Why is My Yucca Plant Turning Yellow?

It's spring, and many of your plants are pushing out new growth and blooming. But your yucca plant, on the other hand, has started turning yellow! What's going on? Well, you might need to take a closer look at the leaves because you MIGHT just have a few uninvited guests dropping by to suck them dry!

Yucca plant bugs Halticotoma valida (order Hemiptera), which are cousins to other sucking insects such as stink bugs (but much tinier), have already been out in force on my yucca plants for a good month already this spring. I usually try to stay on top of it but I’ve been busy…so the REALLY yellow foliage color today really caught my attention.

Yucca plant bugs are in the order Hemiptera and are related to other sucking pests such as stink bugs and leaf-footed bugs, but are much smaller. Both stages of the insect pierce plant tissue and suck out water and nutrients. This leaves small black dots and a progressive chlorosis (yellowing) of the foliage.

How to Treat Yucca Plant Bugs

Now, I usually don’t treat them - nature usually takes it’s course and my plants are fine; but I’d say this is the worst I’ve seen them. It’s been a particularly cool and humid spring and that’s meant a boom in insect populations.

Due to the severe impact the infestation is now having on the plants, I’ll probably try hitting them with a rotation of alternating insecticidal soap and horticultural oils. But, you really have to get good coverage for these to work as they smother the pests they come in contact with.

There’s no need for me (in this context) to use anything stronger or a systemic pesticide, as again, usually the yuccas pull through with no treatment. In fact, I’ll probably just start with a hard water blast from the hose.

Want better results when starting seeds indoors?

When starting seeds indoors, it's important to manage several environmental conditions...one of those being humidity during germination and sprouting.

When seeds germinate, and the initial but shoot and root initial (radical) emerges, that seedling doesn't yet have root branching or root hairs with which to take up water and nutrients yet. It's surviving off of the resources stored in the seed.

So as the seedling emerges it can quickly shrivel and die if the relative humidity in the space is too low (especially if temperatures around the seedlings are warm or hot due to close grow lights).

You'll need to keep relative humidity (RH) pretty high - around 60% - to keep your young seedlings healthy. You'll need to maintain this RH for several weeks, at least until the seedlings put on their first set of true leaves. Now, this is relative also the species you're growing. Even so, when you're germinating cactus seeds and seedlings, you'll still find higher humidity is going to get you the best results.

Same goes for when you're germinating microgreens seeds.

So, how do you manage to temporarily keep RH high around you seedlings to get them off to a good start? Use a humidity dome!

PC: Leslie F. Halleck

Many humidity domes have top or side vents. After the seeds have germinated and begin growing or cuttings begin to root, you can open the vents to slowly reduce humidity. If too much water begins condensing inside the dome, or you keep the humidity dome on too long, fungal diseases or rot could set in. So if you're using a humidity dome with no vents, you may need to take it off for short periods of time now and then.

Some humidity domes come in taller sizes with a vent you can open and close to conserve or vent moisture and heat. I prefer to use tall humidity domes with vents because it makes moisture management easier, allowing you to grow the seedlings under cover for longer.

If you're germinating a smaller group of seeds, say in a single pot or cup, you can use clear plastic cups turned upside down, or even plastic bags, as a humidity dome. You can also use glass jars or cloche.

If you want to learn more about successful seed starting, check out my book "Plant Parenting" where I guide you step-by-step!

Is your cutting only growing roots with no shoots?

Plant propagation is pretty popular these days. Much of the latest houseplant craze was fueled by a couple specific plants: coin plant (Pilea peperomioides) and fiddle leaf fig (Ficus lyrata). Once more plant parents got their hands on these plants, they of course wanted to try propagating them. But wouldn't you know, not all vegetable plant cuttings are alike, and these two species just happened to throw a lot of first time propagators for a loop. When people tried taking leaf cuttings of both of these species, they may have gotten roots on their cuttings....but never a bud shoot!

Why?

Blind cuttings.

An important thing to know about leaf cuttings is that just because a certain plant can easily develop a callus or adventitious roots on a leaf, petiole, or stem section, it may not be able to develop a new adventitious bud or shoot from that same location. In fact, some leaf cuttings can survive on their roots for a very long time, yet never develop a new bud or shoot. We call these blind cuttings. You'll also see this same phenomenon in Hoya kerrii or heartleaf hoya.

The petiole of this fiddle leaf fig plant can develop roots, but it won’t develop a new bud shoot.
PC: Leslie F. Halleck

Totipotency in Plants

Did you know that not every part or tissue (cell) of every plant can grow roots, or shoots, or both? In each species, each part of the plant - be it the stem, the petiole, or the leaf, each have (or do not have) a particular type of reproductive potential or potency. A seed is an example of plant tissue that has totipotency. That means it contains ALL the necessary genetic information to differentiate into EVERY different type of tissue that plant species will develop. But some plant tissues are pluripotent or multipotent, meaning they may be able to differentiate into some different tissues, not all.

Coin plant, fiddle leaf fig, and heartleaf hoya (to name just a few popular species) won't develop new bud shoots from leaf-petiole cuttings alone. (and none can be propagated just from a leaf cutting). The petiole tissue (the small stem that connects the leaf to the main stem) can differentiate into new root tissue. But not new bud tissue. There needs to be at least some main stem meristem cells attached to the petiole in order to form new bud tissue.

In fact, some leaf cuttings can survive on their roots for a very long time, yet never develop a new bud or shoot.

So if a few of you say "oh you're wrong, MY Hoya kerrii leaf-petiole cutting DID make a new but shoot!" that's likely because there was at least a small piece of main stem tissue still attached to the petiole. It's all about the different types of meristematic tissue that are present in each plant tissue, and that's species specific. (try and take a leaf petiole cutting from your citrus plant and it won't root at all).

And THIS my, plant friends is also why vegetative cuttings of variegated plants that are chimeras (for example variegated Monstera deliciosa) is so difficult. It all comes down to which cells happen to have the genetic code for the variegation (which you won't be able to discern) and are those cells included in the cutting you took.

If you want to learn more about how to take and grow cuttings of your plants, check out my book "Plant Parenting", and if you want to dig deeper into the science of plants and botany, you can join me in my UCLA Botany for Gardeners course.

ICE CUBES: Do they belong on your houseplants and orchids?

You may have heard the recommendation that you water your orchids using ice cubes---that the slow melt of the ice cube will deliver the right amount of water to the orchid over the right amount of time. Lately, I've noticed this recommendation is now migrating over to foliage houseplants. I get asked all the time, especially by my students, if they should or shouldn't use ice cubes to water. I touched on this in my book "Gardening Under Lights", but let's dig a little deeper.

How did ice watering come about?

Well, as a professional horticulturist and long-time avid orchid grower, I watched the ice cube watering method develop as a marketing campaign from within the industry. WHY? Well, orchids used to be few and far between in terms of commercial availability. When I was a budding horticulturist, I had to shell out a pretty penny...we're talking $60-80 for what are now very common butterfly orchids (Phalaenopsis spp.) Eventually, growers figured out production methods that worked well in commercial greenhouse production that allowed for the mass growing and availability of moth orchids (which are generally some of the easiest to grow, that's why the are EVERYWHERE now).

This growing method involves using clear plastic growing pots tightly filled with sphagnum moss. This method allows growers to successfully manage water and nutrients in a controlled environment (versus the looser orchid bark type mix you'll typically see recommended for home growers). These plastic growing pots are then set inside a water-tight cachepot for sales.

The tight root-bound pots with moss, set inside a dark watertight container, become a very-than-ideal long-term growing medium that holds too much moisture for these orchids once they arrive at your home.

• You likely have a LOT less light in your home than where they were professionally grown.
• That likely means you could easily overwater, given the lack of light and the watertight cachepot.

• When customers kill certain plants too quickly, they stop buying them, so...

Orchid growers started actively marketing the "ice-cube watering method" as a strategy meant to mitigate potential over-watering in a less than ideal growing media and environment for moth orchids long term.

As epiphytes, moth orchids generally thrive with much more air flow around their roots, which are adapted to absorbing moisture from humid air and nutrients from surrounding debris that settles around the roots. While professional growers have developed successful management strategies for growing moth orchids en masse using these specific materials and methods, it’s much more likely a home grower will overwater the plants, unless they transplant the orchids to a looser orchid bark mix or more porous containers. The tight plastic sleeve containers and dense moss simply don’t allow for as much air space or drainage. In fact, you may not even be “overwatering”, but the nature of the container and media simply doesn’t allow for enough air flow, even if you aren’t watering too frequently. The ice cube watering method was developed by the orchid-growing industry to help customers to slowly water the plants enough, without overdoing it. Essentially, it’s a work around for a less than ideal long-term growing media and container combination.

If you want to keep your plant long-term and rebloom it, then after it finishes it's initial bloom, I'd remove it from the plastic container and repot your orchid in a container that provides good drainage and aeration, and use a loose orchid potting mix. I guarantee you'll never overwater your orchid again!

Is There any Research on Ice Cube Watering

University research has been performed with ice cube watering methods specifically on moth orchids (but not other houseplants as far as I can find). If done properly, which requires you use three ice cubes and do not allow them to touch any part of the plant, there shouldn’t be any damage to the plants from the initial cold temperature of the ice.

Should You Water Your Orchids with Ice Cubes?

I prefer to use and recommend the standard run-water-over-the-roots-and-out-of the-pot method for a few minutes to introduce more oxygen to the root zone. A good soaking with drainage once per week for moth orchids is standard, but that depends on your environment and if you’ve repotted your orchid into a looser mix. A dry home or repotted plant often requires more frequent watering. (This method of course may not apply to other species of orchids that you grow under glass or in orchidariums, etc...all care methods depend on the species and environment).

Most orchids need a wet/dry cycle---they shouldn’t stay wet all the time.

This is not to say the ice cube method won’t work fine for you for a while. If you keep moth orchids as temporary décor, and expect to replace them every few months after they’ve finished blooming (you can put your spent plants into a compost bin), then the ice cube method is perfectly fine if it’s easier for you. If you’re having success with it long term, then great. But, you’ll need to keep an eye on root health and compaction, and make sure you aren’t seeing root rot develop on plants you intend to keep and re-bloom.

Should You Water Tropical Houseplants With Ice Cubes?

The bigger issue I see at this point is using ice cubes to water all your other tropical houseplants. Temperature aside, throwing a few ice cubes on top of the soil in your potted plants could cause you to underwater the plant (you might be surprised how little water 3 ice cubes may deliver given the size of your container), or simply never get enough saturation lower in the pot. This can cause both lower roots to start to die off from drying out, and result in a build up of salts in your containers. Both are bad news for your plants.

If you use ice watering on houseplants now and then, fine, but make sure your plants get a thorough watering when they need it and you let water run out of the bottom of the pot to leach out built up salts.

DON'T Make Ice Cubes for Your Plants!

Lastly...remember that it takes energy to freeze ice. Environmentally speaking, if there is no real benefit to using ice cubes to water your plants....why burn fossil fuels and put your freezer to work like that? Sure, use the ones you have left over in your cup if you want....but skip making ice cubes just for your plants!

Ever Seen Moss Spores?

Omg LOOK AT HER RECEPTACLES! I admittedly get a case of the vapors when I find such ripe receptacles on liverwort gametangia nestled in a soft tuft of Bryophyta (mosses). HOT.

If you want to hear more dirty talk like THIS you’ll have to join me in botany class through UCLA Extension Horticulture, which starts 4/1/24! A new online section is OPEN for registration (if you’ve been on the waitlist), and you'll learn all about what's going on in this video.

Botany sections fill fast so don’t wait…Let’s go botanize!!

Confused About Garden Soil Health?

You might be a plant and garden nerd if you get a case of the vapors when you get your hands into soil like this! (NO JUDGEMENT HERE!)

Soil structure, biology, and ecology - as well as nutrition in the context of plant growth - is fairly complex. But without it, and a descent basic understanding of soils, it’s tough to live out your gardening (and houseplant) dreams the way you may envision.

NEW Soil is Not Instantly Better

Did you know? “NEW soil”, be it in the form of bagged or bulk mixes and compost, aren’t “better” right out of the gate. I know, it’s a little counterintuitive. I can’t count the number of times I’ve een asked “why are my new vegetable transplants turning yellow when I just planted them in all new soil!” Well, it’s the microbes that are really doing all the heavy lifting for you and breaking down matter to relsease nutrients for your plants.

Until there are enough of them to do the job, which happens over time in the garden after you’ve added new soil or amendments, you’ll probably need to supplement your plant’s diet.

Want to learn more about soils, be it for native ecosystems, managed landscapes, or indoor plants and growing?

I cover soils and plant nutrition, in different depths and contexts, in each of my UCLA Extension courses: Botany for Gardeners, and Indoor Plants: Care and Maintenance. Both are open for spring registration now!

Why Humidity Matters for Houseplants

Check out my recent appearance on Episode 222 of the Growing Joy Podcast

Here is a short synopsis from Maria of my latest appearance on the Growing Joy Podcast, to talk about humidity for houseplants:

Do I really need a humidifier? Why do my plants get crispy leaves when I crank the heat? When it comes to houseplant care, humidity is right up there with water and light as one of the most important factors. I’ve spent years stressing about humidity levels. Fortunately, I invited my dear plant friend and humidity expert, Leslie Halleck, for another session of Grow Better. Understanding the science behind humidity has helped me stop freaking out and keep my plant collection happy.

Many popular houseplants are tropicals that originate in humid, rainy environments. Places like the steamy rainforest floor where humidity levels hit 90%! No wonder these plants rebel when we try to grow them in our homes.

Humidity affects the process called transpiration, which is the movement of water from the soil, through the roots, up the stem, and out through pores in the leaves. When humidity is too low, transpiration happens faster than plants would like.

Their leaves lose moisture quickly, which causes wilting, yellowing, and crispiness (I see you, my diva Calathea).

Click Image to Listen to the Podcast Episode

OR download in your favorite podcast App HERE

Understanding Key Humidity Concepts such as Vapor Pressure Deficit

RH: Relative humidity tells you the percentage of moisture in the air based on temperature.

AH: Absolute humidity is the actual volume of water vapor droplets that are in a given amount of air.

VPD: Lastly, vapor pressure deficit (VPD) indicates how far the air is from saturation – like how much moisture is missing. So the higher the VPD, the faster your plant will lose moisture from its leaves.

Example: Warm air can hold more moisture than cooler air. So even if relative humidity reads 40% in a hot room and 40% in a cold room, plants will lose water faster in the hot room because of the higher VPD.o check out her new book, Tiny Plants.

What Doesn't Work to Boost Humidity

I was shocked to learn that some popular blogs about boosting humidity just don't work and can be more of a placebo.

Misting plants only increases humidity briefly before settling back down, risks inviting fungal diseases onto leaves, and misses most leaf pores anyway.

Pretty pebble trays are equally lackluster despite their popularity on blogs. Any moisture they add to the surrounding air dries up quickly without noticeably impacting VPD.

Leslie says humidifiers can work but only in large capacity, used regularly, and grouped closely with plants. But even humidifiers struggle to overcome the dry air gushing inside from HVAC systems all day long.

Why Winter Air is Drier

Winter often brings dry air indoors. Natural humidity outdoors is typically lower, and heating furnaces dry the air as it passes through them. Plus the warm air running across leaf surfaces increases transpiration. All that blasting heat from furnaces keeps VPD high indoors, and HVAC systems are built to actively strip moisture out. This explains why our homes tend to be MORE dry in winter than in the summer, even though it's more humid outdoors in the summer.

Managing Moisture Indoors

So what does work to boost moisture for houseplants? Here's what really works:

Growing Under Glass: For humidity-loving plants like aroids and Calathea, Leslie said the best tactic is growing them under glass to contain moisture around the plants.

Creating mini greenhouse environments seals in moisture availability. This could involve a glass cabinet, terrarium, cloche, or Wardian case. I was amazed to learn that these diva plants thrive with less maintenance when grown this way!

The glass recycles humidity at ideal levels to support healthy transpiration. As a bonus, this method prevents messy watering spills around your home.

As Leslie reminded me, getting to know your plants means catering conditions to their preferences. For many indoor plants, high humidity is non-negotiable

PC: Leslie F. Halleck, "Tiny Plants" - Microgramma heterophylla

Light Still Rules

Leslie reminded us that sufficient light still governs everything for houseplant health. No amount of perfect humidity can save a plant that doesn't get enough light! So address both moisture AND adequate sunlight to help plants thrive indoors. I hope you found this episode as helpful as I did! Leslie is truly a humidity expert. If you want to know more, she offers online plant classes via UCLA Extension open to all. You can also book private online plant parties with her on her website to get advice from fellow plant lovers.

Important Links mentioned in the episode

• Botany for Gardeners Course, UCLA Extension
• Indoor Plants: Care and Maintenance Course, UCLA Extension
• Leslie’s Book Tiny Plants
• Growing Joy Ep 169: How To Prep Your Houseplants For Winter
• Growing Joy Ep 107: How To Care For A Large Plant Collection
• How to Care for Alocasia // My Favorite Alocasia Varieties! // Alocasia Care Guide
• Leslie’s Architectural Digest interview on indoor greenhouses

Struggling With Your Fiddle Leaf Fig? Here's Why

Why are you FLF’ing yourself?

I hereby absolve you of any responsibility for growing a beautiful fiddle leaf fig (Ficus lyrata) indoors, and any green guilt you may be carrying around because you keep killing them (or they constantly look so sad in your house that you feel like a bad plant parent).

See, here’s where knowing a little botany and understanding some light science comes in handy. Folks, FLFs are BIG ASS trees, and not only that they are what we consider fruit bearing trees (you know, figs?). They are in the FIG and MULBERRY family, which should tell you something about the kind of light (ENERGY) these beasts really need.

Not only are these trees getting exponentially more light outdoors (even in a dense rainforest type situation, and many FLFs will grow quite happily out in full sun) but they are also semi(hemi)epiphytic (which is why the root zone needs some drying between waterings ) and like warm humid environments to sustain that form of growth.

We stick them in our dark, cold, dry homes and then wonder why they always look so sickly.

I guarantee you, 99% of you that have a happy FLF have an FLF that lives OUTSIDE on your patio, at least for a good part of the year! OR you have several grow lights, or one more powerful grow light, that is keeping your FLF from the edge of death.

While FLFs might not like super hot direct sun coming through a window and hitting it’s leaves (very high PPFD), that doesn’t mean it doesn’t need a substantial DLI (daily light integral) - meaning the overall quantity of light it receives through the day - to thrive, rather than just barely survive.

1. You never have as much light in your house as you think you do.

2. It’s ok to get rid of plants that constantly frustrate you.

3. Your compost pile is your friend.