The Leaves Are Curling Over But even though they're known to do this (and many other plants too), sometimes it's a sign that they're being over-watered. Overwatering causes the leaves to curl up at the tips and edges, as seen here.
Low Humidity
Many common houseplants require high humidity to thrive. Dry indoor air can starve them of this humidity, causing the leaf edges to curl or brown. Boost humidity levels by misting your plants regularly or using a humidifier.
1. If a plant is overwatered, it will likely develop yellow or brown limp, droopy leaves as opposed to dry, crispy leaves (which are a sign of too little water). Wilting leaves combined with wet soil usually mean that root rot has set in and the roots can no longer absorb water.
The leaves curl when the plant senses that it is dehydrating too fast. There are multiple causes: low humidity, extreme heat or light, dry soil, or inability to draw insufficient water from the soil because of root damage (root rot, transplant shock).
Leaf curl can usually be controlled satisfactorily by a spray of a suitable registered fungicide at any stage of dormancy. Most effective control is achieved by spraying when the buds are swelling but before they have opened. It's not possible to control the fungus once it's entered the leaf.
Leaf curl disease shows up in spring, does the damage, and then disappears (until next spring). If your trees were infected last spring, they should have completely recovered by mid to late summer. They usually grow lots of healthy new leaves (like the photo below).
Prevent Leaf Curling
Another sign of magnesium deficiency, this problem can also be tackled with the help of Epsom Salt. Mix two table spoons in a gallon of water and spray your curled foliage with the solution until the leaves look perfect again.
The Leaves Are Curling Over
But even though they're known to do this (and many other plants too), sometimes it's a sign that they're being over-watered. Overwatering causes the leaves to curl up at the tips and edges, as seen here.
During spring and summer in Southern California, she suggests drip watering three times a week for 18 minutes, then adjusting from there if the soil is too wet or dry. Don't know how to tell? Stick your finger in the soil. If it's consistently moist 2 inches down, your plants are fine.
Feel the Soil: The simplest way to gauge your plant's watering needs is by feeling the soil. Turns out that old trick of sticking your finger into the soil up to the knuckle has merit! If it's soggy or has standing water, you're overwatering. If it's dry an inch below the surface, it's time to water.
Overwatered plants can sometimes recover on their own, but it depends on the plant and the extent of the overwatering. If the roots have rotted, the plant will likely not recover. However, if the overwatering has only caused the leaves to wilt, the plant may be able to bounce back.
How to check for root rot. The main signs to look out for are: yellowing, wilting leaves, wet soil that isn't drying, black, wet roots, quickly declining health, and stunted growth.
Organic Control Methods for Leaf Curl
Spray at bud-swell with copper hydroxide or copper oxychloride and ensure thorough coverage over all branches. Timing is critical to prevent the fungus entering the plant when new leaves are vulnerable.
Phosphorus deficiency commonly causes older leaves to curl, distort, and remain smaller than normal. Unusually purple leaf veins and tip dieback from phosphorus deficiency. Purpling of leaf undersides due to phosphorus deficiency.
Curled leaves are due to a light problem
The plant is probably trying to fight against too much light. This tends to happen in summer, when you take your plant out into the garden to enjoy the fresh air. Unaccustomed to these new growing conditions, your plant will fold up its leaves to limit its exposure to the sun.
Each plant has its specific needs, but as a starting point plants in pots with: 3-4 inch diameter containers need about half a cup of water. 5-7 inch diameter containers need about 1 cup of water. 8-10 inch diameter containers need about 2-3 cups of water.
Morning watering is actually preferable to evening watering as the plant has time to dry before the sun goes down. At night, water tends to rest in the soil, around the roots, and on the foliage, which encourages rot, fungal growth, and insects.
The best way to tell if your plants need water is to stick your finger about an inch into the potting mix—if it feels dry, break out the watering can. If you detect dampness, check back again in a day or two. For smaller houseplants, you can also pick up the whole container. If it feels light for its size, add water.
Plants often recover on their own from leaf curl caused by transplant shock within a couple of weeks.
When a plant is first becoming overwatered, leaves turn yellow. If soil doesn't have a chance to dry out before you water again, leaves start to wilt. When overwatering is the problem, wilted leaves are soft and limp. (If too little water is the issue, wilted leaves are dry and crispy.)
Treat trees with a fungicide in late fall and winter. The fungal spores that cause the disease spend the winter on twigs and buds and germinate in the spring. For effective control, treat trees just after leaves have fallen, usually late November or December.
For potted plants, Scott recommends applying the Epsom salt solution once per month. When planting a magnesium-loving plant in your garden, such as roses, Scott recommends adding a light sprinkle of Epsom salt to the soil before planting it into the garden.
Best Treatment for Leaf Curl
Where Leaf Curl has been severe in the previous year, trees can also be sprayed with Yates Liquid Copper Fungicide in autumn when the leaves drop, plus at the first sign of bud swell and again one week later.
Leaf Curl, prevention is the only way to deal with it that I know of. I start spraying my trees with Neem oil / Bonide Copper Fungicide mixture when dormant 3 times last spray when the buds start swelling but are still gray colored. Blossom Blight is a problem too, this spray takes care of that too.