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Helen Chesnut's Garden Notes: How to increase moisture retention in potted plants

Adding a heavier, more substantial all-purpose sterilized soil adds substance and “staying power” to potting mix.

Dear Helen: How can I increase moisture retention in the soil of my hanging baskets and patio pots? I used a commercial planting mix.


Pot size is a first consideration. The larger the pot the easier it is to keep the soil adequately damp.

Most purchased potting mixes are lightweight and fast-draining. Adding a heavier, more substantial all-purpose sterilized soil adds substance and “staying power” to a mix. Another useful addition is coconut fibre (coir), which enhances moisture retention and acts as a buffer against temperature extremes.

For hanging baskets, which are not usually large, I sometimes use worn kitchen sponges, dried and saved, at basket bottoms to help hold moisture and release it into the soil as needed. I’ve heard of people using the absorbent, “working” section of (unused) diapers in the same way for moisture retention.

Dear Helen: I have read gardening advice to hold off removing mulching materials, preparing the soil and planting until overnight temperatures remain consistently above 10 C. This is to ensure the soil has warmed sufficiently. That did not happen until very recently. Do you follow this guideline?


The general guideline of waiting to plant until overnight temperatures stay above 10 C applies to cold-sensitive (“tender”) flower and vegetable transplants like tomatoes, peppers, marigolds and zinnias. Cold April nights did delay many plantings, but not all. Optimal soil and air temperatures for planting vary widely among the many different plants we grow.

How soon you remove winter mulching materials to prepare the soil and start planting depends on what you are seeding or transplanting. A simple soil thermometer is a useful guiding tool.

For outdoor seedings, cucumbers need a minimum soil temperature of 15 C, while peas, radish, carrots and beets need only a minimum of 5 C.

Because of high temperatures arriving suddenly in May in recent years, I tend to push for the earliest possible sowings of all but the most heat-loving plants. The peas, carrots, beets and bok choy I seeded at the beginning of April are all up and growing. My hope was to get these plantings to a sturdy enough stage to stand up to high heat if it comes again this spring. As well, I have shade cloth on hand to protect heat sensitive plantings.

You’ll find charts of minimum and optimum soil temperatures for various seedings on the internet. The Oregon State University (OSU) Extension Service site is one source

Weekend column. I’ll be taking a break from writing the Saturday column on the upcoming holiday weekend. May you enjoy a splendid spring long weekend. Happy gardening!


Rock and Crevice gardens. The Victoria Hardy Plant Group will meet this evening (Wednesday, May 15) at 7 p.m. in the Couvelier Pavilion at the Horticulture Centre of the Pacific, 505 Quayle Rd. Avid plant explorer and photographer Paul springs will speak about Cracks and Crevices: The Art of Rock and Crevice Gardening. Paul will present a beautiful and easily grown palette of plants that are rarely seen in gardens.

Tomato Day. The Horticulture Centre of the Pacific, 505 Quayle Rd., is hosting its annual Tomato Day sale of transplants for tomatoes, other vegetables and herbs on Saturday, May 18, 9 a.m. to 12 p.m. New this year is a 10 a.m. Master Gardener presentation on growing tomatoes.

Plant sales. The first of two Garden Babies for Birthright plant sales will be at St Elizabeth’s Church in Sidney (10030 3rd Ave.) on Saturday, May 18, from 11 a.m. to 2 p.m.. The second sale will be on Saturday, May 25, from 9 a.m. to 1 p.m. at St. Patrick’s Church, 2060 Haultain in Oak Bay. Both sales feature perennials, flowering shrubs, a large selection of heirloom tomato plants, all at irresistible prices.

Plantaholics sale. Interesting and unusual plants from local propagators will be sold at Robin Denning’s Brentwood Bay Nursery, 1395 Benvenuto Ave. on Saturday, May 25, 10 a.m. to 2 p.m. Information at [email protected].

Introduction to new plants. The Horticulture Centre of the Pacific’s next workshop in the monthly series Plant Identification and Culture with Jane Tice will be on Saturday, May 25, 1 to 4 p.m. Each session introduces 20 new plants that fit in with our local climate. Cost for HCP members $40 per session, others $50. Register at or call 250-479-6162.

[email protected]