Fertilizer Basics for Hosta Growers

Hostas make extraordinary ground cover perennials and are by far the most popular plant for shady areas, such as under deciduous trees.

Offering seventy species and hundreds of hybrids, gardeners can create striking borders of any color between vibrant greens, chartreuse, and blue.

Though not high feeders, applications of nitrogen and potassium will boost the growth and foliage health of your Hosta if used correctly. When fertilizing your Hosta beds, err on the side of restraint, sticking to less than a pound of 10:10:10 per 100 square feet.

Should You Fertilize Hostas

Hosta plants are considered low maintenance plants, so fertilizing hostas (and all plants) should be informed by soil tests rather than your desire to boost their growth.

Too much fertilizer can cause more harm than good. A healthy approach is cultivating soil health rather than focusing on leaf growth. The former precedes the latter.

Soil Testing

There’s an adage, “measure what matters.” Healthy soil matters as it’s the foundation for all your gardening dreams, so knowing its condition is vital.

Soil tests inform you of the nutrient levels of your soil, the soil pH, and even the soil’s ability to manage those nutrients – the Cation Exchange Capacity (CEC).

Soil samples should be randomly taken from across the yard and submitted to a laboratory. If you’re really interested in knowing the health of your soil, you may want to have the microorganism population reviewed as well, but that’s a whole different topic.

Soil pH

The test will tell you if your soil is sour (acidic) or sweet (alkaline) – it even tells you if lime is needed and how much. Often, an iron deficiency is caused by too alkaline soil – a high pH.

The test results will indicate the actual pH compared to the target range – based on soil type and what you intend to grow.

Generally, lime is added to increase the pH, and sulfur decreases the pH (increases the acidity). Lime Lifts and Sulfur Suppresses it. As per the graph below, a pH of 7.00 is considered neutral.

Hosta plants prefer a pH range between 6.50 and 7.5, with a bias toward slight acidity (6.5). Remember that 6.5 is five times more acidic than basic.

Lime Recommendation – If pH is low for the crop you intend to grow, lime will be recommended. The advised rate is in pounds per 1000 square feet.

The general rule is that adding lime is safer than adding sulfur – lifting your pH is safer than suppressing it. Remember that added lime may take as long as six months to move the needle.

Sulfur suppresses the pH – apply only the amount recommended by the report, remembering that it takes two to three months to have an effect. A good rule is only to add sulfur if the pH is over 7.5 and plants show symptoms of micronutrient deficiency.

Epsom salts (magnesium sulfate) lowers the pH and provides magnesium and sulfur, two nutrients often deficient in alkaline soils. A better course of action is to add compost, but if you do apply Epsom salts, do it no more than twice per growing season.

Understanding Soil Test Fertilizer Recommendations

The test’s recommendation will specify the quantity per thousand square feet and the NKP ratios. Let’s review the numbers:

  • N – nitrogen – boosts growth, but not flower growth necessarily.
  • K – phosphorus – essential for flowers, fruit, and roots.
  • P – potassium – also called potash, it boosts plant hardiness and flavor.

The test measures the phosphorus and potassium content – nitrogen is too volatile to measure accurately and is often the most limiting nutrient.

Nitrogen

  • Promotes green, leafy growth
  • Most commonly deficient
  • Tends to leach into the soil, polluting surface and groundwater
  • Insufficient nitrogen stunts growth and cause leaves to yellow – starting with the oldest leaves
  • Too much nitrogen:
    • Burns plants
    • Can increase pest problems
    • Reduces flower yields
    • Reduces plant resilience to stress

Phosphorus

  • Promotes root growth, flower, fruit, and seed production
  • Held tightly by CEC action in soil – leaching rare
  • Causes pollution when soil erodes
  • Needs to be incorporated before planting – at least a month before
  • Frequently fertilized soils often have too much phosphorus, which negatively affects nitrogen fixating microorganisms
  • Insufficient phosphorus causes:
    • Reduced growth
    • Plants dark green – Purple or reddish color to older leaves

Note: Phosphorus is not taken up well in cold or wet soils. Deficiency symptoms in winter are usually due to cold weather rather than a lack of nutrients in the soil

Potassium

  • Increases drought tolerance, disease resistance and improves winter hardiness
  • Deficiency is hard to observe, though levels are often low

The Essential Nutrient Element – Organic Matter (Compost)

Compost has several benefits that only it can provide – improved soil health, microbe support, tilth improvement, pH buffering, and aggregate formation.

It adds some nutrients, but its primary benefit is the soil’s improved water and nutrient-holding capacity – it improves the efficacy of fertilizers.

Organic Fertilizer Alternatives

  • Nitrogen – blood meal is 12% nitrogen
  • Phosphorus – Bone meal is 10% phosphorus
  • Potassium (Potash) – Greensand is 5% potassium and 1% phosphorus.

Cation Exchange Capacity (CEC)

Your report will provide you with a CEC measurement, the ability of your soil to hold and release water and nutrients to your plants – think of it as your soil’s nutrient magnetism.

There’s a whole science to CEC, but the answer is always organic matter – compost. Compost can boost your CEC beyond what any other substance can.

Closing Thoughts

Cultivating healthy soil is a long-term investment in a beautiful garden, irrespective of what you grow. Plants grown for their foliage, such as Hosta, benefit from a limited nitrogen application, but excessive nitrogen does more harm than good.

This holds true for both your garden and for the environment.

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