Edamame (Soybean)

Direct-sow after your last frost, once soil has warmed to at least 55 degrees F, placing seed only a quarter to a half inch deep, 2 to 4 inches apart, in rows about 30 inches apart. Handle the seed gently - breaking the seed skin cuts germination. Sow into soil that is already moist rather than watering hard afterward; if the soil is dry, irrigate before you plant. Direct seeding is the norm, but a short-season grower can raise transplants under cover instead. Edamame seed is mostly sold untreated and germinates less reliably than field soybean seed, so it rots or emerges in patches in cold wet ground. The seedlings are weak and cannot push up from a deeper sowing, and irrigating after planting crusts the surface and traps them.

If soybeans or edamame have never grown in this ground, dust the seed with Bradyrhizobium japonicum soybean inoculant before sowing. Bean, pea, and clover inoculants are different bacteria and will not work. On an inoculated crop skip the nitrogen fertilizer entirely, and set phosphorus and potassium from a soil test instead. Aim for a soil pH of 6.0 to 6.5. Only Bradyrhizobium japonicum forms the root nodules that fix nitrogen for soybean. Feeding an inoculated crop extra nitrogen does not reliably raise yield, and it can suppress that fixation and delay flowering, which on a short-day crop can cost you the harvest.

Net or row-cover the seedlings as they emerge and leave it on until the plants have several trifoliate leaves. If deer or elk range on your land, fence the planting before pods start to form. Birds do extensive damage to young edamame plants, and deer and elk can be devastating to a mature stand right when the pods are worth eating.

From the first flowers through full pod fill, supply about three quarters to 1 inch of water a week in humid regions and 1 to 1.5 inches a week in arid ones. Water deeply on a 3 to 5 day interval rather than lightly every day. Flowering through pod fill is the crop's peak water demand, losing up to 0.3 inch a day, and a shortage in that stretch shows up directly as flat pods and aborted beans. Watering more often than every 3 to 5 days keeps the canopy damp and raises disease risk.

Walk the rows weekly once the plants are up, looking for chewed leaves and the beetles themselves: bean leaf beetles are small and often tan with black borders and spots, and Mexican bean beetles are copper-colored ladybeetle look-alikes whose larvae are yellow and spiny. Only act at about 20 percent defoliation or 2 to 3 beetles per plant. Catching these beetles while they are still working on leaves keeps their numbers from building into the later generation that chews directly into the filling pods, which is the damage that actually costs you the crop.

In hot dry weather check leaf undersides weekly, starting at the edges of the planting, for fine white stippling along the veins. Press a damaged leaf against white paper to confirm crushed mites. Spot-treat the affected edges with sulfur as soon as you find it rather than treating the whole planting. Escalate to treating the whole planting only if live mites are numerous, about 20 to 30 per leaflet, and more than half the plants show stippling, yellowing, or defoliation. Mites build up first on the hot dry margins and work inward, so treating the edges at the first stippling holds the infestation there and spares you a full-cover spray. If it has already gone planting-wide it is still treatable, but it then takes a whole-planting application rather than spot work.

Once pods are filling, check for stink bugs on the pods and for soybean aphid colonies at the growing tips. Treatment thresholds are 7 stink bugs per 50 sweeps during pod development, and 250 aphids per plant while pods are still filling. Brown and brown marmorated stink bugs shrug off pyrethroids more than green ones do. Stink bugs and pod-feeding caterpillars pierce and chew straight through the pod wall, and a holed pod is simply lost. Once pods have finished filling the plant tolerates over 1000 aphids with no yield loss, so a late aphid flare rarely justifies spraying at all.

Harvest when the pods are plump and bright green and the beans almost touch inside the pod, soybean stage R6. Check every day or two as they approach it, because the window can be as short as 3 to 4 days. Pull whole plants once most pods are ready, pick in the cool of early morning, and get the pods down to 32 to 37 degrees F right away. The moment pods show any yellow the sugars have gone to starch and the sweet nutty flavor with them. Sugars and amino acids also start breaking down within 3 to 10 hours of picking if the pods stay warm, which is the only reason garden edamame beats frozen.