Jostaberry

Taper the water after harvest to harden the bush for winter

Irrigate steadily through fruit development for best berry size and yield, then gradually reduce the water once the last picking is done rather than cutting it off all at once. Adequate irrigation is what sets fruit size and yield, but water carried late into the season keeps pushing soft growth that has not hardened off by the time the first freezes arrive.

Two to three inches of mulch, topped up every year

Spread two to three inches of straw, wood chips, compost, lawn rakings or grass clippings over the root zone and replenish it yearly. Grass clippings are the best of them here. Jostaberry is a cool-climate plant with a shallow root system that wants moist soil and a high water-holding capacity. Mulch keeps the soil cool through summer, holds that moisture, suppresses weeds, and reduces winter injury to the roots.

⚠ If you mulch with fresh straw or sawdust, apply extra nitrogen fertilizer alongside it. These high-carbon materials tie up soil nitrogen as they decompose, and this plant is already a heavy nitrogen feeder.

Feed only a bush that is not vigorous, and keep the chloride out

Work manure into the soil before planting, and topdress with composted manure annually - on soil already in good heart that is the whole job. Then look at the bush each spring: if it is pushing strong new canes from the base, it does not need fertilizer. Only if plants are not vigorous, lightly broadcast about a quarter to a half pound of 10-10-10 per plant. Cornell recommends the 10-10-10 only for plants that are not vigorous. Ribes are heavy nitrogen feeders and the cane-renewal system does depend on the plant pushing new basal shoots every year, but feeding a bush that is already making them buys soft growth and a denser canopy, and poor air circulation is what favors powdery mildew - the disease that most commonly plagues Ribes.

⚠ Avoid any fertilizer containing muriate of potash (potassium chloride).

Renewal pruning: cut out everything older than three years

While the bush is dormant, in late winter or early spring before the buds swell, cut every cane older than three years off at ground level, along with any branches lying on the soil and anything broken or hollow. Thin the young shoots so a mature bush carries about eight bearing canes, roughly three or four of each age class. Ribes fruit at the base of one-year-old wood and most heavily on the spurs of two- and three-year-old wood, so a fourth-year cane takes light and space without paying for it. Removing the oldest class every year forces the new canes that will carry the next crop.

⚠ Destroy the prunings instead of stacking them nearby. Currant borer larvae overwinter inside the pith of old canes, and cutting infested canes out before the clearwing moths emerge in June is the best control available.

Imported currantworm: check the undersides of the first leaves

As the leaves expand, turn over the leaves low in the center of the bush and look along the major veins for rows of small eggs, then a week to ten days later for half-inch greenish larvae with black spots chewing holes and eating in from the leaf margins. Hand-pick the egg-bearing leaves and the larvae. If a bush is being stripped after fruit set, an azadirachtin product such as AzaGuard or Molt-X, or a horticultural oil such as SuffOil-X, will bring the larvae down. The sawfly adults lay on the undersides of the first leaves to expand, and if the larvae are numerous they can strip a bush of its foliage in a few days. A defoliated bush cannot finish the crop it is already carrying.

⚠ Bt will not touch these. Sawfly larvae resemble lepidopteran caterpillars but are not related, so Bt products do not control them. The materials above carry a 0 to 3 day pre-harvest interval. Do not spray anything insecticidal while the bush is in bloom - jostaberry flowers are bee-visited.

White pine blister rust: check your state law first, then watch the leaves

Before you plant at all, check with your state department of agriculture or local extension office - jostaberry is a black currant hybrid and several states regulate it as one. Through summer, look on the undersides of the leaves for orange-yellow pustules, the sign of Cronartium ribicola, and report a find to your extension office. The rust cannot complete its life cycle without both a Ribes and a five-needle pine, which is why states with white pine still control Ribes planting. Jostaberry cultivars are rated resistant, but resistance is not immunity, and a newer rust strain in New England has already overcome resistance in some black currant varieties previously considered safe.

⚠ Jostaberry is not permitted in Virginia or West Virginia. Maryland permits it. Michigan requires a permit and issues them only for varieties rated highly resistant, listing Josta and Orus 8 among them. Check your own state before you buy a plant.

Dormant oil for San Jose scale, only if you saw scale last year

Scale is easy to spot on bare dormant wood as small hard grey bumps crusting the older canes. If you find them, spray a dormant horticultural oil or lime-sulfur while the bush is fully dormant and before the buds swell. Pruning out and destroying the infested canes handles a light infestation on its own. Oil kills scale by smothering it, so it only works against the armored overwintering stage, and only while there is no foliage or swelling bud tissue for the oil to burn.

⚠ Dormant oil carries a 0-day pre-harvest interval. Apply before buds swell, and never within 48 hours of a frost - oil plus freezing injures the wood.

Pick when fully colored and soft - the window is forgiving

Pick jostaberries once they are fully colored, reddish-black to black, and soft to the touch. Timing is not critical the way it is with most soft fruit, so a bush can be picked over more than one pass without losing the crop. Jostaberries have a long shelf life, so the fruit does not punish a late picking the way a raspberry does.