Bigleaf hydrangea care guide: when & how to prune, fertilize and spray

When: from 2 weeks before to 2 weeks after the last spring frost.

• zones 3–5: about Apr 26 – May 24
• zones 6–7: about Apr 1–29
• zones 8–10: about Feb 11 – Mar 11

Next summer's flower buds sit at the tips of last year's stems. Wait until the buds swell so you can see exactly which wood is alive, and cut only above them.

1. Wait until leaf buds are green and swelling. You'll be able to see which stems survived winter.
2. Cut each old, dried flower head back to the first pair of fat, healthy buds below it. Those buds hold this year's flowers.
3. Cut stems that are completely dead (no green buds, brittle and gray) back to the base.
4. On mature plants, remove 1–2 of the oldest, thickest stems at ground level every few years to renew the shrub.
5. Don't shear or shorten healthy stems. You'll remove this year's flowers.

Tools: Bypass pruners, Small pruning saw

Caution: Never prune in fall or winter, and never cut all stems back. That removes the flower buds.

When: 2–5 weeks after the last spring frost, then every 8 weeks (2 times a season).

• zones 3–5: about May 24 – Jun 14
• zones 6–7: about Apr 29 – May 20
• zones 8–10: about Mar 11 – Apr 1

A light feeding in spring and early summer is plenty. Over-feeding gives lush leaves and fewer flowers.

1. Use a slow-release, balanced fertilizer at the label rate, spread around the drip line.
2. For blue flowers, choose a low-phosphorus, acidifying feed. For pink, a higher-phosphorus feed.
3. Water it in well.

What to use: 🌿 Organic acid-loving plant food (e.g. Espoma Holly-tone 4-3-4) · Slow-release balanced fertilizer (e.g. Osmocote Smart-Release 14-14-14)

When: 6–8 weeks before the first fall frost.

• zones 3–5: about Aug 6–20
• zones 6–7: about Aug 30 – Sep 13
• zones 8–10: about Oct 6–20

Late feeding pushes soft new growth that won't harden off before frost and gets killed back.

1. Don't apply any more fertilizer until spring. If you were planning one last feeding, do it now or skip it.
2. Keep watering as normal. Well-watered plants handle winter better.

When: 3–8 weeks before the last spring frost.

• zones 3–5: about Mar 15 – Apr 19
• zones 6–7: about Feb 18 – Mar 25
• zones 8–10: about Dec 31 – Feb 4

This plant prefers a pH of 5.0–5.5 for blue, 6.0–6.5 for pink. A soil test tells you whether you need lime, sulfur or extra nutrients, so you aren't guessing.

1. Take 6–8 small samples from the root zone, 4–6 in (10–15 cm) deep, using a clean trowel.
2. Mix them in a clean bucket, let the soil air-dry, and send about 2 cups to your local extension office or a soil lab. A home kit works but is less accurate.
3. Follow the lime or sulfur amounts in the report. Change pH slowly: no more than 1 unit a year.

Tools: Clean trowel, Plastic bucket, Soil test kit or lab mailer

When: from 2 weeks before to 3 weeks after the last spring frost.

• zones 3–5: about Apr 26 – May 31
• zones 6–7: about Apr 1 – May 6
• zones 8–10: about Feb 11 – Mar 18

Bigleaf hydrangea flowers turn blue in acidic soil where aluminum is available, and pink in neutral or alkaline soil.

1. For blue: water in aluminum sulfate (1 Tbsp per gallon / 15 ml per 4 L) or scatter soil sulfur around the drip line. Repeat in fall.
2. For pink: scatter garden lime at the label rate and use a fertilizer higher in phosphorus.
3. Expect it to take a full season or more. White varieties won't change color.

What to use: 🌿 Elemental sulfur or aluminum-free soil acidifier, to lower pH · 🌿 Garden lime (dolomitic limestone), only if soil test says pH is low · Aluminum sulfate (for blue)

When: 2–6 weeks after the last spring frost.

• zones 3–5: about May 24 – Jun 21
• zones 6–7: about Apr 29 – May 27
• zones 8–10: about Mar 11 – Apr 8

Mulch keeps roots cool and moist, suppresses weeds and slowly feeds the soil. Applying it after the soil warms lets roots wake up first.

1. Spread 2–3 in (5–7 cm) of mulch over the root zone.
2. Keep mulch 2–3 in (5–7 cm) away from stems and trunks. Mulch piled against bark invites rot and rodents.
3. Don't go deeper than 3 in (7 cm) in total. Too much mulch suffocates roots.

What to use: 🌿 Shredded hardwood or pine bark mulch

When: 3–6 weeks after the first fall frost.

• zones 3–5: about Oct 22 – Nov 12

In cold winters, the flower buds at the stem tips get killed. That's the most common reason a healthy bigleaf hydrangea doesn't bloom.

1. Leave the dried flower heads on. They give a little protection, and they're pretty in snow.
2. After a few hard frosts, circle the shrub with chicken wire or burlap.
3. Fill the cage loosely with dry shredded leaves or straw to cover the stems.
4. Remove it gradually in spring once hard freezes are past.