Few things kill an herb garden faster than soil that holds water like a bowl. Rosemary, thyme, oregano, and most culinary herbs evolved on dry, rocky slopes — their roots rot quickly in soggy ground. Before you plant a single seed, a 20-minute soil drainage test tells you whether your garden bed needs work or is ready to grow. Here is exactly how to run the test and what to do with the results.
What You Need for a Soil Drainage Test
The perk test requires simple tools you probably already own. You do not need a kit, a lab, or any special equipment.
- A shovel, post-hole digger, or flat garden spade — the flat spade is best for cutting vertical walls
- A ruler, tape measure, or yardstick
- A straight stick, scrap board, or flat piece of wood to lay across the hole
- A hose or bucket of water
That is it.
The Step-by-Step Percolation Test
The procedure from Iowa State Extension is the most widely trusted method, and the steps are straightforward.
1. Dig the Test Hole
Dig a hole 12 inches deep and 4 to 12 inches wide. For the easiest math, make it a 1-foot square hole. The walls should be as vertical as possible — sloping walls let water escape sideways and skew the results. Use a flat spade to cut clean edges.
2. Saturate the Soil First
Fill the hole with water and let it drain completely. This step is critical: dry soil absorbs the first fill of water, so measuring that fill gives a false reading. Wait until the hole is empty before moving on.
3. Refill and Measure
Immediately after the first drain, refill the hole to the top. Lay the stick or board across the hole and measure the distance from the water surface to the bottom of the stick. You can also stick the ruler straight into the water and note where the surface hits.
4. Wait 15 Minutes
Set a timer for 15 minutes. Do not disturb the hole during this time.
5. Measure Again
After 15 minutes, re-measure the water depth from the same reference point.
6. Calculate the Drainage Rate
Subtract the second measurement from the first to get the total drop in inches. Multiply that drop by 4 to get the drainage rate in inches per hour.
Formula: (First measurement − second measurement) × 4 = drainage rate in inches per hour
Example: If the water dropped 0.5 inches in 15 minutes, multiply 0.5 × 4 = 2 inches per hour. That is ideal for an herb garden.
What the Drainage Rate Means for Your Herbs
The number tells you exactly what your soil needs — or whether it needs nothing at all. The table below shows what each range means for an herb garden.
| Drainage Rate (inches/hour) | Soil Classification | What It Means for Herbs |
|---|---|---|
| 1 – 3 | Ideal | Perfect drainage for most herbs. No action needed. |
| Less than 1 | Poor | High root rot risk. Soil holds water too long. |
| 4 – 6 | Very well-drained | Herbs may need more frequent watering in dry spells. |
| Over 6 | Fast-draining | Water runs through too quickly. Heavy soil amendment needed. |
How to Fix Poor Drainage for Herbs
If your test shows poor drainage, do not add sand — it binds with clay to form a concrete-like layer that makes drainage worse. The fix is organic matter.
Spread 2 to 4 inches of compost over the soil surface and work it into the top 6 to 12 inches. Compost improves both drainage and moisture retention at the same time by creating pore spaces in clay and binding particles in sand. For raised beds, drainage is handled by the bed design itself — you control the entire soil mix. If you are starting a new raised bed and want a proven soil recipe, see our tested roundup of the best soil for herb gardens.
One more thing: do not over-till. Excessive tilling destroys soil structure. Work the compost in gently and let the soil biology do the rest.
Common Mistakes That Ruin the Test
A few errors produce results that look good but lie to you. Avoid these.
Digging in wet soil. Soggy soil glazes the hole walls, creating a false barrier that slows drainage and gives a poor rating even when the soil is fine. Test when the soil is moist but not muddy.
Skipping the saturation step. If you measure the first fill, you are measuring how fast dry soil absorbs water, not how fast the soil drains. That overestimates drainage and hides drainage problems.
Measuring from the wrong reference. Without a stick or board across the top, it is hard to get the same measurement point twice. A consistent reference eliminates guesswork.
Using a shallow hole. If you plant herbs with deep root balls — like lavender or rosemary — test at the depth those roots will actually grow. An 18-inch hole may be more useful than a 12-inch one for deep-rooted herbs.
Alternative Method: The Time-to-Drain Calculation
Some gardeners prefer to track how long it takes the hole to drain completely rather than measure a 15-minute drop. This method gives a “minutes per inch” rate that is useful for very slow or very fast soils.
After the saturation step, refill the hole and time how many minutes it takes for all the water to disappear. Divide the total minutes by the depth of the hole in inches. For example, 120 minutes to drain a 5-inch-deep hole equals 24 minutes per inch.
How to Test Soil Drainage for Herb Gardens — Final Checklist
Follow this sequence for a clean, reliable result on your first try.
- Dig a hole 12 inches deep with vertical walls.
- Saturate and wait until the hole drains completely — do not skip this.
- Refill, mark a reference point, and measure the water depth.
- Wait exactly 15 minutes and measure again.
- Subtract, multiply by 4, and compare to the 1–3 inches per hour target.
- Amend with 2–4 inches of compost if drainage is too slow or too fast.
- Retest after amending to confirm the fix worked.
FAQs
Can I test drainage in a raised bed the same way?
Raised beds usually drain fine because you control the mix, but you can run the same perk test inside the bed. If drainage is slow, the issue is often the soil against the bed walls or a compacted base layer.
How deep should the test hole be for shallow-rooted herbs?
A 12-inch hole works for most herbs, including basil, cilantro, and chives. For deep-rooted herbs like lavender, rosemary, and sage, extend the hole to 18 inches to match the eventual root depth.
Do I need to test more than one spot in my garden?
Yes. Soil can vary significantly across a single yard. Test at least two or three spots in the area you plan to plant, especially if some patches stay wet longer after rain.
What if water still sits in the hole after 24 hours?
That level of poor drainage means the soil is effectively waterlogged. Avoid planting herbs in that spot without serious amendment — consider a raised bed or a mound instead to lift roots above the saturated zone.
References & Sources
- Iowa State University Extension. “Testing and Improving Soil Drainage.” Primary source for perk test dimensions, procedure, and 1–3 in/hr ideal rate.
- TreePeople. “How to Test Soil Drainage.” Alternative 1ft x 1ft hole method and time-based drainage chart.
- Western Garden Nursery. “Testing Soil Drainage.” Confirms 15-min wait and “multiply by 4” formula.
- Garden Culture Magazine. “Easy Home Soil Tests.” Ideal 2 in/hr rate and compost application tips.
- High Country Gardens. “Garden Soil Type Testing.” Lab testing recommendation for blended soils.