Gardening with Xpress: Leaf mulch, mushroom compost and dahlias!

SOCIAL DISTANCE: When planting multiple dahlia tubers, it's best to keep them about 12-18 inches apart, as dahlias get quite large. Photo courtesy of HummingBlooms Flower Farm

Hi there neighbors, Chloe here, back again with some more gardening guidance. This month’s reader questions were about mulch, compost and dahlias. We’ll explore the qualities and uses of leaf mulch versus mushroom compost, how to separate and plant dahlia tubers and some tips for caring for dahlias once they’re in ground.

A reminder to email me your questions at I can’t wait to hear from you!

Which is better for a vegetable garden, leaf mulch or mushroom compost?

Both leaf mulch and mushroom compost can be great garden amendments. In fact, they have a lot in common. Each one adds some of the same compounds and qualities to your garden, such as organic matter and trace minerals. On the other hand, they are distinct, and one or the other may be more suited to your particular situation.

First, let’s dive into some definitions: What are leaf mulch and mushroom compost?

Leaf mulch is just what it sounds like: decomposed leaves that have fallen from the many beautiful and generous deciduous trees that share these mountains with us. Often, you can find bags of leaves free for the taking on curbsides during the fall. Sometimes these leaves will be mixed with dry pine needles, which are less ideal to add to the garden due to their acidity. A few pine needles are fine, but my definition of leaf mulch includes a majority of broad-leafs. When you pile up the dry leaves and let them crumble and break down, they transform into leaf mulch.

Mushroom compost is a little less straightforward. It’s basically the leftover substrate that commercial culinary mushrooms have been grown on. After mushroom farmers are done with it, they compost it, then bag it up and sell it to us gardeners. According to the Oregon State University Extension Service, “the recipe for mushroom compost varies from company to company but can include composted wheat or rye straw, peat moss, used horse bedding straw, chicken manure, cottonseed or canola meal, grape crushings from wineries, soybean meal, potash, gypsum, urea, ammonium nitrate and lime.”

Some of these ingredients, notably horse bedding straw, cottonseed, canola and soybean meal, may harbor significant pesticide residues and/or genetically modified contamination. When you buy mushroom compost, there may or may not be a specific list of ingredients on the bag, so it’s a bit of a gamble as to what you’re getting.

As I already mentioned, both are very high in organic matter, which helps loosen the soil, feed soil microorganisms, increase water retention and drainage (isn’t that magical‽) and boost root growth and nutrient mobility in the soil. Leaf mulch is higher in trace minerals, since the leaves grew on deep-rooted perennial trees. Mushroom compost is higher in nitrogen, which boosts green growth of all plants, and phosphorus, which supports fruiting and flowering.

The downsides of leaf mulch are that it’s low in nitrogen, so it doesn’t act as a fertilizer per se, and it takes a bit of elbow grease to collect and make. Mushroom compost has its flaws, too. For one, you can’t just collect it from the forest or neighborhood yard cleanups; it costs money. Additionally, mushroom compost can be high in dissolved salts. This means it’s not gentle enough for germinating seeds, very young plants or salt-sensitive veggies like turnip, radish, carrot, onion and lettuce.

How to use leaf mulch and mushroom compost in the garden?

Both well-broken down leaf mulch (not just crumbled-up fresh leaves) and mushroom compost can be mixed into garden soil during bed preparation, though mushroom compost shouldn’t be added like this if you plan to direct-sow seeds. Less-broken down leaf mulch works well when spread atop the soil surface, around veggie plants. Applied in this way, it can suppress weeds and improve water retention while slowly breaking down to feed the soil. To apply mushroom compost after the fact, it’s best to dig it in gently around the base of established plants. These rich sources of organic matter and other plant nutrients can be added any time of year and will benefit your garden soil for years to come.

What’s the best way to separate dahlia tubers?

Dahlias, like potatoes, store their energy through the winter in underground tubers. Once the soil warms up in spring, sprouts pop up out of “eyes” that grow into the luscious flowering beauties we adore. Dahlia tubers, which are living, breathing storage organs, will die if they freeze. So, many gardeners dig them up in the fall and store them in cool (but not freezing), moist conditions until spring planting.

THE EYES HAVE IT: Unlike potatoes, dahlia tubers don’t have a smattering of eyes all over their bodies. Their eyes, circled in red, are just up top, near where the tubers connect with the stalk. Photo by F.D. Richards

Each year, dahlia tuber clusters expand, leading to larger and more robust plants the following spring. A benefit of this natural growth is that we can separate or divide dahlia tubers to produce more individual plants. Depending on how big the cluster of tubers is at the base of a dahlia stalk, we might be able to turn one plant into two, three, four or more. The trick is to cut off an entire tuber (or tuber cluster) at a time, including the eyes. Unlike potatoes, dahlia tubers don’t have a smattering of eyes all over their bodies. Their eyes are just up top, near where the tubers connect with the stalk.

To divide dahlia tubers, use a very sharp and clean knife; a shorter curved blade is ideal for dahlia dividing, but a straight one will do. Work at the base of the stalk and identify eyes. They are small bumps and can be hard to discern; waiting until sprouts begin to emerge makes this much easier. Slice around an eye or group of eyes, being careful not to break off the juicy tuber beneath. After you’ve cut off an eye-with-tuber pair or group, leave them exposed to air for a day or two to help heal the wound, then either plant them in the ground or restore them to storage conditions. Here’s a great video on dividing dahlia tubers from Swan Island Dahlias, the nation’s largest dahlia grower.

How to plant and support dahlias?

To plant dahlias, dig a hole 4-6 inches deep in rich garden soil, place a tuber on its side, eye facing up, and cover with soil. Plant dahlia tubers about 12-18 inches apart, as they get quite large! Don’t water them much until they’ve poked through the soil, as the tubers can rot in overly moist soil if they’re not actively growing. Also, don’t feed them with high-nitrogen fertilizers; instead, use composted cow manure or a product called “bloom food.”

Once they’re about 1 foot tall, snip or pinch off the top 3-4 inches to encourage a bushier growth habit that will yield more blooms.

To support tall, top-heavy, abundant dahlia plants, you can stake each one individually by tying its main stalk to a piece of wood or bamboo planted near the tuber as it grows. Alternatively, if you have a large dahlia patch, corral all of the plants together by pounding stout T-posts every several feet along the perimeter and running two courses of string around the whole party.

A special thanks to Michelle Morrision of HummingBlooms Flower Farm for sharing her dahlia-growing wisdom!


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