How to Care for a Crown of Thorns

Potting and Moving a Crown of Thorns

Choose a sturdy pot with numerous drainage holes. Crown of thorns plants come in a range of sizes based on age, variety, and other factors, so select a pot that looks to be appropriately sized for your plant. Pick a sturdy pot material, such as terra cotta, especially if you plan to keep the plant outdoors part- or full-time. Most importantly, choose a pot with lots of holes in the bottom so that it drains quickly. A smaller pot should have at least 4 holes in the bottom, while larger pots should have 6 or more. Place a 1–2 in (2.5–5.1 cm) layer of gravel or broken terra cotta in the bottom of the pot to prevent the drainage holes from getting clogged up with soil. If you plan to keep the plant indoors, place a tray underneath the pot to collect the water that drains through the holes.

Put a fast-draining growing medium in the pot, then add the plant. For outdoor growing, fill the remainder of the pot (on top of the gravel or broken terra cotta) with cactus soil mix. For an indoor pot, use all cactus soil mix or a 2:1 ratio of cactus soil mix to perlite. Create a “well” in the soil mixture, place the plant’s root ball into the well, and cover the root ball completely with the soil mixture. Look for cactus soil mix at any garden center. A crown of thorns plant can be grown directly in the ground if the soil is sandy and well-drained, and the climate is sunny and stays above 55 °F (13 °C) year-round. Even in these ideal conditions, however, it may grow better in a pot.

Set an indoor pot in a spot that gets 4+ hours of sunlight. If the crown of thorns receives at least this much daily sunlight indoors, it may flower all year round instead of only from late spring through early fall. Even if this isn’t possible in your case, aim to give the plant as much sunlight as possible. Ample direct sunlight can, however, heat up a windowsill beyond the plant’s preferred temperature range of 60–96 °F (16–36 °C). If you see wilted or discolored foliage due to overheating, move the plant back a bit from the window or relocate it to a somewhat less sunny spot.

Place an outdoor pot in full or partial sun, depending on your climate. If the temperature rarely rises above 90 °F (32 °C) or you often get less than 4 hours of sunlight per day, locate the plant pot where it will get as much full sunlight as possible. If, however, the temperature in full sunshine often rises above 90 °F (32 °C) where you live, choose a spot that gets only partial sunlight—4 hours or less per day. In the U.S., for example, place the pot in full sun in Pittsburgh and partial sun in Phoenix.

Bring in an outdoor pot when the temperature falls below 55 °F (13 °C). Basically, when the nighttime temperature regularly dips to 60 °F (16 °C) or below, it’s time to bring your crown of thorns indoors for the season. Choose a spot in your home that meets the recommended indoor growing conditions and keep it there until the temperature consistently warms up again. As a temporary measure on a chilly night, move the pot into a garage or cover it with a towel or sheet. If you live in a cooler climate, it’s much easier to just grow your crown of thorns plant indoors!

Re-pot an indoor or outdoor plant every 2-3 years in spring. Pick out a new pot that is 1–2 in (2.5–5.1 cm) larger in diameter and fill it ⅓ of the way with cactus soil mix (start with a layer of gravel for an outdoor pot). Grasp the plant near the base of the stem and pull the entire root ball up and out of the old pot. Nestle the root ball into the new pot and cover it completely with either cactus soil mix alone (for outdoors) or equal layers of cactus soil mix and perlite (for indoors). Re-pot the plant at least every 3 years. Do it earlier if the plant simply appears to be too big for the pot. Whenever possible, re-pot the plant in early-to-mid spring, before it begins its rapid growth for the season. As the name indicates, this plant has lots of thorns! Put on thick gardening gloves before trying to transplant it.

Watering and Fertilizing Your Plant

Water the plant whenever the top half of the soil is dry. To check the soil moisture in a smaller pot, stick your index finger straight down into the pot and feel for where the soil transitions from dry to moist. Alternatively, stick a wooden skewer or chopstick into the soil, pull it back out, and look for signs of moisture and damp soil on the wood. Estimate the depth of the transition point from dry to damp soil. If the top half (or more) of the soil is dry, add just enough water to moisten it. You may only need to water the plant every 1-2 weeks.

Check for over-watering if many leaves turn yellow or fall off. As a desert plant, the crown of thorns tolerates under-watering better than over-watering. The first sign of over-watering is often yellowish leaves, which will start to fall off when the problem becomes more severe. Cut back on your watering frequency immediately as a first step. If signs of over-watering continue, transfer the plant to a pot with better drainage. If the root ball looks soggy, swollen, and possibly moldy when you transfer the plant, it’s probably suffering from “root rot.” The plant is unlikely to recover at this point.

Add a water-soluble, diluted fertilizer monthly. Buy a general-purpose houseplant fertilizer that’s made to be mixed in water. Follow the mixing instructions, but double the amount of water in order to dilute the mixture by half. Pour the mixture onto the soil in the plant pot. If, for example, the instructions say to use 4 fl oz (120 ml) of water for the mixture, use 8 fl oz (240 ml) instead. As a desert plant, the crown of thorns thrives in poor soil.

Maintaining and Pruning Euphorbia Milii

Wipe away any small pests with a soapy cotton ball or swab. Crown of thorns plants typically don’t draw in a large number of pests, but you may occasionally spot some spider mites, aphids, or other small critters on the stems and leaves. Take a good look at the plant every 2-3 days. If you see any pests, soak a cotton ball or swab in a mixture of water and dish soap, then simply wipe away the little critters! If there are more than a handful of pests, bring a dish of soapy water to the plant and keep re-dipping the cotton ball or swab as you work. The soapy water will quickly kill the pests.

Remove and discard fallen leaves and flowers regularly. If you have kids or pets around, it’s essential to get rid of all fallen leaves and flowers. Crown of thorns plants are poisonous, and a child or pet who eats any leaves or flowers may develop severe mouth irritation, gastrointestinal distress, and, in rare cases, internal bleeding. Contact a poison control center or doctor if you suspect a child has eaten crown of thorns leaves or flowers, and call your pet’s veterinarian in the same scenario. Clearing away fallen leaves and flowers helps the soil to "breathe" and reduces the risk of the plant developing "gray mold," which is a fungus that can kill your crown of thorns. Immediately remove any leaves, flowers, or stems that have a fuzzy gray-brown mold on them.

Put on protective gear before pruning and disinfect your tools afterward. While it’s safe to pick up dried, fallen leaves or flowers with bare hands, never pluck or cut living parts of the plant bare-handed. Put on thick gardening gloves and, as an extra safety measure, wear eye protection. Clean your tools thoroughly when you’re finished as well. The viscous, milky ooze inside of the leaves and stems contains the plant’s poison. It will cause severe irritation to any scrapes or cuts on your skin, and can even cause temporary blindness if it gets into your eyes. If you use pruners or scissors, rinse them under warm water and wipe them thoroughly with a rag soaked in rubbing alcohol.

Prune the plant lightly for shaping in the spring. A crown of thorns plant doesn’t really require any pruning, so do only minimal pruning if you want to adjust its shape slightly. Use sharp pruning shears to create clean cuts about 0.25 in (0.64 cm) above the base of any stems or branches you want to trim. Discard the cuttings right away. The best time to prune for shaping is in early-to-mid spring, before the plant’s primary growing season. Remember to put on your gloves and eye protection first, and to disinfect your tools afterward.

Snip off branches with wilted leaves during the growing season. Throughout the main growing season—which is typically late spring through early fall—you’ll see branches here and there with wilted leaves. To encourage growth and flowering elsewhere, snip off these branches at the base with your sharp pruning shears. Once again, remember to put on your safety gear, discard the cuttings, and disinfect your tools.