Gadenin日本語Get the appRepot while the plant can rebuild roots
Repotting always tears some roots, however careful you are, and the plant has to replace them before it can get back to growing. That only happens in warmth and active growth, which is why the season matters far more than convenience does.
| Season | Repot then? | Reason |
|---|---|---|
| Late spring | The best window | Rising warmth, roots regenerate within days |
| Summer | Yes, with shade afterwards | It transpires heavily, so cut roots means wilting |
| Early autumn | Only early in the season | Needs weeks of warmth to root before the cold |
| Late autumn to early spring | Avoid entirely | Roots do not move and cut surfaces rot in cold wet compost |
A newly bought plant is an exception worth knowing. If it arrives in midwinter sitting in soaking peat, get it out of that compost rather than waiting for spring. Saturated dormant roots do not survive three months.
The signals that it is pot bound
Philodendrons do not need repotting every year, and the trailing types are perfectly happy slightly cramped. Work from the signs rather than the calendar.
- Water runs straight through Water appears at the drainage holes almost instantly and the compost barely darkens. The roots have filled the pot and water is running down channels instead of soaking in.
- Roots emerging from the base White roots pushing out of the drainage holes, or a rootball that lifts out as a solid block holding the shape of the pot, both mean it has run out of room.
- New leaves keep coming smaller If the light is right and you have been feeding, and each new leaf is still a size down on the one before, the root system cannot supply what the top wants.
- The plant tips the pot over When the top growth outweighs the pot, the roots are equally cramped. On an upright type this is often the first thing you notice.
One size up, never two
Moving into a much bigger pot surrounds the rootball with a mass of compost that has no roots in it, and that compost stays wet for weeks after every watering. It is the single most common cause of root rot in a plant that was previously healthy.
| Current pot | Move up to | Typical plant |
|---|---|---|
| 5 in (13 cm) | 6 in (15 cm) | A young trailing plant or a rooted cutting group |
| 6 in (15 cm) | 7 in (18 cm) | An established trailing plant, vines to 24 in (60 cm) |
| 8 in (20 cm) | 10 in (24 cm) | An upright type around 24 to 32 in (60 to 80 cm) tall |
| 10 in (24 cm) and larger | Same pot, fresh compost | When you do not want the plant any bigger |
To hold a plant at its current size, shave roughly a third off the outside and bottom of the rootball with a clean knife, return it to the same pot with fresh compost, and cut back a similar proportion of the top so the reduced roots are not overloaded.
A mix that drains
Philodendron roots are thick and demand air. A dense water retentive compost of the sort plants are sold in is the commonest slow killer of indoor philodendrons, so build the mix around drainage and accept that you will water slightly more often.
- A mix that works for both habits Roughly five parts houseplant compost, three parts orchid bark or coarse fine grade chips, and two parts perlite. It drains freely, holds enough moisture for four or five days, and does not collapse in a season.
- Add more bark for a plant on a pole A climbing philodendron taking water through aerial roots on a damp pole wants an even more open mix at the base. Push the bark fraction up towards a third.
- Do not simply reuse old compost Old compost has broken down and lost its structure, so it packs and holds water. Replace at least half of it with fresh material even when the plant looked fine in it.
- Crocks are not drainage A layer of stones in the bottom does not stop rot, it just raises the saturated zone. Real drainage comes from the structure of the mix and from emptying the saucer.
Doing the job
Water the plant thoroughly the day before so the rootball holds together and the roots are not already stressed. Slide it out rather than pulling on the stem, supporting the base of the plant with one hand and tipping the pot with the other.
- Loosen the outside only Tease the outer roots free with your fingers and remove the loose compost that falls away. Do not wash the whole rootball clean unless you are dealing with rot.
- Cut out anything black or soft Healthy philodendron roots are firm and pale. Trim any black, hollow or mushy root back to firm tissue with clean pruners and let the cuts dry for a few minutes before potting.
- Set it at the same depth The point where stem meets root should sit at the same level as before. Burying an upright type deeper to make it stand up straight rots the buried stem.
- Firm gently and water once Fill around the ball, tap the pot down to settle rather than pressing hard, and water thoroughly once. Then leave it alone.
The fortnight afterwards
What you do in the two weeks after repotting decides whether the plant sails through or sulks for a season. Keep it out of strong light in slightly warmer air than usual, and do not water again until the top of the compost has dried. Cut roots in wet compost are exactly how rot begins.
Do not feed for about a month. Fresh compost carries enough nutrition, and fertilizer applied to freshly cut root ends burns them. Some leaf droop for the first few days is normal and passes once new roots take hold.
Do not repot and prune hard on the same day, and do not repot and divide on the same day. One disturbance at a time, with three or four weeks between them, and record the date you did it so next year you know when the plant was last moved on.