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Measure the seedling's diameter 1 inch (2.5 cm) above the root collar, where the roots meet the trunk. Select seedlings with a minimum diameter here of 1/4 inch (0.64 cm), and preferably larger. This is the most important measurement for predicting quality.  Bare-root seedlings, sold without any soil, should be planted in early spring, before bud growth, and should be planted immediately after acquiring. Containerized seedlings can handle later planting dates and drier soils, but are typically much more expensive. Choose well-draining, loamy soil, avoiding steep slopes and hilltops. Place the seedlings in holes that are twice as wide in diameter as the seedling roots, and just deep enough to bury the roots. For best results, backfill with one part compost for every three parts normal soil. Tamp down the soil and water thoroughly. Plant the seedlings 12–17 feet (3.7—5.2m) to get the most nuts from your trees. Keeping seedlings 10–12 ft (3.0–3.7 m) apart is best for timber production. For at least the first two years after planting, whether grown from nut or seed, the walnut tree needs supplemental watering, especially during dry or warm weather. Weekly waterings are best for newly planted trees, but you can water less as the tree grows. Give the plant a thorough watering, but do not water again until the soil has mostly dried. Frequent watering can harm the plant. After two or three years, the trees only need to be watered during the hottest time of year or during a drought, about one to three times a month. Care for seedlings by keeping the area around them free of sod and weeds, which will compete with the growth of small seedlings. Remove sod and weeds by hand or by laying fabric weed barrier. Larger seedlings can be treated with mulch to keep weeds at bay, using about 2 or 3 inches over the root zones. Do not use mulch on plants that have not yet emerged from the soil, as it can block the sprout from growing. Wait until the seedling is woody and has developed roots. If you are raising the walnut for timber, it is important to prune early to ensure a straight trunk, leaving one "leader" branch at the top of the tree and guiding it straight and upright over the next one or two growing seasons. Saplings grown for nuts can be left alone until after thinning, but subsequent pruning is wise for black walnut trees, as these are usually sold for timber eventually, even nut varieties.  If you have not pruned trees before, especially saplings, finding an experienced pruner to help you identify leaders and important branches is recommended. If the top of the tree is forked, bend the best leader upright and tape it to other branches as support, then cut off the tip of the supporting branches to prevent growth. Most orchards begin with more plants than the area can support. Once the trees are large enough that the branches are beginning to run into each other, select the healthiest trees that display the characteristics you value, typically a straight trunk and rapid growth. Remove the rest, but avoid clearing too much space that can cause weeds or even competing trees to grow. You may wish to use a crown competition formula to help you make your decision. Fertilization is somewhat controversial, at least for black walnuts, because it can assist competing weeds more than the tree if the soil is already rich in nutrients. Wait until the trunk is "pole" size, or at least 4 inches (10 cm) in diameter measured 4.5 feet (1.4 m) above the ground. Ideally, send soil or leaves to a forestry laboratory to identify exact nutrient deficiencies. If this is not possible, apply fertilizer containing 3 lbs nitrogen, 5 lbs triple super phosphate, and 8 lbs muriate of potash to each tree in late spring. Leave a few trees unfertilized to compare the effect, and, if positive, reapply every 3–5 years.   Test soil pH after fertilizing to see if you need to adjust it back to normal levels. Squirrels are a common sight in walnut groves, and can take an entire crop of nuts if not controlled. Cover the trunks with plastic tree guard to keep them from climbing them, and prune away branches less than 6 ft. (1.8 m) from the ground if you are able to do so without causing knots that diminish timber value. Other pests such as caterpillars, aphids, and flies vary by region, and may not harm your tree if they are active late in the growing season. Consult a nearby forester or experienced walnut grower for information specific to your region.  Keep livestock away from walnut trees of any size, as the damage they cause may even make the timber value of adult trees worthless.

Summary:
Select seedlings (if not growing from nuts). Plant the seedlings in the spring. Water occasionally. Deal with weeds. Learn about pruning walnuts. Thin the stand of trees to select the best examples. Use fertilizer once the tree has grown past sapling size. Control pests.