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When to give your houseplants the boot outside

When is it time to take the houseplants outside? It’s not about the calendar. It’s about the weather, especially nighttime outdoor temperatures.

Most houseplants are comfortable when night temperatures remain above 55 degrees. Many survive lower temperatures, but some plants do not. Each genus has its own limits of tolerance.

Timing is also about you – how warm you keep the house and when you turn the furnace off for the season. To avoid “plant shock” – a series of plant responses to radical changes in their environment – it’s best to move plants when inside and outside temperatures aren’t much different.

If you keep your rooms cool and turned off the furnace in April, the plants won’t be surprised by the trip outside in May. But if you’ve kept the place at 70 degrees, then you’d best wait awhile until the outside and inside highs and lows have become similar.

The timing is also set by the plants. Some yell at you: “Get me out of here, now!” Some of mine have simply had enough dry heat, even though I try to add humidity in winter.

In late winter it’s typical for many plants to develop problems such as mealy bugs or scale, or the leaves begin to yellow or drop. That’s basically because stressed plants – needing more light and humidity – are vulnerable to pests and diseases.

People ask where the scale insects come from late in the season. The answers are: I don’t know; the eggs were just there waiting; it’s a miracle.

Healthy plants slough off the problems and exhausted plants get sick. It will all be better outside!

Making the move

Our last average frost date for a couple of decades has been about May 20, obviously a variable. Frost would kill many tender plants and damage others.

But I’ve watched the weather predictions and see that most nights will remain in the 50s. It’s time for me to move most plants out, and I will prepare to throw sheets over them if I see a frost or even a 45 degree night coming.

You might consider two stages for the moving project and keep very tender plants inside until June – especially if you won’t have time to pay attention to nighttime temperature predictions.

Damaged by temperatures lower than 45 or 50 degrees: African violet and many other gesneriads; Anthurium; basil; begonias (most species); Caladium (keep above 60 degrees); coleus; Gardenia; impatiens; Maranta (Prayer plant); Sansevieria (Snake plant); Florist Gloxinia (Sinningia); palms (most houseplant types), and rosemary.

Placement outside also requires a two-stage or gradual approach. The brightest window or artificial light setup inside is usually the equivalent of shade outside. And radical change is always unacceptable. Therefore do not move any plants – even “full sun” plants – straight into positions in full sun. Maybe they’ll be ready later in June but they will get sunburned, literally, until their tissues adapt to the new environment.

Typical late-spring rainy weather also poses risk for plants that get root-rot from excess water. If you put begonias or jade plants and other succulents outside, be prepared to put them under cover during rain storms. Even during the growing season many plants must dry out between soakings.

Triage time

Houseplant moving days are also the right time to make decisions about their health and whether they need potting up or not.

Occasionally a plant is so stressed by indoor winter conditions that it’s not worth saving. Most minor pest problems are solved simply by getting the plants outside into better light and humidity, and many beneficial insects will quickly take care of pest insects (or the latter will just fly off into the big wide world).

My clivia (that needs a cooler winter rest period than it had) developed mealy bugs to my amazement, and I will wipe them off. But I see some scale on a 5-foot Dracaena and the kumquat tree, and I expect natural forces to solve that. It’s happened before.

Now it’s time to review the entire plant collection. I received some new plants for trialing from Proven Winners, and they are also ready to be grouped and planted in mixed containers. Why shouldn’t the old scented geranium spend summer with the new Cuphea (Firecracker) ‘Vermillionaire’ and a baby green-and-white striped spider plant?

Why not put a couple of the new trailing Evolvulus hybrids ‘Blue My Mind’ in the lime green pot with a precious gold hosta? This creative mixing and matching is a joy.

Planting and replanting

1. Have bags of fresh potting mix standing by – bought from local garden centers.

2. Fill a pail or tub with air-temperature water to soak extremely dry root balls.

3. Fill another tub with soapy water to wash used pots.

4. Gather all the containers and pots you have for possible filling.

5. Clean up or keep isolated any plants that have pests or possible diseases. They can wait to make new friends another time.

6. Cut back and groom plants with overgrown or straggly shoots. Some plants need dividing but remember that many (clivia) flower best when crowded.

7. Group or plant individuals in new housing with new soil mix. General rule: Put individual plants in pots about one pot-size up; never put a little plant in a huge pot.

8. Unless your container mix includes fertilizer, add houseplant fertilizer to all the plants that lived through the winter indoors. Follow instructions and apply slightly less – never more – than instructed.

9. Water well and plan for a new outside watering routine. It’s often helpful to group plants together according to heavy or light watering needs.

For plant lovers this isn’t a duty. It’s playing with plants and we waited all winter!

Sally Cunningham is a garden writer, lecturer and consultant.

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