Drilling Guide

A drill is a machine that sows seeds by burying them in rows in the soil.  A seed drill uses a specialized disk set-up that cuts through the thatch on top of the soil, places the seed into the furrow, and then presses the soil back over the seed for good seed-to-soil contact.

For a successful stand, seeds need good seed-to-soil contact.  Planting the seed too deep is often the main cause of stand failure in a drilled planting.  This guide will give answers to some commonly asked questions about drilling native seed.

Do I Need a Special Drill for Fluffy Seeds?

Several of the native grasses are fluffy (think about trying to sow feathers), which can interfere with seed flow in the hopper.  Fluffy seeds tend to bridge up over the exit holes.  Some drills have native grass boxes with agitators or picker wheels to help combat this issue.

How Deep do I Sow the Seeds?

An ideal planting depth for native grass and wildflower/forb seeds is 1/8 inch – ¼ inch.  The exception is Eastern Gamagrass, and it prefers to be buried about 1” deep.  If seeding a mix, it is safer to plant on the shallow side because large seeds that do fine with deeper planting depths will almost always still emerge when planted at a shallower depth, but small seeds will not emerge if planted too deeply. 

To be on the safe side, it is ideal to see 50% of the seed on top of the soil (except for a pure stand of Eastern Gamagrass).  Some drills do not have good depth control, and as topography and field conditions change, the depth that the drill is burying the seed can change, which can result in a patchy stand because in certain areas the seed was buried too deep.  It is best to err on the side of the seed being on top of the soil rather than burying it too deep!

How do I Set/Calibrate the Drill?

Adjustments will need to be made to the drill to reach the desired seeding rate and planting depth.  How the drill is set varies from manufacturer to manufacturer. There is no “recipe” that fits every drill type. A drill manufacturer typically provides a manual for calibrating or setting a drill. Many times, the local NRCS/USDA/SWCD office can also provide advice. And, in this modern age, it is usually possible to find a good how-to video on the Internet.  However, in general, there are usually 2 things to adjust for: amount of seed and depth.

1.  Amount of Seed:  The flow rate calibration is a setting that controls the amount of seed (or seed + carrier) that is dispersed at one time.  A general rule of thumb when calibrating the drill for a mix is that the opening of the seed metering unit should be set to distribute the largest seed in the seed mixture. If this means that the mixture will flow too freely to reach the desired seeding rate, carrier can be added to “dilute” the mixture.
2. Depth:  Getting the depth set properly is very important with native seeds.  There are two settings that help control the depth.
   • Coulters – This part of the drill opens up a “ditch” in the soil where seed will be placed.  By adjusting the coulters, the “ditch” is cut deeper or shallower.  Each coulter can be adjusted.  This, along with the packing wheels, determines seed planting depth.
   • Packing wheels – close the “ditch” made by the coulters after the seed has been placed.  The packing wheels impact planting depth because they can be set to press the seed into the soil more or less.

What if the Drill Can’t be Set This Shallow?

Some drills won’t drill shallow enough for native seeds.  If that is the case, there are a few options:

• If the coulters cannot be set shallow enough, try to set the coulters to skim the top of the soil (in other words, they do not actually touch the ground) and set packing wheels to press seed into the soil. 
• If the packing wheels cannot be adjusted, take seed drop tubes off the coulters to in effect place seed on top of the soil.

Advantages of Drilling

• A seed drill not only distributes the seed but also ensures seed-to-soil contact.  (Unless there is a lot of thatch on the soil…read more in the words of caution below.)
• Drilling minimizes soil disturbance.
• With some government cost-share practices, less seed is required when drilling than when broadcasting.  It may also extend the planting deadline of some cost-share plantings.
• Because drilling presses the seeds into contact with the soil, it can be a great seeding method when a planting deadline is near.  The better contact with the soil, the quicker the seed can take up water and either begin the process of germination or stratification.
• A seed drill places the seeds in a row, which can make it easier to determine if the seeds have germinated and how intense the weed pressure will be.

Words of Caution When Drilling

• A common concern is that seeds of differing sizes may separate and settle as the hopper is jostled while planting.  To resolve this potential issue, periodically remix the seed within the hopper or only fill the hopper with the amount of seed that can be planted in an hour’s time.
• Adding a weed-free carrier to the mix will help to “bulk up” the mix volume which allows the seed to be spread over a greater area at a lower seeding rate. Common carriers include pelletized lime, dried distiller’s grain (usually available at local feed stores) or rice hulls.
• If possible, divide the seed mix into quarters before planting.  Do the same with the area to be planted.  This will allow you to determine if the drill has been calibrated correctly during the first quarter of planting.  It is best to take a conservative approach initially.
• Do not run the seed drill in reverse.  Doing so will clog the drill and inhibit the seed from flowing freely.
• Make sure that the seed tubes and hopper are free of undesirable seed (fescue & other introduced pasture grasses and legumes).
• Make sure that the seed tubes are free of debris.
• Setting a drill can be incredibly frustrating, especially when there are no clear settings meant for native plantings.  A resource such as a knowledgeable technician at the Soil & Water Conservation District (which often rents drills to landowners) or NRCS or your state’s wildlife agency can be extremely helpful.
• It is easy to bury the seeds too deep with a drill.  Most native plants do best when the seed is buried no more than 1/8” to ¼”.  To be on the safe side, it is ideal to see 50% of the seed on top of the soil.
• Too much thatch on top of the soil means that the drill won’t be able to get the seed in contact with the soil.  If there is a lot of thatch (e.g. a full growth of cover crop), consider grazing the growth off with livestock or using a controlled burn.   
• Drills do not love rocky soils.  
• Drills do not love stumps and other obstacles commonly found in savanna restorations or silvopasture plantings.