Caption: An established stand of PAm Annual Clover mix growing in a field. Image by: Project Apis m.
Cover crops are an affordable tool which can solve multiple issues on the farm. This practice can be used to combat soil erosion, reduce dust, improve water infiltration, slow or prevent nutrient leaching, and many other benefits. Popular places to plant cover crops include orchard drive rows, field edges, winter fallow crop lands, and unused areas.
There are many factors to consider when planting a cover crop, even before the seed has been delivered. Seed planting rate is a crucial part of this practice’s long-term performance. There are many considerations when deciding the planting rate of a mix. Plants which are seeded at too dense a rate can become stunted and have over all poor performance. While cover crops which are seeded at too low a rate will not achieve optimal soil coverage and their full benefits will not be realized.
Seed size and mature plant size are two factors which play deciding roles in planting rate. Typically, the smaller the seed size, the fewer pounds to the acre are needed. This is because there are more individuals per pound than larger seeds. For example a cup of mustard seeds will contain far more individual seeds than a cup of peas.
Seed weight plays a role in determining species percentages in cover crop mixes. The composition of a mix is presented as a weight percentage, but this may not directly correspond to the actual number of plants that will grow in the field.
Calculating the amount of seed required to plant a cover crop is based off the planting rate. If the planting rate for a selected mix is 30 lbs/acre and the area to be planted is 10 acres then 30 times 10 would equal 300lbs of seed. Additional calculations will be required if the grower wants to plant only a portion of their farm, such as the drive rows in an orchard. In this instance acreage is determined by dividing the desired cover crop strip width by the total spacing between trees. If a grower wants a 10 foot wide cover crop and their tree to tree spacing is 20 feet then divide 10 by 20 to get 0.50. This number shows the percentage of the orchard which will be planted into cover crop. The final step is to multiply the total orchard acres by 0.50 to get the number of cover crop acres. If the orchard had a total of 100 acres then the grower will need enough seed to cover 50 acres.
10 foot wide cover crop strip ÷ 20 foot tree to tree spacing = 0.50
100 orchard acres X 0.50 = 50 cover crop acres
50 cover crop acres X 30lbs/acre seed rate = 1,500lbs of seed
There are a few basic requirements for planting winter cover crop seed. First and foremost is the planting date. Seeds for Bees has found that the optimum planting window for winter cover crops is early to mid-October though cover crops can be planted as early as September 10 to as late as November 10. This timing ensures that cover crops are likely to be blooming when honey bees are present later in the winter and early spring, giving the bees a boost in nutrition, which in turn helps with almond pollination.
There are two main ways cover crops are planted in California, either with a seed drill or by broadcasting. The main difference between these two methods is how the seed is planted. A seed drill opens up the soil and buries the seed at a predetermined depth while a broadcaster scatters the seed over the soil surface.
Seed drills ensure better seed germination by planting each seed. Because of this they can plant cover crop at a lower seed rate and save costs per acre. They also offer greater control over where the seed is placed which can be beneficial in orchards and vineyards. However seed drills come with a steep up front cost and are more complicated to manage and maintain. Seed drills can also be difficult to operate in orchards where the machine can break low hanging limbs or be damaged by branches hitting the unit.
A mix of cover crop seeds in a grain box (left) ready to be planted via see drill (right). Photo by: Project Apis m.
Broadcasters can vary greatly in size and cost making them more versatile for small to mid-sized growers. These units can be small, easily fitting to a quad, while others can be larger and are often pulled behind a tractor. A drawback with this unit is that it often results in a lower germination rate, needing more seed, and requires the seed to be incorporated into the soil by another unit. An economic fix for this issue is to drag a piece of chain link fence behind the broadcaster while planting.
Like all crops, cover crops require water to germinate and grow. Seeds for Bees’ mixes are all winter cover crops and designed to survive off of rain fall which can affect when the seed is planted. Rain is exceedingly important for germinating the crop. If the weather forecast calls for a dry October, Seeds for Bees recommends waiting to plant the seed until a few days before a storm. Growers with a solid set irrigation system or a micro-sprinkler system which reaches the drive rows can plant their seed regardless of rain fall so long as they plan to irrigate the seed in case of dry weather. Once the seed has germinated there is no need for irrigation unless the leaves begin to curl, which is a sign of water stress.
The final factor to consider before planting is how cover crops will affect other activities on the farm. A common issue almond growers face is timing seed planting with winter sanitation for Navel Orange Worm and cleaning up after pruning. Farmers have devised a number of solutions for this timing issue. Growers can plant the seed and shortly after take care of their winter sanitation and brush clean up before the seed has sprouted. Other farmers plant their cover crop every other row, allowing for a clear row to be available throughout the season.
In the grand scheme of things, a cover crop's success and timing hinge on several factors, with the weather playing a pivotal role. It's essential to approach cover crop planting with care, striking a harmonious balance between the farm's requirements, the cash crop, and the optimal timing for cover crop sowing. By aligning these elements thoughtfully, we set the stage for a flourishing and productive future.