Indoors or outdoors?

Three trees from this course's cases, all instructional examples, each need to move.

The gift juniper, bought in mid-December, has spent weeks in a heated shop and a warm home, and now sits on a windowsill above a radiator in the west balcony apartment. Lesson 2 showed that its “INDOOR BONSAI” tag is wrong: the junipers sold as bonsai are temperate trees, and this one, most likely Juniperus procumbens 'Nana', belongs outdoors. Its owner is ready to carry it out to the balcony, but the forecast shows four hard frosts in a row, down to −9 °C (16 °F).

The house with the small backyard has two moves to plan. In early spring, its owners bring home a trident maple from a heated greenhouse, already in soft new leaf while the neighbor's maple is still bare. In late spring, they want the ficus, which spent the winter in a bright window, out on the sunny lawn.

Each owner knows where the tree should go. Moved there today, the juniper would meet a frost it is not ready for, the ficus a sun its leaves were not built for, and the maple's new leaves a frost that would blacken them. Where a tree should spend its year is half of this lesson's question; the other half is how to get it there.

Where each kind of tree belongs

Placement starts with what the tree is. As Course 01, Lesson 13 showed, a tree's year follows the climate it is adapted to, and broadleaf or conifer, evergreen or deciduous, do not settle where it should live. First find out whether it is temperate, tropical, or subtropical.

Temperate trees, such as the junipers, the trident maple, and the Chinese elm, spend the whole year outdoors. There they get full daylight, feel the shortening days and cooling nights of autumn, and then get the cold winter rest that meets their buds' chilling requirement.

Tropical trees, such as the ficus, have no winter rest, and frost damages or kills them. Where winters bring frost, they spend the cold months indoors, in warmth and the strongest light available. Subtropical trees vary; many do best in winter in a cool, bright, frost-free place rather than a warm room.

So the groups differ most in their overwintering, how they are kept through winter. A temperate tree overwinters cold, outdoors or in an unheated shelter, with its roots protected from hard freezes; a tropical tree overwinters warm and bright. In the course's example climate, with frost from mid-October to late April, the three example trees spend their year like this:

What a warm room takes away

For a temperate tree, a heated room takes away three things.

Light. Indoors, light comes from one side, through glass, for only part of the day. In the Northern Hemisphere, where all the course's cases are, a south-facing window gets the most direct sun (in the Southern Hemisphere, swap north and south), yet even there the sun reaches the sill for only a few hours on a clear winter day, and a step or two back from the glass the light falls to a small fraction of what open sky gives. Leaves build sugars from light (Course 01, Lesson 6), while evergreen foliage keeps respiring, faster in warmth (Course 01, Lesson 7), so a juniper on a warm sill can spend more sugar than it makes for months while it still looks green. Lesson 4 shows how to measure the light a spot really gets.

Winter. A warm room gives no chilling, so the next spring bud break comes late and weak, and over several winters the tree declines. Nor does the tree build cold acclimation, the readiness for freezing that develops over weeks of cooling.

Moist air. Winter heating makes indoor air very dry. That raises the evaporative demand on foliage (Course 01, Lesson 8) and dries a pot above a radiator fast; Lesson 5 looks at humidity.

Tropical trees indoors, and summers outside

A ficus can live indoors all year where the light is strong: close to a bright window, often with a grow light as well (Lesson 4). Compact new shoots with closely spaced leaves suggest the light is enough; long, thin shoots with small, widely spaced leaves suggest it is not.

Many growers in cold-winter climates also move tropical trees outdoors for the summer. Even in bright shade outdoors, a tree usually gets more light over a day than in a window, so it grows more strongly and builds reserves for the next winter indoors. Summering outdoors is a choice, and it has three rules: go out only once nights stay reliably above about 10 °C (50 °F), move into stronger light gradually, and come back in before autumn nights fall below that again. The rest of this lesson explains why.

Leaves are built for the light they grew in

Whenever a tree moves, its surroundings change faster than the tree can. Plants do adjust to stronger light, cooler nights, or drier air, but over days to weeks. That gradual adjustment is acclimation, and cold acclimation is one kind of it.

A leaf's structure is set while it grows, to suit the light around it. Leaves that expand in strong light become sun leaves: smaller and thicker, with a thicker cuticle and, in the mesophyll, two or more layers of tall palisade cells packed with chloroplasts (Course 01, Lesson 4). They can use a great deal of light and safely get rid of much of the excess.

A shade leaf expands in dim light: in a window, under greenhouse glass, or deep inside a canopy. It is larger, thinner, and softer, often darker green, with a thinner cuticle and usually a single, shorter layer of palisade cells. It is efficient in weak light and poorly protected in full sun.

Conifer needles also differ between sun and shade, less visibly. Nobody knows where the gift juniper grew, but junipers are usually raised outdoors at nurseries, so its needles most likely grew in sun. Winter sun is also weak, so light is not its main risk.

In full sun, a shade leaf absorbs more light energy than it can use or safely shed. The excess damages its light-capturing machinery and breaks down its chlorophyll, so patches turn pale, almost white: bleaching. The thin leaf also heats up and loses water fast, and the damaged patches dry and turn brown: scorch. Both show on the leaves, and the parts of leaves, that face the sun, and both are worse on hot or windy days.

Badly bleached and scorched patches do not turn green again, and a severe episode can strip much of a canopy, leaving the tree to regrow from its reserves. Similar patches can come from drought, cold, pests, or chemicals, so read them alongside what changed: a recent jump in light, with the damage on the sunny side, points to scorch.

Existing leaves adjust only partly, building more protective pigment within days; they cannot rebuild their structure. The full change comes from new leaves grown to suit the new light. That is why it takes weeks, and why a tree moves into stronger light in steps.

Hardening off: one step at a time

Hardening off is the gardener's name for getting a plant that grew indoors or under glass used to outdoor conditions gradually. It is not the late-summer hardening of shoots into wood that Course 01, Lesson 13 described; the word is shared, the process is not. For leaves that grew in a window or a greenhouse, hardening off means stepping up the light:

1. Bright shade first: open sky above but no direct sun, for a few days or longer.
2. Then part sun: some hours of direct sun each day, ideally morning sun with afternoon shade, for a few days or longer.
3. Then full sun, for a tree that wants it.

Allow one to two weeks or more in all, and longer after very dim light. Start in mild or cloudy weather, somewhere sheltered from wind, which these leaves have never met, and hold at the current step through a heat wave or strong wind. Check the leaves facing the sun every day, and go back a step if pale or brown patches appear. The tree dries out faster at each step, so check its root zone more often (Course 03, Watering and Daily Observation). New leaves that come out smaller and firmer than the old ones are a good sign that it is adjusting.

Cold readiness takes weeks to build

Cold works the same way. Course 01, Lesson 13 described how shortening days and then cooling nights prepare a temperate tree's cells for freezing over several weeks of autumn. Three consequences matter whenever a temperate tree moves:

• Warmth undoes it. Weeks in a heated shop and home strip away much of the readiness a tree built outdoors, and a tree kept warm since autumn never built it. Carried straight into a hard frost, a tree that would have been hardy outdoors can be badly damaged. This is the gift juniper's position.
• It returns slowly. Cool nights a few degrees above freezing rebuild readiness over weeks, not overnight. How quickly depends on the tree and on how long it was warm; nobody can give you an exact schedule.
• New leaves are tender, whatever the tree. A temperate tree loses its cold readiness as it starts to grow, so soft new leaves are damaged by frost even on a tree whose bare twigs came through a hard winter. The greenhouse trident opened its leaves weeks early, and in the example climate frost is possible until the last spring frost, usually in late April. Until then, its leaves need shelter on every frost night.

Roots need their own protection. As Lesson 1 showed, they are generally less cold-hardy than branches, and in a pot they get far colder than roots in the ground. So even a fully acclimated tree in a container needs its roots protected from hard freezes, in ways that keep the tree cold: a sheltered spot outdoors, or an unheated space (Course 06, Seasonal Care and Timing). A tropical tree, by contrast, builds little frost tolerance however gradually it cools, and many tropical plants are harmed by cold above freezing, which is why many growers keep a ficus above about 10 °C (50 °F).

Moving day: plan it with the forecast

Now plan the three moves from the start of this lesson. In Moving day: plan the transition, choose a tree under Tree and season, read its ten-day forecast, and pick a place for every day, from Warm room and Cool bright room to Outdoors, shade, Outdoors, part sun, or Outdoors, full sun; each choice covers that day and the night after it. Press Run the plan and watch the Used to strong sun, Ready for cold, and Condition readouts and the Day by day log. Try these:

1. With Gift juniper in December, first try the move its owner was about to make: outdoors from day 1. Then plan a version that keeps it safe.
2. With Ficus in late spring, keep it in its Warm room (its bright window) for days 1 to 3, then put it in Outdoors, full sun from day 4, when the nights turn mild. Then try stepping it up instead.
3. With Trident maple from a heated greenhouse, find the two frost nights in the forecast and plan around them.
4. For any tree, press Show one sensible plan, read Why this plan works, and run it to compare with your own.

A few rules decided every result. Light worked in steps: a place one step brighter than the leaves were used to was safe, and after about three days they had adjusted, but a jump of two or more steps on a sunny day scorched the sunny side, worse in heat or wind, while cloud softened a jump. The ficus carried straight to the open lawn scorched badly; stepped through shade and part sun, it came through. Put outdoors on days 1 to 3, when nights were below about 10 °C (50 °F), it was stressed by the chill; waiting in its window was the right call. The juniper, whose needles most likely grew in sun, never scorched.

Cold readiness built slowly, a level every few cool nights, and the warm room undid it. So in the sensible plan, the juniper waited out the frosts of days 1 to 5 in the cool bright room and went out in the milder spell, and the trident maple spent the frost nights of days 2 and 6 in the garage, because frost damaged its new leaves however it had been prepared. Staying put was safe for ten days but never the goal (the juniper left in the warm room ended “Safe, but no winter rest yet”), and many different plans worked. Real trees adjust at their own pace, and many real transitions take one to two weeks or more.

Correcting a placement, with the weather in mind

A tree in the wrong place should move, but not always today. Before each move, ask three questions:

1. Where did its current leaves grow? If the new place is more than a step brighter, use an in-between place.
2. How ready is it for cold, and what do the next ten nights bring? A temperate tree from a warm room has little readiness, and a tropical tree has almost none.
3. What is tender right now? New spring leaves and any tropical tree need shelter on frost nights, however well prepared.

Here is how the answers play out for the three trees, beyond the simulation's ten days.

The gift juniper in December

Its destination is the west balcony in full sun, but its first move is indoors: off the radiator to the coolest bright, frost-free place the apartment has, here a bedroom with a bright window and its radiator off. It stays sheltered from the hard frosts there, and the cooler the room stays without freezing, the more readiness the cool nights can rebuild. It will dry more slowly there, so check before watering, starting with the weight of the pot, rather than keeping to the tag's twice a week (Course 03).

When the milder spell arrives, with nights near or above freezing, it moves out to the balcony: sheltered at first, then into its sunny spot. That step can be quick, since sun is not its main risk. From then on the forecast decides. Protect its roots before the next hard freeze (Course 06), and in its first weeks outdoors, bring it back into the cool room, never the warm one, for any night much colder than it has met since the move. If its foliage turns bronze or purplish in the cold, that is a normal winter color for many junipers, not damage.

Its pot has no drainage hole, so rain and winter watering would collect around the roots outdoors, and its glued pebble crust hides the root zone; Lesson 7 corrects both while it waits in the cool room. And although it missed the start of winter, the rest of the winter still brings the cool weather its buds need.

The ficus in late spring

In the example climate the last frost usually comes in late April, but nights often stay reliably above about 10 °C (50 °F) only well into May. Until then the ficus waits in its window: one night at 8 °C (46 °F) is unlikely to kill it, but it is stress for no gain.

Then it steps up: a few days in bright shade, for example under shade cloth; a few days in part sun, ideally in the western third of the yard, which gets morning sun while the neighbor's maple shades it on summer afternoons; then full sun on the open lawn. It dries much faster outdoors, so moisture checks become daily.

It comes back in before autumn nights fall below about 10 °C (50 °F), in the example climate often by late September or early October, weeks before the first frost. First, inspect it outdoors: under the leaves, along the stems, and where leaves join the stems, for webbing, sticky residue, cottony patches, or small bumps that could be scale insects, which can multiply indoors and spread to houseplants (Lesson 8 covers inspecting a tree and keeping it apart). Then give it the brightest place at once, close to its window and often with a grow light; going in needs no steps, since the light falls whatever you do. Some leaf drop as it adjusts is common. Its new indoor leaves will be shade leaves, so next spring the stepping starts again.

The trident maple from a heated greenhouse

Bought in early spring, perhaps early April in the example climate, it belongs outdoors, and every frost-free day there in good light helps it adjust. Its leaves grew under glass, so it hardens off like the ficus, starting in bright shade out of strong wind. On frost nights it shelters in the unheated garage, by its window, which stays frost-free on light frosts, and goes back out in the morning; for a harder frost, it spends the night in the coolest room of the house.

Keeping it in the dimmer garage until the last frost would be safe but costly, because its leaves would stay unready for sun. After the last spring frost, usually in late April, it settles into its outdoor year. A Chinese elm, or any temperate tree bought in leaf before the trees around you, needs the same care. A tree from an outdoor nursery is used to local light and weather, though its new leaves still need watching on frost nights; ask where a tree was grown before you buy it (Lesson 6).

A placement plan you can follow