Gadenin日本語Get the appPowdery mildew: spacing beats spraying
Powdery mildew is the problem verbena is known for. It shows first as a faint gray or white dusting on the upper surface of older leaves low in the plant, usually on the shaded inner foliage rather than at the tips. Left alone it spreads outwards, the affected leaves yellow and drop, and the plant thins from the middle while still flowering at the ends.
The condition that causes it is still, humid air around damp foliage, not rain. Mildew often appears in a dry spell with heavy dew and no wind, and it is worst where plants are packed tightly, backed against a wall, or sitting under an overhang where air never moves. This is why spacing is the real control. A verbena with a foot (30 cm) of open air on every side, in a spot where you can feel a breeze, will often go the whole season clean while an identical plant crowded into a sheltered corner is gray by midsummer.
If mildew has started, do the physical work first and reach for a spray last. Strip out the worst affected leaves and bin them rather than composting. Thin the plant by cutting whole stems out from the base to open the interior, rather than shortening every stem, which only makes the canopy denser. Move a container somewhere with more air. Switch watering to the base in the morning so the foliage is never wet overnight. Those four actions stop most outbreaks; a fungicide applied into a plant still sitting in still air will not.
| Condition | Mildew risk | What to change |
|---|---|---|
| Plants at 6 in (15 cm) spacing against a wall | Very high | Replant at 12 to 18 in (30 to 45 cm) and pull away from the wall |
| Evening overhead watering | High | Water at the base in the morning only |
| Warm days, cool dewy nights, no wind | High | Thin the plant by removing whole stems from the base |
| Open bed, breezy site, well spaced | Low | Nothing; keep the spacing next year |
| Container under an eave or balcony roof | High | Move it out where air crosses the plant |
| Heavily fed, dense soft growth | High | Stop feeding; soft crowded growth is the most susceptible |
Spider mites in hot dry weather
Spider mites turn up in the opposite conditions to mildew: hot, dry and sheltered, particularly on containers against a sunny wall. The early sign is a fine pale stippling across the leaf, as though the color has been dusted away in pinpricks. Turn the leaf over and you will see the mites as moving specks, and in a bad case fine webbing where the leaf meets the stem.
- Check the undersides weekly in a heatwave Turn over a few of the older lower leaves once a week whenever the weather is hot and dry. Mites double in numbers fast, and a colony caught in the first week is trivial to deal with.
- Hose the plant down forcefully A strong jet of water aimed up under the foliage removes most of a light infestation. Do this in the morning so the plant dries by evening, and repeat every three days for two weeks.
- Cut out the worst affected growth Stems that are heavily stippled and webbed will not recover their color. Remove them entirely and bin them. This is quicker than treating them and it opens the plant up at the same time.
- Raise the humidity around the plant Mites thrive in dry air. Grouping containers, standing pots on damp gravel, and keeping the soil evenly watered all make the site less suitable for them without needing any treatment.
Aphids and whitefly on the soft tips
Aphids cluster on the newest shoot tips and on the flower stalks, and they are most obvious in the soft growth that follows heavy feeding. Whitefly appear as small pale insects that lift in a cloud when you brush the plant, and are more of a container and greenhouse problem than an open ground one. Neither usually kills verbena, but both leave sticky honeydew on the leaves which then grows a black sooty film.
Squashing them by hand or knocking them off with water handles small numbers, and ladybirds and hoverfly larvae usually arrive within a couple of weeks in an open garden. The more useful long term response is to stop pushing soft growth: a verbena kept lean and airy is far less attractive to both pests than one fed hard into a dense soft mat.
Crown rot and collapse from wet feet
The most abrupt way to lose a verbena is rot at the crown, where the stems meet the soil. The plant looks normal, then wilts across a whole side or the entire plant within a day or two, and the base is soft and brown when you touch it. There is no treatment once it has taken hold; the plant is finished.
The cause is soil that stays saturated around the crown, which is why it hits container plants with a blocked drainage hole, plants in heavy clay, and plants that have been watered daily by habit rather than by need. Prevention is the whole of the answer: plant on soil that drains, never bury the crown deeper than it sat in its original pot, keep mulch pulled back an inch or two from the stems, and let the surface dry between waterings.
If one plant in a group collapses at the crown, lift it and look at the site rather than blaming the plant. A hollow that holds water after rain will take the next plant you put there too.
Knowing when to stop trying
Late in the season a verbena that is more than half mildewed, thin in the middle and flowering only at the tips is not worth rescuing. Cutting it back that late leaves too little time to rebuild before the cold arrives, and the diseased foliage is a source of spores for anything nearby. Pull it, bin the top growth rather than composting it, and start the next season with better spacing.
Write down which plants failed and where they stood. Mildew and rot both follow the site far more than the plant, and a note saying which corner it happened in is worth more next spring than any amount of general advice.