Whether you purchased seedlings from a garden centre or have been nurturing them indoors from seed, these plants need a little toughening up before being planted outside.
Most garden centres have roofs or walls that may be opened to allow some exposure to the local environment. While this helps harden up the plants, it still provides more protection than the plant will receive in your garden. So, what should you do with that tray of tender annuals you’ve just purchased from the garden centre?
Start by getting the plants used to being outdoors. This gradual acclimatization of plants to the outdoor environment is called “hardening off.” If the plants have been started in your home, then begin by placing them outside in a protected, shady area for a couple of hours. Each day increase the length of time outside by one or two hours. Also increase their sun exposure. Start in the shade, then move into dappled light and final into full sunlight. In seven to 10 days, the plants should be ready for life in the great outdoors. Plants that are purchased from a garden centre have already started the hardening off process, so they should require less time to be ready for outdoor growing. Ideally, while transitioning the garden centre plants to life outside, keep these plants in cool but protected space such as a garage or greenhouse overnight rather than bringing them into your home. If they are in a mini greenhouse overnight, adding a fan and heat mat will help prevent damage to the plants when frost is predicted.
May’s weather is very variable. As we’ve just seen, there can be snow followed by temperature highs in the teens, followed again by snow. This year, the start of the frost-free season is predicted to be May 25. Tender annuals should be planted outside after this date.
Tender annuals commonly planted in Calgary include begonia, vinca, salvia, zinnia and impatiens. Tender vegetables and herbs may also require additional protection and should also be planted or seeded in June. These plants include cucumbers, tomatoes, squash and beans. Tender herbs include basil and stevia. In Calgary, many of our container plants are perennials in warmer climates and are also frequently treated as houseplants. These plants, referred to as tender perennials include geranium, lantana, coleus and various tropical plants such as dracaena.
The plants that may tolerate a mid-May outdoor planting are those considered half-hardy annuals. These plants can tolerate growing in cool soils and air temperatures but are damaged by light frost. If you plant these half-hardy annuals out now, keep frost protection handy. Half-hardy annuals planted for summer colour include ageratum, dianthus, lobelia, petunia and gazania.
All the plants mentioned thrive in containers. A container may be a large pot or a raised bed. When preparing the container, use a quality potting medium. Mix into the medium a slow release fertilizer, compost and/or worm castings. Work in the shade, plant the container on a cloudy day, or plant the container in the morning or evening. Exposing the roots and plants to the harsh mid-afternoon sun will unnecessarily stress them.
The next morning be sure to water the container. Plants that bloom profusely require a lot of nutrients to keep them looking their best. Water with a half strength fertilizer (meaning use twice as much water as recommended on the fertilizer package) or water with a compost tea.
To grow well, tender plants need warm air and warm soil. If they are planted outside before June, be prepared to provide additional protection such as a pop-up greenhouse, cloche, row cover, frost blanket or heating cable.
Pay attention to the weather reports. If temperatures below 5°C are predicted, protect the plants from the cold. Containers and frost protection are tools for pushing the season’s boundaries and getting a start on your garden but be sure your plants have been hardened off before moving them to grow in the great outdoors.
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