Growing hostas is easy if you satisfy their single, non-negotiable demand – healthy garden soil rich in organic matter is the shortest route to meeting this need.
The organic matter and microorganisms in compost help create the soil characteristics required by hosta plants. Compost introduces a diverse and essential population of organisms that boost several beneficial features of a healthy soil – and as we all know, healthy soil produces healthy plants.
While some hosta plants can tolerate sun, all varieties require soils that drain well yet retain moisture – a feature best achieved by adding compost.
Well-drained, moist soil and shade are generally the only requirements for successfully growing hostas. With the what already established, let’s explore the why, when, and how of cultivating the best soil to grow hosta.
Why Compost – What Are the Benefits
While compost is generally viewed as the material added to boost the percentage of organic matter in the soil, maybe to lighten clay soil or improve the water retention of sandy soil, it is much more potent (and essential) than that.
Below are five garden soil-health characteristics that compost improves.
Increases Plant Nutrient Availability for Hosta
The concept of bio-available nutrients applies – complex mineral compounds need to be simplified for plant availability.
In the composting process, billions of microorganisms work to decompose organic matter, consuming carbon and nitrogen (and some of the other 17 essential plant minerals) in the process.
The carbon is burned off as CO2, and the nitrogen is used for their growth and reproduction.
Most of these organisms die in the composting process, and their microbodies, laden with simplified nitrogen, form part of the organic matter (humus) you add to your soil.
However, billions survive and head straight for the plant roots – a source of simplified carbons and amino acids.
They’re eaten by predators right at the roots, releasing the bio-available essential minerals exactly where the growing hostas need them.
Maximizes Cation-Exchange Capacity (CEC)
Cation-exchange capacity (CEC) is a vital soil attribute affecting nutrient availability, soil structure stability, fertilizer, other ameliorant reactions, and soil pH.
Negatively charged soil particles attract positively charged molecules – water, soil amendments, and fertilizer. When the CEC value of the soil is low, only a few fertilizer molecules are attached to the soil particle surface.
Your soil’s CEC is given when you do a soil test (hopefully, every three years). The lowest CEC is sand (about 10 cmolc/kg), and the strongest CEC is found in clay soil (25 cmolc/kg).
Adding organic matter boosts CEC levels to 100 cmolc/kg. Leaf mold, for example, has such intense electrical activity that it can power a small device.
A strong CEC prevents water from draining from clay soil, and a low CEC lets water (and fertilizer with it) flow through the sandy soil at 20-inches per hour – leaving little fertilizer at the roots.
The link is clear: soils with low organic matter levels have low cation-exchange capacity. They cannot hold on to water and fertilizer—plants in pots or beds where the ground has a low CEC wilt sooner and generally perform poorly.
Improves Soil Composition and Structure
Soil is a three-part open system consisting of solids, liquids, and gases to provide a life-sustaining environment for plants and organisms.
Where soil solids aren’t cemented together in peds or aggregates (silt and sand), they are prone to erosion. The shape and strength of the aggregates influence pore size and distribution and water, gas, and nutrients.
The peds’ binding substance is a product of bacteria, fungus, and actinomycetes – organisms abundant in compost. These microorganisms increase aggregate formation with built-in channels, enabling the retention and flow of air and moisture.
Improves Water Infiltration, Drainage, and Retention
Compost is a preferred management strategy to raise infiltration, drainage, and water retention capacities – a function of the stability and number of fine aggregates it produces.
With the capillary action within soil pores, this water resists evaporation and percolation and is readily available for your hosta’s use without causing wet feet.
For healthy hosta plants with bright hosta leaves of green and white (Hosta, Curled Leaf), bluish-green (Hosta, Siebold), yellowish-green (Hosta, Fragrant), or glossy green (Hosta, Lance Leaf), the soil needs to be moist, but not wet, and there needs to be ample shade.
The shady spot is your choice; the well-draining, moist soil will be taken care of by adding compost – the result: a beautiful and healthy hosta.
Much More Mulch for Hydrated, Healthy Hostas
Shredded bark is an excellent mulch for the garden – it’s organic and displays well.
Leaf mold is often a suggestion for a good mulch, but remember that hosta roots require moisture, and leaf mold is hydrophilic – it draws water from its surroundings. Leaf mold is better added to pots of hosta or other potted plants – more on this later.
Hostas will benefit from organic mulches such as crushed bark, shredded leaves, or pine needles, which will help prevent moisture evaporation. Mix in some coffee grounds for added carbon content.
A word of caution – too much mulch creates a habitat for slugs to breed.
While adding Epson Salt (Magnesium Sulfate) may be beneficial to growing larger hosta plants that accentuate the dramatic hosta leaf, the truth is that merely adding compost addresses all the soil needs hosta plants may have.
What Is the Best Type of Compost to Use on Hostas
Firstly, let’s establish that there are several types of compost: aerobic compost, anaerobic compost, leaf mold compost, vermicompost, and actively aerated compost tea, to mention the most popular varieties.
An additional factor to consider is the final product’s fungi to bacteria ratio – read on and learn.
Fungi populations are essential for plants with rhizomes – like the hosta plants – which are members of the lily family.
Hosta plants do well with compost which is slightly higher than a 2-to-1 fungi-to-bacteria ratio.
Getting that ratio right during the composting process is challenging but is more easily manipulated by using the compost to make an actively aerated compost tea.
Actively aerated compost tea (AACT) is made by submerging a horticulture teabag filled with composts into a pail of water and pumping air through the mix for 24-hours.
The air ensures that the microorganisms don’t drown, helps humic acid production, and, if you add a protein like fish hydrolysis, the fungi population is boosted. Adding molasses promotes bacteria proliferation.
What Are the Best Ways to Use Compost
It’s important to note that not all composts are equal.
If you’re making your own compost, always aim for a 30:1 carbon to nitrogen ratio, a sustained temperature above 146 °F for at least four days, and managed water and oxygen level – water 50% and oxygen above 6% – integrated by turning the batch.
After composting the whole batch, keep turning and keeping the pile moist until there are no further temperature spikes – an indication that it is well cured.
The target ratio of carbon in your soil is 5% – unachievable in a single application of compost.
Perennials allow you the opportunity to add a compost application twice a year – two weeks before the growing season, and at the end of the growing season.
Compost tea can be added more frequently as a hosta foliage application – monthly ought to suffice.
What to Look out for When Using Compost
If you add uncured compost to your garden or potted hostas, the microorganisms will use the available nitrogen in the ground as an energy source to consume the available carbon.
It’s advised that in a fresh garden bed, you add your compost a fortnight before planting your hosta roots, and allows the compost to stabilize and benefit your soil and hosta health.
Use a compost brand you trust – preferably one approved by USDA with the organic seal.
Using Compost in a New Hosta Bed
If you’re growing hostas in a fresh bed or pots, test your soil and use a recommended fertilizer to adjust the nutrients required before adding compost – do this two weeks before you plant your small hosta root balls in early spring.
Select a site where there’s partial to full shade – depending on hosta variety.
The initial fertilizer application should be high in phosphorus (5-10-5) to boost blooming, brighten hosta foliage, boost leaf production, and make them more resilient to pests and pathogens.
Watch out for too much nitrogen fertilizer – two ounces per 125-square-foot bed of hosta is ample.
Too much nitrogen, and you’ll need to manage diseases on the hosta foliage.
Recommended fertilizer rates for growing hostas are 2-pounds of 5-10-5 per 100-square-feet.
Growing hostas with foliage are more manageable if the starting soil is a healthy balance of nitrogen, phosphorus, and potassium (NPK fertilizer).
Don’t add fertilizer unless you have a soil test informing the relevant quantities. Once the base nutrition is established, add compost that further enhances nutrient availability.
Add three to four inches of compost and work it into a depth of 10-inches.
Adding Compost to an Existing Hosta Bed
For most perennials, a top dressing of compost of 1 to 2-inches will provide season-long fertility.
Hosta is not a heavy feeder, unlike Daylilies, mums, and tall phlox. A lightly incorporated inch of compost will be ample to keep your hosta foliage looking pristine.