Gadenin日本語Get the appHow cold they actually take
A well rooted pansy is far tougher than it looks. Established plants come through repeated frost and through short spells down to around 5F (minus 15C) at the root, and they flower whenever the weather relents in between. What limits them is not the low point but the length of it: a hard freeze that lasts two days does very little damage, while three weeks of the same temperature with frozen soil and no thaw kills plants outright because the roots cannot take up water.
During a freeze the plants go limp and dark and lie flat on the soil, looking dead. As the temperature comes back up they stand up again within a few hours, usually with the flowers intact. This alarms people every winter, and the correct response is to do nothing at all. Do not water a frozen plant, do not cut off the collapsed foliage, and do not pull anything out until you have seen it fail to recover through two or three mild days.
Flowering slows or stops below about 40F (4C), which is a growth response rather than damage. Buds already formed hold and open in the next mild spell. In a winter with regular mild breaks a good plant flowers on and off the whole way through; in a continuously cold one it holds until late winter and then surges.
| Condition | What happens | What to do |
|---|---|---|
| Light frost overnight | Foliage limp, upright again by midday | Nothing |
| Hard freeze, two or three days | Plants flat and dark, flowers hold | Nothing; do not water or cut back |
| Weeks of frozen soil | Roots cannot take up water; real losses | Mulch around, not over, the crowns |
| Freeze and thaw cycles | Plants heaved partly out of the ground | Press each plant firmly back in on a mild day |
| Cold and permanently wet | Crowns rot; plants collapse and do not return | Improve drainage; keep mulch off the stems |
| Mild winter spell | Flowering resumes within days | Deadhead, resume half strength feed |
| Snow cover | Insulates; plants often come out unharmed | Brush heavy wet snow off, otherwise leave it |
Wet, not cold, is what kills them
The plants lost over winter are almost always lost to water rather than to temperature. A crown that sits wet through weeks of cold rots at the base: the stems go brown and soft right at soil level, the plant collapses, and it does not recover. This is why the drainage of the bed and the depth of planting matter more than any frost protection you might add.
Three things cause it. Planting too deep, so that the crown is below the surface and soil washes in around the stems. Mulch heaped up against the plants, which holds water exactly where it does the most harm. And containers standing in saucers or on solid ground where the drainage holes silt up, so the compost never drains after rain. All three are fixable in ten minutes and all three are commonplace.
Mulch is still worth applying, but around the plants and not over them. A one to two inch (2.5 to 5 cm) layer of bark or leaf mold across the bed steadies the soil temperature and reduces frost heave, with a clear two inch (5 cm) ring of bare soil left around each crown. Raise containers off the ground on pot feet or bricks so water runs out of the base freely, and tip out any saucers before a cold spell.
- Clear dead material out of the crowns Go over the plants every couple of weeks and pull out fallen leaves, spent flowers and any yellowing foliage lodged in the center. Wet debris sitting in the crown through winter is where rot starts.
- Check for frost heave after every thaw Freeze and thaw cycles push shallow rooted plants partly out of the soil, leaving the roots exposed to drying wind. Press each one firmly back down on the next mild day.
- Water only in the morning, and only if dry In a dry frosty spell with no rain, plants can genuinely run short. Water in the morning so the foliage and crown are dry before nightfall, and only when the soil is dry an inch (2.5 cm) down.
- Raise containers off the ground Pot feet or two bricks under each container keep the drainage holes clear. A pot sitting flat on paving through a wet winter holds water in the base whatever compost you used.
- Stop feeding while growth is stopped Feed applied to a dormant plant is washed straight through by winter rain. Resume a half strength feed at the first sustained mild spell, not before.
Containers need more protection than the open ground
A pansy in the ground has the whole mass of soil around it holding warmth. The same plant in a container has a few liters of compost that freezes solid from all sides, and a frozen root ball is the situation in which pansies actually die. Small pots and window boxes are the most exposed, and a shallow trough will freeze right through on a night that barely touches the border.
Group containers together against a house wall for the coldest weeks, which is both warmer and more sheltered from drying wind than an open position. Wrapping the outside of a pot with bubble wrap, hessian or an old towel insulates the root ball without touching the plants themselves. Do not wrap the foliage; pansies do not need it and covered foliage stays wet and rots.
If a container is going to freeze solid for a week or more, moving it into an unheated porch, garage or shed for that spell is legitimate, provided it gets light and comes back outside as soon as the freeze breaks. Do not bring pansies into a heated room. Warmth without light produces long pale drawn growth in days, and the plant that comes back out is worse than the one that went in.
Note which containers froze solid and which did not, and where each was standing. Positions behave the same way every winter, and after one season you will know which two or three spots need moving or wrapping and which can simply be left alone.
Coming out of winter
At the first sustained mild spell, go over everything properly. Pull out any plant that stayed limp through three mild days, since that one has rotted at the crown and will not return. Clear all the accumulated debris from between the plants. Firm back anything that has heaved. Then resume the half strength liquid feed every two weeks, because winter rain will have leached the bed and the plants are about to make their heaviest growth of the year.
This is also the right moment to cut back. Plants that have stretched over winter, with long bare stems and flowers only at the tips, should be reduced by half, cutting each stem just above a leaf joint with two or three leafy joints left below the cut. New shoots break within seven to ten days and the plant flowers again in two to three weeks, denser than before. Done at the start of the mild weather, this gives you a compact plant for the whole spring display; left until mid spring, it costs you the best weeks of the year.
Watch for aphids from the first warm days. They build up fast on the soft new growth that follows the cut back and the resumed feeding, and a colony that establishes at this point undoes the plant's spring surge. Check the buds and shoot tips at the same time as you deadhead, which by now should be back to twice a week with the whole flower stalk taken down to the crown each time.