Fortnight Lily (Dietes iridioides): How to Grow and Plant Care

Fortnight lilies are unusual, captivating plants which produce gorgeous flowers every two weeks, hence the name. They are also referred to as the Japanese iris, the African iris, the butterfly iris, or the wood iris. 

They are reliable plants that bloom for an extended period of time, providing color and interest from spring all the way through to the later days of autumn, and if the weather’s kind enough, even into winter.

At a Glance: What You Should Know about Fortnight Lilies

Fortnight lilies belong to the Iris plant family, or Iridaceae. They are part of a dedicated genus called Dietes (see also Dietes Plant Guide), which translates from Greek to mean “two affinities”. 

They were originally placed in the Moraea genus of the Cape tulips, but their habit of forming rhizomes separate them into their own group. Like the plants in the Moraea group, fortnight lilies are nature to South Africa.

These incredible plants look like irises, but you can tell the main difference in the flowers, where the six tepals don’t join together at the base. 

The flowers will either feature brilliant white petals, with a hint of yellow and lilac hearts, or creamy-white flowers speckled with maroon. Whichever you go for, they’ll brighten up any area of your garden.

Depending on where you live, they’ll either grow as perennials (USDA 8 – 11), or annuals which provide long-lasting color during the summer in colder parts of the world.

Despite their delicate appearance, fortnight lilies are fairly resistant to pests and disease, but there are some to keep an eye out for, which is covered further in the article.

To get the best out of fortnight lilies, plant a few of them close together, to really show off the form and the color of these stunning perennials. 

How to Grow Fortnight Lilies

While they may look complicated, fortnight lilies are easy to grow, and easy to care for, so they’re perfect for people who are just starting out on their gardening journey. 

It helps that they look beautiful, and others might assume that they’re hard to look after!

Fortnight lilies are great at adapting to different conditions, but if you want the most flowers possible, it’s best to provide them with the conditions they adapted to over hundreds of years.

Positioning and Soil

Fortnight lilies need somewhere where they can soak up as much sun as possible, for as long as possible. 

As they hail from central and South Africa, you don’t have to worry about them getting too much sun exposure. They’ll be just fine.

You can also grow them in partial shade, but you may not see as many flowers. 

Fortnight lilies will thrive in fertile, well-draining soil, but they will also tolerate soil with poor levels of nutrients, especially if you feed them regularly. 

You can try adding some grit or sand to a hole meant for fortnight lilies in order to improve drainage. If your soil is really wet, you are better off growing them in pots, where you can control the type of soil and how well it drains.

Things to Consider When Choosing Where to Plant Fortnight Lilies

While you can put fortnight lilies nearly anywhere in your garden, there are some things you should think about before you plonk them anywhere in your garden.

In the Ground

You can put them in garden beds as part of a planting scheme, where the drainage might be better if there are lots of surrounding plants, but make sure there’s enough light for them to thrive.

Planting them against darker colors will really highlight the beauty of these blooms and the flowers around them.

In Pots

Pots are a great way to display fortnight lilies, especially because they stay so compact, they may get lost amongst bigger plants in large garden beds. 

It helps that you can control the level of drainage in pots, and containers tend to hold onto less water, which stops the roots from rotting. 

Another great benefit of planting fortnight lilies in pots is that you can move them inside during winter in colder areas, and plant them out again when the risk of frost has passed.

This saves you money, as you won’t have to replenish the fortnight lilies every year, while also adding greenery into your house for temporary color. 

It is worth noting that pots will lose their nutrients within the soil much faster than in the ground, so you will need to feed your flowers more regularly if you do decide to plant them into containers.

Pond Edges

Sounds odd, doesn’t it? Fortnight lilies can also survive round the edges of water, such as ponds and pools. They brighten up these areas and add interest to what otherwise could be sparse landscaping.

Resistance to Fire

Fortnight lilies are resistant to fire, which helps if you live in an at-risk area for wildfires. It’s not fireproof, but it will survive significantly high temperatures, heavy smoke, and some ash.

If the fire does damage your fortnight lilies, cut the plant back, and it should reappear in the following year.

Watering and Feeding

Fortnight lilies will adapt to both dry spells and environments where water drainage is less than ideal. The growth and flower production may be stunted, but they will survive.

For best results, try not to let the soil dry out completely in between watering. You want it to be moist, but not water-logged.

During the summer or hot and dry spells, you can water them up to twice a week in order to keep the growth of the fortnight lilies vigorous. 

Fertilizer

Feeding fortnight lilies isn’t required if the soil is fairly nutritious, but they will benefit from the occasional feed during the summer months. This will help support the flower production, and reduce the energy lost.

Be careful not to overfeed them, otherwise they may not flower at all. 

If the soil doesn’t have much goodness in it, you can add a slow-release fertilizer when you plant the fortnight lilies in order to help sustain their growth.

How to Prune Fortnight Lilies

Pruning fortnight lilies helps support vigorous growth, while also preventing disease and promoting the most flower production possible.

If you’re not someone who collects seeds directly from the plant, remove fading flowers and seed pods as they appear. Seed pods take a lot of energy to generate, and this is energy that the plant can use to make more flowers.

As a general rule, cut off any fading flowers, as this will help the plants produce more, and it will also keep the fortnight lilies looking neat. 

Refrain from waiting until they fall from the plant, as dead plant matter on the floor can invite disease, and attract pests to your plants. 

While it normally helps to leave dying foliage to die back into the bulb for next year’s growth, this is not the case for fortnight lilies.

In spring and autumn, it’s recommended to get rid of old flower heads as well as dead or dying foliage, as fortnight lilies form clumps. If they get too crowded, the flowers won’t appear at all. 

To help prevent this, once the plant goes completely dormant, you should cut the fortnight lily back hard. Do this every few years, and this will help encourage new and vigorous growth.

The Toxicity of Fortnight Lilies

While not as poisonous as other plants, you should still be careful with fortnight lilies, as they are toxic.

The sap from the plant can be an irritant, so wear gloves if you’re going in to deadhead or otherwise prune the plant. 

Always wash your hands after handling a fortnight lily, as you would with peace lilies, digitalis, or anything else which has a risk of poisoning you or others.

Parts of the plant are poisonous to both animals and humans, and the plant itself can be deadly if eaten. 

Signs of poisoning include stomach pain, and vomiting, and while some cases may be mild, it’s best to get them checked out.

So when it comes to planting fortnight lilies out in your garden, be wary of children and pets who might get too curious. Make sure they are supervised, so that no accidents can happen.

Pests and Diseases to Watch Out For

Fortnight lilies aren’t usually bothered by pests or disease, but it’s still a good idea to keep a look-out for them, as catching any early will mean that the plant will recover quicker, and there will be less damage overall.

Foliar Nematodes

Also known as leaf or bud nematodes, these creatures are parasites which feed on leaves, stems, and any part of the plant that sits above the ground.

Most nematodes are a necessary part of the ecosystem, where they recycle organic matter within the soil and other areas. Some are parasites which cannot live without living off a host.

Foliar nematodes are unfortunately microscopic. They can travel between different parts of the plant, feeding off plant cells, juices, and sap. 

You will notice the damage, however. It varies between types of plants, but most growth will become deformed, leaves yellowing before turning brown, and flowers become deformed, cutting their lifecycle short and dropping before they should. 

There aren’t any pesticides you can use to get rid of them, unfortunately. The only real way is to prevent them in the first place, by making sure that every plant you buy is from a respected source, and it looks completely healthy before you introduce it into your garden.

Nematodes need a film of water to settle over the plant, so avoid using sprinklers and overhead watering.

Crown Rot

Crown rot is a fungus which is attracted to heavy, set soil. It decimates plants, and there’s not usually anything you can do once it takes hold of a plant.

Small signs of rot may begin to show only on one side of the plant, but it will spread to the rest. Dark splotches or tan discoloration indicates that the plant tissue has died, and something is very wrong.

If you notice any signs of crown rot, your best bet is to pull the affected plant from the soil, and discard it responsibly. 

Don’t put it on a compost heap or in a garden waste bin, as this will spread the disease. 

Get rid of any affected soil, and sanitize the area. 

If your garden is full of heavy clay soil, improve the drainage by adding grit. 

Make sure to only water your plants when they really need it, watering them rarely but deeply. This will also improve the overall health of the roots.

Rust

Rust is a nasty fungus which decimates a plant from the inside out. You’ll often find it in warm, humid conditions which don’t have much in the way of proper ventilation. 

Rust may not seem like much of a problem to begin with, but leave it unchecked and it will spread to every plant in the proximity, leading to premature plant death, and a very bare bed or border. 

It makes matters worse that rust spores are carried on the wind, which can affect your whole garden. 

You can recognize signs of rust by yellowing, stunted growth, usually appearing on foliage. Orange or yellow blister-like spots may appear on the underside of leaves, and white or yellow splotches on the top leaves. 

If you notice your plants shedding their leaves prematurely, you may have rust. You can use neem oil or a light sulfur spray to keep the infection away, but you also risk killing beneficial insects and microbes living on the plants and in the soil.

To prevent rust, don’t overwater your plants, and when you do water them, water them deeply and infrequently, allowing the water to fully evaporate. 

Prune any plants which are too close to each other to improve air circulation, and this will also stop the plants from getting injured. Only use clean tools to do this, and make sure to sanitize them afterward.

Check any new plants for signs of disease before you introduce it into your garden, and these steps will go a long way to ensure that you don’t have to try and remove rust or any other difficult disease from your garden.

Scale Insects

One of the worst pests which affect fortnight lilies is the scale insect. These tiny insects can cause a lot of damage in a very short period of time.

You’ll notice powdery residue on the leaves, and discoloration, leading to premature leaf drop. If it’s a mild infection, dip cotton swabs in rubbing alcohol and sweep the leaves, on both the underside and the tops. 

If there’s a bigger infestation, use a mixture of water and alcohol to get rid of them, or a solution of neem oil and water.

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