The Navajo blackberry, a cultivar developed at the University of Arkansas in 1989, yields the highest sugar content of any Arkansas variety. Ripe Navajo blackberries store for three to four weeks instead of the usual three to four days for other blackberry varieties. Fruits ripen from the middle of June to the end of July on thornless vines. A good variety for the home garden or U-pick farms, the Navajo requires an extended period of winter chill. Between 800 and 900 hours of winter temperatures between 32 and 45 degrees F. are needed for vigorous growth.
Select a planting site for the Navajo blackberries that has full sun, good drainage and deep soil, rich in organic matter. Planting a thick cover crop and tilling it under helps prepare the ground for blackberry growth. Plan a bed 5 feet wide to allow plenty of space for the plant's shallow roots. Allow enough space between rows for easy access--10 or 15 feet between centers.
Test the soil pH to be sure it falls in the range of 5.5 to 6.5. Lower the pH one point by adding 1 lb. of powdered sulfur over 100 square feet of ground. Raise the pH by adding 5 pounds of lime for every 100 square feet of bed. Till the soil deeply and test again in a few weeks.
Plant Navajo blackberries in late winter, 4 inches deep and 3 to 4 feet apart down the center of the row. Keep the canes upright and prune the plants back to 6 inches high.
Set steel fence posts 15 feet apart down the row. Drive support stakes 3 feet or more from the ends of the row, leaning out at a 45-degree angle. Fasten two support wires to the row at 2 feet and 4 feet high on the posts. Anchor the wires to one of the support stakes, tighten the wires and fasten them to the stake at the opposite end of the trellis.
Mulch the blackberry bed 6 inches deep with straw rather than hay if possible to prevent adding weed seed to the bed. Sawdust, bark or wood chips also work well. Add more mulch when the layer reduces to 3 inches. Mulch protects the top layer of soil where the berry roots grow and reduces the plants' water needs. A thick layer of mulch also reduces competition from weeds.
Select two new canes for attachment to the trellis. When the canes pass the first wire, tip-prune one and loosely attach the cane to the lower wire with a wire tie. Allow two horizontal branches to grow in either direction on the trellis wire and tie them to it later in the season. Tip-prune the other cane when it passes the higher wire and train it the same way. Cut out any other canes at ground level in mid-summer.