or bury them, and cut the knots out of any wild plum or chokecherry thicket within throwing distance. The knots are the fungus itself, and they pump out spores every spring rain until they are gone; prunings left lying under the tree keep releasing spores as if they were still attached, and wild Prunus in the fencerow reinfects the tree you just cleaned.

⚠ Prune when temperatures are below freezing so the fresh cuts are not open to spores; disinfect the saw between cuts.

American Wild Plum

Open the center before the buds moveFeb – Mar

Prune after the coldest weather has passed but before growth begins. Train to an open center, vase-shaped, cutting close to the branch collar and taking out crossing and inward-growing wood so light and wind reach the middle of the tree. Wild plum and its hybrids throw long, upright branches with few laterals, and a closed canopy stays wet after rain, which is exactly what brown rot and black knot need to infect.

American Wild Plum

Cut the root suckers unless you want a thicket

Through the growing season, cut the shoots coming up from the roots off flush with the ground if you are training a single-trunk tree. If you want the thicket for wildlife cover and a hedge, leave them and let the colony fill in. Prunus americana suckers freely and will build a dense colony from its roots - that is how it lives in the wild, and it is a choice, not a defect, but a thicket is far harder to prune knots and curculio out of than a tree.

American Wild Plum

Pick up the drops twice a week while curculio fruit falls

During the early fruit drop after petal fall, walk the tree at least twice a week and promptly collect every fallen plum, looking for the crescent-shaped scar. Destroy them - do not compost them under the tree. The larva rides the fruit to the ground and spends only about two to three weeks feeding inside it before exiting to pupate in the soil and emerge as the next generation of adults; picking the drops up promptly, before the larva leaves the fruit, breaks that generation instead of raising it.

American Wild Plum

Strip the mummies after the last picking

After harvest, pull every shriveled, hard, gray-fuzzed plum off the tree and rake up the ones on the ground, and cut out the twigs with sunken, gummy cankers. Finish before bud break next spring. The fungus overwinters in those mummies and in the infected wood, and they are the entire source of next spring's blossom blight - a mummy left hanging is next year's inoculum sitting directly above the flowers.

Plum

Dormant oil at green cluster for scale and aphids

On plum, spray horticultural oil at the green cluster stage, after buds break but before bloom, at a 2 percent rate, using the dilution your product's label gives for it. Cover the bark and crotches thoroughly; the oil only works where it lands. Oil kills San Jose scale, aphid eggs, and mite eggs by smothering them while they are exposed on the bark and before leaves shelter them. Once the canopy fills in, coverage of overwintering stages becomes impossible.

⚠ Only spray when temperatures will stay above freezing, ideally above 40F, for 12 to 24 hours afterward, and skip it if rain is expected within 24 hours - freezing or washoff after an oil application injures the tree. Oil does not control borers or codling moth, so do not expect it to.

Plum

Black knot: start fungicide as flower buds crack open

Begin fungicide applications when the flower buds are just starting to open, using captan, chlorothalonil, thiophanate-methyl, or lime sulfur, and repeat per the label until the new shoots have matured and hardened off. Spores are shed from last year's knots during spring rains and infect young green shoots and wounded branches, which is why the spray has to be running before the new growth is out and why fresh pruning cuts made in wet spring weather are an open door. Once shoot growth hardens, susceptibility drops and spraying stops paying.

⚠ Confirm plum is listed on the product label before applying - not every formulation of these actives is cleared for stone fruit. Fungicide alone will not save a tree whose knots were never pruned out.

Plum

Brown rot: blossom blight spray as flowers open

Start a fungicide when the first blossoms open, using myclobutanil, propiconazole, chlorothalonil, captan, or sulfur, and repeat per label through petal fall. Monilinia spores infect through the open flower, and the blighted blossom becomes the base camp from which the fungus moves into the spur and twig and produces the spores that rot the fruit months later. Stopping it at bloom is what makes the summer manageable.

⚠ This is a fungicide only - never add an insecticide to a bloom spray, because the flowers are full of foraging bees. Chlorothalonil is cleared on stone fruit for blossom blight only; do not carry it forward into fruit-rot sprays.

Plum

Plum curculio: spray at shuck split, once the petals are gone

On stone fruit, time the plum curculio spray for shuck split, after the petals have dropped and the dried flower remnants are splitting off the young fruit. Kaolin clay (Surround) is an option for home plantings. Repeat if warm weather continues, tightening the interval during peak activity, and pay extra attention to trees along a woods edge. The overwintered adults walk out of the leaf litter and hedgerows and cut crescent-shaped egg scars into the young fruit; the grub then tunnels the fruit centre and the fruit drops. The first few days above roughly 75F after petal fall are when the beetles are actually active and hittable - below 70F they barely move.

⚠ Do not apply insecticide during bloom - curculio adults are present then, but so are pollinators, so wait until the petals are off. Follow the label's pre-harvest interval and application limits for whatever product you choose.

Plum

Brown rot: protect the ripening fruit

Start fruit-rot sprays 2 to 3 weeks before harvest as the fruit begins to soften and colour, using captan, myclobutanil, propiconazole, or sulfur, and repeat per label right up to harvest. Green fruit is largely immune; susceptibility climbs steeply as sugars rise during ripening, which is why a clean tree can go to mush in the last fortnight. Warm wet weather in that window is what turns a few infections into a total loss.

⚠ Do NOT use chlorothalonil for fruit rot on stone fruit - it is cleared for blossom blight only. Observe each product's pre-harvest interval on the label; these are the sprays closest to the day you eat the fruit.

American Wild Plum

Black knot: start fungicide as the flower buds crack open

On a tree with a history of knots, begin fungicide when the flower buds are just beginning to open, using captan, chlorothalonil, thiophanate-methyl or lime sulfur, and continue on the label interval until the new shoots harden off and the weather turns consistently warm and dry. Black knot spores are shot out of old knots during spring rains and can only infect the current season's green shoots, so the whole spray program is about protecting that soft new wood while it is still soft.

⚠ Sprays work only if they are on the wood before the rain and above 60F; follow the label interval and the plum entry on the label, not the ornamental one.

American Wild Plum

Brown rot: protect the blossoms at 25 percent bloom

If the tree has rotted fruit before, spray a fungicide labeled for plum, captan, myclobutanil or propiconazole, when about a quarter of the flowers are open, then again 10 days after shuck fall and once more 10 days after that. Monilinia infects open flowers, and a blossom needs only about three hours of wetness near 70F to take the fungus in; the blighted flower then becomes a twig canker that seeds the fruit rot in August.

⚠ This is a fungicide, not an insecticide - never put an insecticide on an open flower. Alternate myclobutanil or propiconazole with captan rather than using the sterol inhibitors for every spray, because they share a mode of action and the fungus builds resistance.

American Wild Plum

Plum curculio: spray at petal fall, not before

Once three quarters or more of the petals have fallen and the fruit is around a quarter to a half inch, spray malathion or permethrin, or use kaolin clay for a non-insecticide option, and repeat on a 10 to 14 day interval. Trees next to woods need the follow-up most. The weevil overwinters in leaf litter at the woods edge, walks into the tree during bloom, and cuts a crescent-shaped slit in each young fruit to lay an egg - the first warm spell after petal fall, above about 70 to 75F, is when nearly all of that egg-laying happens. Mississippi State calls malathion the most effective material available to a homeowner for this weevil.

⚠ Do not apply any insecticide during bloom - it kills the bees the crop depends on; Mississippi State states plainly 'Do not apply insecticides during bloom.' Mississippi State lists a 7-day pre-harvest interval for malathion and a 7-day pre-harvest interval for permethrin on peaches; read the plum line on your own label and stop spraying at least that long before picking. Avoid applying malathion in overcast or highly humid weather - the spray dries slowly and can injure the tree.

American Wild Plum

Brown rot: cover the fruit in the month before picking

Begin fungicide about one month before the plums start to color and repeat every 7 to 10 days through harvest, using a product labeled for plum such as captan. The second brown rot infection window opens as sugar rises in the ripening fruit - one rotted plum sporulates and takes the whole cluster down within days of picking weather.

⚠ Pre-harvest intervals differ sharply between products - captan and the sterol inhibitors are not interchangeable this close to harvest, so check the days-to-harvest figure on your label before every late spray.