Cranberry

Before the ground freezes hard, protect the vines from winter drying by flooding the bed or by laying on a mulch such as pine boughs. Where snow cover is dependable, the snow itself may be enough. The conditions to protect against are at least three consecutive days of frozen soil, freezing temperatures and drying winds. Winter kills cranberry by dehydration, not simply by cold - once the soil is frozen the roots cannot replace the water that freezing temperatures and drying winds pull out of the evergreen leaves. Flood water or a blanket of boughs or snow keeps that wind off the vines.

Set rooted cuttings about one per square foot into sand or sandy soil with a pH below 5, and acidify with sulfur first if your soil tests higher. Water daily but lightly for the first week or two, then stretch to every few days. Skip fertilizer in the planting hole if the roots are ample. Cranberry is a bog plant that only takes up iron and other nutrients in strongly acid soil, and spring planting gives the roots cool nights and warm days to establish before summer heat. At one cutting per square foot the vines knit into a mat in one or two years.

Once the buds break dormancy, protect the bed whenever a frost is forecast, either by running sprinklers through the freeze or by flooding the bed. Tolerance falls fast as the buds open: for Ben Lear, Stevens and the newer hybrids it runs about 20F at spring dormant, 25F at bud swell, 27F at bud break, and only 29.5 to 30F from bud elongation through bloom. Early Black and Howes hold roughly two to three degrees more from spring dormant through bud elongation - about 18F, 22F, 25F and 27F at those same stages - but that margin disappears at bloom, where every variety is equally tender at 29.5 to 30F. The tissue that will become this year's flowers is the tissue that loses hardiness first, so a bud that shrugged off 20F in March can be killed at 30F in May. Every degree of hardiness the plant sheds is crop you can lose in one night.

Split the season's nitrogen across bloom and set rather than dumping it early. On Early Black and Howes budget 25 to 40 lb N per acre for the year, on Ben Lear and Stevens 35 to 50 lb, and on the Rutgers and Wisconsin hybrids like Crimson Queen and Demoranville 50 to 80 lb. With fast-acting sources put 50 to 60 percent on at about 75 percent bloom and the remaining 30 to 40 percent at 75 percent out of bloom. Keep phosphorus to 10 lb per acre or less unless a tissue test shows a deficiency. Cranberry evolved on nitrogen-poor peat and preferentially takes up the ammonium form over nitrate, so high-nitrate fertilizer is largely wasted. Nitrogen delivered at bloom and set goes into berries, while nitrogen delivered early goes into runners that shade the fruit.

Start when the bed is 10 to 50 percent through bloom, with many growers going at about 30 percent of flowers open, using chlorothalonil (Bravo Weather Stik, Echo 720) or mancozeb, and repeat on a 7 to 14 day interval through bloom. Once fruit has set and is sizing up, further sprays no longer help. The fruit rot fungi infect open flowers and just-set berries months before any rot shows at harvest, and chlorothalonil is a contact material that only protects the tissue it lands on - so it has to be on the flowers while the flowers are open.

Time the first spray to the 50 percent out-of-bloom date on early varieties, especially Stevens; on Howes and Early Black it can wait 7 to 9 days past 50 percent out-of-bloom. Follow with a second application about 10 to 14 days later. Use methoxyfenozide (Intrepid) or chlorantraniliprole (Altacor), which are soft enough on bees to go on right at 50 percent out-of-bloom. The moths emerge in mid to late June and lay their eggs at the calyx of berries that have just set, so the spray has to be on the fruit surface during egg laying - once the larva chews into the berry nothing you spray will reach it.

Sweep at least once a week with a 12 inch net, sweeping 180 degrees down into the vines. One set is 25 sweeps, and you want about one set per acre. Treat when you average 1 to 2 Sparganothis fruitworm per 25-sweep set, and back the sweeps up with a visual search for webbed vines and leaves. The sweep net will not tell you about cranberry fruitworm - that one is scouted by inspecting berries for eggs laid at the calyx end. On a 5 to 7 acre bog, inspect 251 to 350 random berries and treat at 2 viable eggs. Cranberry fruitworm larvae bore in at the calyx and hollow out three to six berries apiece before dropping to the soil, and Sparganothis webs itself into the canopy. Sparganothis you can catch in the net; cranberry fruitworm you cannot, which is why the berries themselves are the sample. Both are invisible from the edge of the bed and untouchable once inside the fruit.

Judge ripeness by looking down onto the vines from above rather than hunting for color under the canopy. As the berries color they toughen against frost, the same for every variety at first: green fruit is done at about 30F, white to light blush at 28F, deep blush on the exposed surface at 27F, deep blush all over at 26F, red at 25F. Early Black and Howes add a dark red stage at 24F. At deep red all varieties hold to about 23F. At maroon the varieties split: Early Black 23F, Howes as low as 20F in some years (returning to 23F two weeks later), Stevens 22F, and Ben Lear only 24F. Protect or pick ahead of any night that undercuts the stage and variety you are at. It is not the color itself that confers frost tolerance but the internal chemical and physiological changes that come with it, so color is simply the readable signal of how much hardiness the berry has banked. Fruit buried in the canopy may be hardier than its pale skin suggests.