Alyssum, Marigolds & Insects: Insights from Research

I ended up writing this blog post twice because my first attempt went totally off course. My mistake was relying on “light” internet research.

A reader had posed a question about attracting beneficial insects and repelling insect pests. I began my research with the intention of finding ways to repel insect pests. One popular topic that came up was using plants to repel mosquitoes. If you do a quick Google search like I did, you’ll find tons of articles claiming that plants like lemon balm and lemon thyme, which contain citronella, can ward off mosquitoes. And while there’s some truth to it, as I soon discovered, simply planting these herbs won’t magically repel pests. According to the Mississippi State University Extension, you need to crush a lot of these plants to release the volatile oils that actually do the job.

Then, I came upon a revelation from the Colorado State University Extension: “Rumors and misinformation abound regarding plants that will repel mosquitoes in home landscapes. Common plants purported to repel mosquitoes include catnip, peppermint, rosemary, marigolds, Eucalyptus and Artemisia species, to name a few. None of them will repel mosquitoes by merely growing in a landscape.”

“Rumors and misinformation abound regarding plants that will repel mosquitoes in home landscapes.”

Colorado State University Extension

The Myth of the Marigold: Is It True?

Now let’s chat about marigolds because I’ve heard from fellow gardeners that these work wonders for keeping pests away. But after digging deeper, I found a statement from the University of Arizona Extension that made me pause: “The data is lacking as to whether marigolds actually deter insect pests.” This is downright disappointing.

However, it turns out that marigolds do contain compounds that can tackle root knot and other plant-parasitic nematodes, which is music to the ears of anyone who’s dealt with these pesky pests.

On top of that, marigolds attract beneficial insects like lacewings and ladybugs, which is a huge win in my book. So, while they might not be the pest-repelling champs we hoped for, they still bring valuable benefits to the garden in addition to being beautiful and cheery.

Embracing Beneficial Insects

Let’s shift gears to something that’s seemingly more achievable than repelling pests with plants; let’s focus on attracting beneficial insects, which are also referred to as “natural enemies” of pest insects. Beneficial insects include bees, lacewings, ladybugs, parasitic wasps and praying mantises, and they perform a number of crucial roles in the garden, including pollinating flowers and vegetables and keeping aphid and scale populations in check.

The University of California Extension has done some extensive research on the role of beneficial insects and the plants who love them, showing that planting hedgerows of diverse plant populations around farms helps reduce insect pest pressure because these plants—which they call “insectary plants”—attract, feed and shelter insect parasites and predators of pest insects.

While their research primarily targets larger-scale agriculture, we home growers can definitely take a page from their book. Alyssum and cilantro, for example, are easy-to-grow plants that can support beneficial insects throughout the seasons. I’ll admit, I never paid much attention to alyssum until now, but I’m definitely giving it a second look.

Here’s a list of flowers and herbs to attract beneficial insects, based on the UC Extension’s recommendations.

Addressing Reader Questions

I have one reader question left to address, about seeding indoors in trays versus direct sowing. I can answer this one pretty quickly here.

Direct sowing. Direct sowing involves planting seeds directly into the garden soil, bypassing the need for seed trays or indoor setups. One of the biggest advantages of direct sowing is its simplicity and convenience. You skip steps like transplanting. The downside to direct sowing is that it’s harder to control the growing environment for the seed such as heat, moisture and light. Weather conditions can impact germination and early seedling growth. And, as I’ve discovered, some pests like to eat small sprouts and seedlings but will ignore the plants once they’re a little larger. So I’d say direct sowing comes with a greater danger of losing plants to pests.

As you can see, I’m a big fan of starting seeds indoors (photo taken February 18, 2024). I like to have more control over the germination and growth rates of my plant starts.

Starting in seed trays. Starting seeds in trays or pots indoors gives you a lot more control over the seedling’s environment, especially if you have a heat mat and growing light. When I use these tools, I see higher and faster germination rates, even for crops that are typically direct sown like beets and carrots. Starting seeds indoors also gives you an extended growing season because you can start seeds in your comfortably warm home while it’s still chilly outside. The challenge with starting indoors is that it takes more work than direct sowing. It also requires more equipment and some plants don’t like the transplanting process. That said, I’ve heard peas don’t like to be transplanted/their roots to be disturbed, but I’ve never had a problem transplanting snap peas or sweet peas. I’m just gentle with the roots in the process.

I transplanted these sweet peas from seed trays into the soil last October, even though sweet peas are “supposed” to be direct-seeded. As of mid-February 2024, they’re healthy and strong–and are even starting to flower!

Most seed packets containing growing information that will recommend whether to start seed indoors or direct-seed into the ground. Ultimately I’d say it’s up to you and your personal preference.

If you’re wanting to learn seed-starting, please join me for a seed-starting workshop hosted by San Diego Seed Company on Saturday, February 24 at 10:30 am to noon at Farmstand 67 in Ramona. This will be a hands-on workshop with all materials provided.

I really appreciate the reader questions that sparked my last couple of posts. If you have questions, please send them my way!