Roots and Shoots: Healthy Soil, Better Drainage

Some plants are fine in this wet, cool weather, but others need more help. (Photo by P. Doan)

By Pamela Doan

Being a gardening columnist during the Anthropocene has meant constantly updating record-breaking weather patterns and having lots of inspiration to figure out how to help things grow in less than ideal conditions.

Since I started writing Roots and Shoots in 2013, the earth has experienced the five hottest years on record. And it’s not your imagination; May has been exceptionally cold and wet. From May of last year to mid-May of this year, precipitation has been 30 percent above normal. Average rainfall is about 50 inches for that time period, and we’re way beyond that.

No matter what scale of gardening you’re attempting, climate change is happening and if you want to garden successfully, the more you understand about soil and plants, the better chance you have.

When water and soil interact, the water flows down into the pore spaces occupied by air. An oversaturated soil has too much water and not enough air. Plants wilt and roots rot in these anaerobic conditions without enough oxygen. Think of an overwatered houseplant sitting in soil that never dries out. It’s not a happy plant.

The soil’s texture, structure, pore space, and the amount of organic matter in it determine how water moves through it. Water can filter down, evaporate, be taken up by plant roots and run off, taking surface material with it.

Some plants are fine in this wet, cool weather, but others need more help. (Photo by P. Doan)

For example, in a clay soil, which is typical for our area, the pores are denser, with little air space in between. It’s less absorbent. A loam soil has a mix of large and small pore spaces and water flows into it faster and stays there longer. Vegetables grow better in loamy soil because the plant roots have more space to spread out and draw up water and nutrients. It holds water and drains well from the balance.

To test your soil drainage, a standard measure is to dig a hole 6 inches wide and 12 inches deep. Fill it with water. After it drains, fill it again and keep note of how long it takes to drain again. If there’s still water after eight hours, use plants that appreciate wet conditions in that location. Other plants will struggle. If the hole drains in three hours or less, the site is sandy and plants that like dry soil will do well. Soil that drains in four to six hours supports the most diverse set of plants and you don’t need to seek out ones with specific requirements.

I’ve found success with growing vegetables in raised beds where I have the most control over soil conditions. They drain better and I don’t have to worry about compacting the soil when it’s wet as I walk on it. Soil in raised beds heats up faster in the spring and if it isn’t draining well, I can amend it more easily.

Cover crops like clover, daikon and vetch can be used to improve clay soils and compacted soils by opening up pore space. Organic matter like compost and shredded leaves increases the humus in soil, increasing nutrients and improving water filtration.

Other than taking measures to improve drainage by building better soil, moving water away from certain areas changes the flow and saturation level. Swales work in this scenario and can be dug out to channel rainfall along the contour of the land, which creates a shallow depression to collect the rainwater for filtration.

Planting on raised berms is another trick to contend with poor drainage. Instead of trying to change the conditions, build on top of the ground — at least 12 inches or higher — with a balanced, well-draining soil that supports a wide variety of plants. In a flat yard, berms can be used as a design element to add layers to the landscape, too.

The wet conditions are ideal for some plant fungus diseases like powdery mildew, downy mildew, botrytis, rusts, leaf spots and anthracnose, which affect many varieties of edible and landscape plants. The best strategy is to closely observe your plants and correctly identify any pathogens for quick treatment. Spacing plants for good air circulation in the garden also gives foliage a better chance to dry out.

These types of gardening challenges from shifting and extreme weather patterns are here to stay. The good news is that for most problems, building healthy soil is usually the answer. If you’re interested in rain gardens, check out this Roots and Shoots column from October for ideas.

Pamela Doan, a garden coach with One Nature, has grown ferns in Seattle, corn on a Brooklyn rooftop and is now trying to cultivate shitake mushrooms on logs. Email her at rootsandshoots@highlandscurrent.org.

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